Chemistry---Part-1---Class-12

E+S E–S E+P centres on the surface of Active site ES enzyme particles. The P molecules of the reactant E+ S (substrate), which have E+ complementary shape, fit into these cavities just like P a key fits into a lock. On account of the presence of Enzyme Substrate Enzyme-Substrate Enzyme Products active groups, an activated (catalyst) (reactants) complex complex is formed which then decomposes to yield Fig. 5.4: Mechanism of enzyme catalysed reaction the products. Thus, the enzyme-catalysed reactions may be considered to proceed in two steps. Step 1: Binding of enzyme to substrate to form an activated complex. E + S → ES ≠ Step 2: Decomposition of the activated complex to form product. ES≠ → E + P 5.2.5 Catalysts in Some of the important technical catalytic processes are listed in Industry Table 5.3 to give an idea about the utility of catalysts in industries. Table 5.3: Some Industrial Catalytic Processes Process Catalyst 1. Haber’s process for the manufacture of Finely divided iron, molybdenum as promoter; ammonia conditions: 200 bar pressure and 723-773K temp- N2(g) + 3H2(g) → 2NH3(g) erature. Now-a-days, a mixture of iron oxide, potassium oxide and alumina is used. 2. Ostwald’s process for the manufacture Platinised asbestos; of nitric acid. temperature 573K. 4NH3(g) + 5O2(g) → 4NO(g) + 6H2O(g) 2NO(g) + O2(g) → 2NO2(g) Platinised asbestos or vanadium pentoxide (V2O5); 4NO2(g) + 2H2O(l) + O2(g) → 4HNO3(aq) temperature 673-723K. 3. Contact process for the manufacture of sulphuric acid. 2SO2(g) + O2(g) → 2SO3(g) SO3(g) + H2SO4(aq) → H2S2O7(l) oleum H2S2O7(l) + H2O(l) → 2H2SO4(aq) Intext Questions 5.4 In Haber’s process, hydrogen is obtained by reacting methane with steam in presence of NiO as catalyst. The process is known as steam reforming. Why is it necessary to remove CO when ammonia is obtained by Haber’s process? 5.5 Why is the ester hydrolysis slow in the beginning and becomes faster after sometime? 5.6 What is the role of desorption in the process of catalysis. 135 Surface Chemistry 2019-20

5.3 Colloids We have learnt in Unit 2 that solutions are homogeneous systems. We also know that sand in water when stirred gives a suspension, which slowly settles down with time. Between the two extremes of suspensions and solutions we come across a large group of systems called colloidal dispersions or simply colloids. A colloid is a heterogeneous system in which one substance is dispersed (dispersed phase) as very fine particles in another substance called dispersion medium. The essential difference between a solution and a colloid is that of particle size. While in a solution, the constituent particles are ions or small molecules, in a colloid, the dispersed phase may consist of particles of a single macromolecule (such as protein or synthetic polymer) or an aggregate of many atoms, ions or molecules. Colloidal particles are larger than simple molecules but small enough to remain suspended. Their range of diameters is between 1 and 1000 nm (10–9 to 10–6 m). Colloidal particles have an enormous surface area per unit mass as a result of their small size. Consider a cube with 1 cm side. It has a total surface area of 6 cm2. If it were divided equally into 1012 cubes, the cubes would be the size of large colloidal particles and have a total surface area of 60,000 cm2 or 6 m2. This enormous surface area leads to some special properties of colloids to be discussed later in this Unit. 5.4 Classification Colloids are classified on the basis of the following criteria: of Colloids (i) Physical state of dispersed phase and dispersion medium (ii) Nature of interaction between dispersed phase and dispersion medium (iii) Type of particles of the dispersed phase. 5.4.1 Depending upon whether the dispersed phase and the dispersion Classification medium are solids, liquids or gases, eight types of colloidal systems Based on Physical are possible. A gas mixed with another gas forms a homogeneous State of Dispersed mixture and hence is not a colloidal system. The examples of the Phase and various types of colloids along with their typical names are listed Dispersion in Table 5.4. Medium Many familiar commercial products and natural objects are colloids. For example, whipped cream is a foam, which is a gas dispersed in a Table 5.4: Types of Colloidal Systems Dispersed Dispersion Type of Examples phase medium colloid Solid Solid Solid sol Some coloured glasses and gem stones Solid Liquid Sol Paints, cell fluids Solid Gas Aerosol Smoke, dust Liquid Solid Gel Cheese, jellies Liquid Liquid Emulsion Milk, hair cream, butter Liquid Gas Aerosol Fog, mist, cloud, insecticide sprays Gas Solid Solid sol Pumice stone, foam rubber Gas Liquid Foam Froth, whipped cream, soap lather Chemistry 136 2019-20

liquid. Firefighting foams, used at emergency airplane landings are also colloidal systems. Most biological fluids are aqueous sols (solids dispersed in water). Within a typical cell, proteins and nucleic acids are colloidal-sized particles dispersed in an aqueous solution of ions and small molecules. Out of the various types of colloids given in Table 5.4, the most common are sols (solids in liquids), gels (liquids in solids) and emulsions (liquids in liquids). However, in the present Unit, we shall take up discussion of the ‘sols’ and ‘emulsions’ only. Further, it may be mentioned that if the dispersion medium is water, the sol is called aquasol or hydrosol and if the dispersion medium is alcohol, it is called alcosol and so on. 5.4.2 Depending upon the nature of interaction between the dispersed phase Classification and the dispersion medium, colloidal sols are divided into two categories, Based on Nature namely, lyophilic (solvent attracting) and lyophobic (solvent repelling). of Interaction If water is the dispersion medium, the terms used are hydrophilic and between Dispersed hydrophobic. Phase and Dispersion (i) Lyophilic colloids: The word ‘lyophilic’ means liquid-loving. Colloidal Medium sols directly formed by mixing substances like gum, gelatine, starch, rubber, etc., with a suitable liquid (the dispersion medium) are called lyophilic sols. An important characteristic of these sols is that if the dispersion medium is separated from the dispersed phase (say by evaporation), the sol can be reconstituted by simply remixing with the dispersion medium. That is why these sols are also called reversible sols. Furthermore, these sols are quite stable and cannot be easily coagulated as discussed later. (ii) Lyophobic colloids: The word ‘lyophobic’ means liquid-hating. Substances like metals, their sulphides, etc., when simply mixed with the dispersion medium do not form the colloidal sol. Their colloidal sols can be prepared only by special methods (as discussed later). Such sols are called lyophobic sols. These sols are readily precipitated (or coagulated) on the addition of small amounts of electrolytes, by heating or by shaking and hence, are not stable. Further, once precipitated, they do not give back the colloidal sol by simple addition of the dispersion medium. Hence, these sols are also called irreversible sols. Lyophobic sols need stabilising agents for their preservation. 5.4.3 Depending upon the type of the particles of the dispersed phase, colloids Classification are classified as: multimolecular, macromolecular and associated colloids. Based on Type of Particles of the (i) Multimolecular colloids: On dissolution, a large number of atoms Dispersed Phase, or smaller molecules of a substance aggregate together to form Multimolecular, species having size in the colloidal range (1–1000 nm). The species Macromolecular thus formed are called multimolecular colloids. For example, a and Associated gold sol may contain particles of various sizes having many atoms. Colloids Sulphur sol consists of particles containing a thousand or more of S8 sulphur molecules. (ii) Macromolecular colloids: Macromolecules (Unit 15) in suitable solvents form solutions in which the size of the macromolecules 137 Surface Chemistry 2019-20

may be in the colloidal range. Such systems are called macromolecular colloids. These colloids are quite stable and resemble true solutions in many respects. Examples of naturally occurring macromolecules are starch, cellulose, proteins and enzymes; and those of man-made macromolecules are polythene, nylon, polystyrene, synthetic rubber, etc. (iii) Associated colloids (Micelles): There are some substances which at low concentrations behave as normal strong electrolytes, but at higher concentrations exhibit colloidal behaviour due to the formation of aggregates. The aggregated particles thus formed are called micelles. These are also known as associated colloids. The formation of micelles takes place only above a particular temperature called Kraft temperature (Tk) and above a particular concentration called critical micelle concentration (CMC). On dilution, these colloids revert back to individual ions. Surface active agents such as soaps and synthetic detergents belong to this class. For soaps, the CMC is 10–4 to 10–3 mol L–1. These colloids have both lyophobic and lyophilic parts. Micelles may contain as many as 100 molecules or more. Mechanism of micelle formation Let us take the example of soap solutions. Soap is sodium or potassium salt of a higher fatty acid and may be represented as RCOO–Na+ (e.g., sodium stearate CH3(CH2)16COO–Na+, which is a major component of many bar soaps). When dissolved in water, it dissociates into RCOO– and Na+ ions. The RCOO– ions, however, consist of two parts — a long hydrocarbon chain R (also called non-polar ‘tail’) which is hydrophobic (water repelling), and a polar group COO– (also called polar-ionic ‘head’), which is hydrophilic (water loving). The RCOO– ions are, therefore, present on the surface with their COO– groups in water and the hydrocarbon chains R staying away from it and remain at the surface. But at critical micelle concentration, the anions are pulled into the bulk of the solution and aggregate to form a spherical Chemistry 138 Fig. 5.5: Hydrophobic and hydrophilic parts of stearate ion 2019-20

OOC COO Ionic shape with their hydrocarbon chains OOC COO micelle pointing towards the centre of the sphere with COO– part remaining outward on Stearate OOC COO the surface of the sphere. An aggregate ion thus formed is known as ‘ionic COO micelle’. These micelles may contain as Water many as 100 such ions. COO OOC COO Similarly, in case of detergents, e.g., sodium laurylsulphate, CH3(CH2)11SO4– (a) (b) Na+, the polar group is –SO4– along Fig. 5.6: (a) Arrangement of stearate ions on the surface with the long hydrocarbon chain. Hence, the mechanism of micelle of water at low concentrations of soap formation here also is same as that (b) Arrangement of stearate ions inside the bulk of soaps. of water (ionic micelle) at critical micelle concentrations of soap Cleansing action of soaps It has been mentioned earlier that a micelle consists of a hydrophobic hydrocarbon – like central core. The cleansing action of soap is due to the fact that soap molecules form micelle around the oil droplet in such a way that hydrophobic part of the stearate ions is in the oil droplet and hydrophilic part projects out of the grease droplet like the bristles (Fig. 5.7). Since the polar (a) (b) groups can interact with water, the oil droplet (c) surrounded by stearate ions is now pulled in water and removed from the dirty surface. Thus Fig. 5.7: (a) Grease on cloth (b) Stearate ions soap helps in emulsification and washing away arranging around the grease droplet and of oils and fats. The negatively charged sheath (c) Grease droplet surrounded by around the globules prevents them from coming stearate ions (micelle formed) together and forming aggregates. 5.4.4 Preparation A few important methods for the preparation of colloids are as follows: of Colloids (a) Chemical methods Colloidal dispersions can be prepared by chemical reactions leading to formation of molecules by double decomposition, oxidation, reduction or hydrolysis. These molecules then aggregate leading to formation of sols. As2O3 + 3H2S Doubledecompostion→ As2S3(sol) + 3H2O SO2 + 2H2S Oxidation → 3S(sol) + 2H2O 2 AuCl3 + 3 HCHO + 3H2O Reduction → 2Au(sol) + 3HCOOH + 6HCl FeCl3 + 3H2O Hydrolysis→ Fe(OH)3 (sol) + 3HCl medium (b) Electrical disintegration or Bredig’s Arc method Ice-bath This process involves dispersion as well as Fig. 5.8: Bredig’s Arc method condensation. Colloidal sols of metals such as gold, silver, platinum, etc., can be prepared by this method. In this method, electric arc is struck between electrodes of the metal immersed in the dispersion medium (Fig. 5.8). The intense heat produced vapourises the metal, which then condenses to form particles of colloidal size. 139 Surface Chemistry 2019-20

(c) Peptization Peptization may be defined as the process of converting a precipitate into colloidal sol by shaking it with dispersion medium in the presence of a small amount of electrolyte. The electrolyte used for this purpose is called peptizing agent. This method is applied, generally, to convert a freshly prepared precipitate into a colloidal sol. During peptization, the precipitate adsorbs one of the ions of the electrolyte on its surface. This causes the development of positive or negative charge on precipitates, which ultimately break up into smaller particles of the size of a colloid. You will learn about the phenomenon of development of charge on solid particles and their dispersion in Section 5.4.6 under the heading “Charge on collodial particles”. 5.4.5 Purification Colloidal solutions when prepared, generally contain excessive amount of Colloidal of electrolytes and some other soluble impurities. While the presence of Solutions traces of electrolyte is essential for the stability of the colloidal solution, larger quantities coagulate it. It is, therefore, necessary to reduce the concentration of these soluble impurities to a requisite minimum. The process used for reducing the amount of impurities to a requisite minimum is known as purification of colloidal solution. The purification of colloidal solution is carried out by the following mehods: Fig. 5.9: Dialysis (i) Dialysis: It is a process of removing a dissolved substance from a colloidal solution by means of diffusion through a suitable membrane. Since particles (ions or smaller molecules) in a true solution can pass through animal membrane (bladder) or parchment paper or cellophane sheet but not the colloidal particles, the membrane can be used for dialysis. The apparatus used for this purpose is called dialyser. A bag of suitable membrane containing the colloidal solution is suspended in a vessel through which fresh water is continuously flowing (Fig. 5.9). The molecules and ions diffuse through membrane into the outer water and pure colloidal solution is left behind. (ii) Electro-dialysis: Ordinarily, the process of dialysis is quite slow. It can be made faster by applying an electric field if the dissolved substance in the impure colloidal solution is only an electrolyte. The process is then named electrodialysis. The colloidal solution is placed in a bag of suitable membrane while pure water is taken outside. Electrodes are fitted in the compartment as shown in Fig. 5.10. The ions present in the colloidal solution migrate out to the oppositely charged electrodes. (iii) Ultrafiltration: Ultrafiltration is the process of separating the colloidal particles from the solvent and soluble solutes present in the colloidal solution by specially prepared filters, which are permeable to all substances except the colloidal particles. Colloidal particles can pass through ordinary filter paper because the pores are too large. However, the pores of filter paper can be reduced in size by impregnating with collodion solution to stop the flow of colloidal particles. The usual collodion is a 4% solution of nitro-cellulose in a Fig. 5.10: Electro-dialysis mixture of alcohol and ether. An ultra-filter paper may be prepared by soaking the Chemistry 140 2019-20

filter paper in a collodion solution, hardening by formaldehyde and then finally drying it. Thus, by using ultra-filter paper, the colloidal particles are separated from rest of the materials. Ultrafiltration is a slow process. To speed up the process, pressure or suction is applied. The colloidal particles left on the ultra-filter paper are then stirred with fresh dispersion medium (solvent) to get a pure colloidal solution. 5.4.6 Properties Various properties exhibited by the colloidal solutions are described below: of Colloidal Solutions (i) Colligative properties: Colloidal particles being bigger aggregates, the number of particles in a colloidal solution is comparatively small as compared to a true solution. Hence, the values of colligative properties (osmotic pressure, lowering in vapour pressure, depression in freezing point and elevation in boiling point) are of small order as compared to values shown by true solutions at same concentrations. Eye (ii) Tyndall effect: If a homogeneous solution placed in dark is observed in the direction of light, it appears clear and, if it is observed from a direction at right angles to the direction of light beam, it Microscope appears perfectly dark. Colloidal solutions viewed in the same way may also appear reasonably clear Tyndall cone or translucent by the transmitted light but they Light source Scattered light show a mild to strong opalescence, when viewed at right angles to the passage of light, i.e., the path of the beam is illuminated by a bluish light. This effect was first observed by Faraday and later studied in detail by Tyndall and is termed as Tyndall effect. The bright cone of the light is called Tyndall cone (Fig. 5.11). The Tyndall effect Colloidal solution is due to the fact that colloidal particles scatter light in all directions in space. This scattering of light illuminates the Fig. 5.11: Tyndall effect path of beam in the colloidal dispersion. Tyndall effect can be observed during the projection of picture in the cinema hall due to scattering of light by dust and smoke particles present there. Tyndall effect is observed only when the following two conditions are satisfied. (i) The diameter of the dispersed particles is not much smaller than the wavelength of the light used; and (ii) The refractive indices of the dispersed phase and the dispersion medium differ greatly in magnitude. Tyndall effect is used to distinguish between a colloidal and true solution. Zsigmondy, in 1903, used Tyndall effect to set up an apparatus known as ultramicroscope. An intense beam of light is focussed on the colloidal solution contained in a glass vessel. The focus of the light is then observed with a microscope at right angles to the beam. Individual colloidal particles appear as bright stars against a dark background. Ultramicroscope does not render the actual colloidal particles visible but only observe the light scattered by them. Thus, ultramicroscope does not provide any information about the size and shape of colloidal particles. (iii) Colour: The colour of colloidal solution depends on the wavelength of light scattered by the dispersed particles. The wavelength of light further depends on the size and nature of the particles. The colour 141 Surface Chemistry 2019-20

of colloidal solution also changes with the manner in which the observer receives the light. For example, a mixture of milk and water appears blue when viewed by the reflected light and red when viewed by the transmitted light. Finest gold sol is red in colour; as the size of particles increases, it appears purple, then blue and finally golden. (iv) Brownian movement: When colloidal solutions are viewed under a powerful ultramicroscope, the colloidal particles appear to be in a state of continuous zig-zag motion all over the field of view. This motion was first observed by the British botanist, Robert Brown, and is known as Brownian movement (Fig. 5.12). This motion is independent of the nature of the colloid but depends on the size of the particles and viscosity of the solution. Smaller the size and lesser the viscosity, faster is the motion. Fig. 5.12: Brownian movement The Brownian movement has been explained to be due to the unbalanced bombardment of the particles by the molecules of the dispersion medium. The Brownian movement has a stirring effect which does not permit the particles to settle and thus, is responsible for the stability of sols. (v) Charge on colloidal particles: Colloidal particles always carry an electric charge. The nature of this charge is the same on all the particles in a given colloidal solution and may be either positive or negative. A list of some common sols with the nature of charge on their particles is given below: Positively charged sols Negatively charged sols Hydrated metallic oxides, Metals, e.g., copper, silver, e.g., Al2O3.xH2O, CrO3.xH2O and gold sols. Fe2O3.xH2O, etc. Basic dye stuffs, e.g., Metallic sulphides, e.g., As2S3, methylene blue sol. Sb2S3, CdS sols. Haemoglobin (blood) Acid dye stuffs, e.g., eosin, congo red sols. Oxides, e.g., TiO2 sol. Sols of starch, gum, gelatin, clay, charcoal, etc. The presence of equal and similar charges on colloidal particles is largely responsible in providing stability to the colloidal solution, because the repulsive forces between charged particles having same charge prevent them from coalescing or aggregating when they come closer to one another. The charge on the sol particles is due to one or more reasons, viz., due to electron capture by sol particles during electrodispersion of metals, due to preferential adsorption of ions from solution and/or due to formulation of electrical double layer. Development of charge on sol particles by preferential adsorption of ions is described below. The sol particles acquire positive or negative charge by preferential adsorption of positive or negative ions. When two or more ions are present in the dispersion medium, preferential adsorption of the ion Chemistry 142 2019-20

common to the colloidal particle usually takes place. This can be explained by taking the following examples: (a) When highly diluted solution of silver nitrate is added to highly diluted potassium iodide solution, the precipitated silver iodide adsorbs iodide ions from the dispersion medium and negatively charged colloidal sol results. However, when KI solution is added to AgNO3 solution, positively charged sol results due to adsorption of Ag+ ions from dispersion medium. AgI/I– AgI/Ag+ Negatively charged Positively charged (b) If FeCl3 is added to the excess of hot water, a positively charged sol of hydrated ferric oxide is formed due to adsorption of Fe3+ ions. However, when ferric chloride is added to NaOH solution a negatively charged sol is obtained with adsorption of OH- ions. Fe2O3.xH2O/Fe3+ Fe2O3.xH2O/OH– Positively charged Negatively charged Having acquired a positive or a negative charge by selective adsorption on the surface of a colloidal particle as stated above, this layer attracts counter ions from the medium forming a second layer, as shown below. AgI/I- K+ AgI/Ag+I- The combination of the two layers of opposite charges around the colloidal particle is called Helmholtz electrical double layer. According to modern views, the first layer of ions is firmly held and is termed fixed layer while the second layer is mobile which is termed diffused layer. Fig. 5.13 depicts the formation of double layer. Since separation of charge is a seat of potential, the charges of opposite signs on the I– fixed and diffused parts of the double layer results in a difference K+ K+ in potential between these layers in the same manner as potential difference is developed in a capacitor. This potential difference K+ I– I– between the fixed layer and the diffused layer of opposite charges I– K+ K+ is called the electrokinetic potential or zeta potential. I– K+ K+ If two particles of an insoluble material (precipitate) do not I– K+ have double layers they can come close enough and attractive Ag I I– K+ van der Waals forces pull them together. When particles possess Solid I– K+ K+ I– double layer as shown in Fig. 5.13, the overall effect is that I– K+ particles repel each other at large distances of separation. This K+ repulsion prevents their close approach. They remain dispersed I– K+ K+ K+ I– and colloid is stabilised. I– K+ I– I– The addition of more electrolytes to sol supresses the diffused Fixed K+ K+ I– double layer and reduces the zita potential. This decreases the layer K+ electrostatic repulsion between particles to a large extent and colloid Diffused layer Fig. 5.13: Formation of precipitates. That is why colloid is particularly sensitive to double layer oppositely charged ions. (vi) Electrophoresis: The existence of charge on colloidal particles is confirmed by electrophoresis experiment. When electric potential is applied across two platinum electrodes dipping in a colloidal solution, the colloidal particles move towards one or the other electrode. The movement of colloidal particles under an applied electric potential is called electrophoresis. Positively charged particles move towards the cathode while negatively charged 143 Surface Chemistry 2019-20

particles move towards the anode. This can be demonstrated by the following experimental set- up (Fig. 5.14). When electrophoresis, i.e., movement of particles is prevented by some suitable means, it is observed that the dispersion medium begins to move in an electric field. This phenomenon is termed electroosmosis. (vii) Coagulation or precipitation: The stability of the lyophobic sols is due to the presence of charge on colloidal particles. If, somehow, the charge is removed, the particles will come nearer to each other to form aggregates (or coagulate) and settle down under the force of gravity. Fig. 5.14: Electrophoresis The process of settling of colloidal particles is called coagulation or precipitation of the sol. The coagulation of the lyophobic sols can be carried out in the following ways: (i) By electrophoresis: The colloidal particles move towards oppositely charged electrodes, get discharged and precipitated. (ii) By mixing two oppositely charged sols: Oppositely charged sols when mixed in almost equal proportions, neutralise their charges and get partially or completely precipitated. Mixing of hydrated ferric oxide (+ve sol) and arsenious sulphide (–ve sol) bring them in the precipitated forms. This type of coagulation is called mutual coagulation. (iii) By boiling: When a sol is boiled, the adsorbed layer is disturbed due to increased collisions with the molecules of dispersion medium. This reduces the charge on the particles and ultimately leads to settling down in the form of a precipitate. (iv) By persistent dialysis: On prolonged dialysis, traces of the electrolyte present in the sol are removed almost completely and the colloids become unstable and ultimately coagulate. (v) By addition of electrolytes: When excess of an electrolyte is added, the colloidal particles are precipitated. The reason is that colloids interact with ions carrying charge opposite to that present on themselves. This causes neutralisation leading to their coagulation. The ion responsible for neutralisation of charge on the particles is called the coagulating ion. A negative ion causes the precipitation of positively charged sol and vice versa. It has been observed that, generally, the greater the valence of the flocculating ion added, the greater is its power to cause precipitation. This is known as Hardy-Schulze rule. In the coagulation of a negative sol, the flocculating power is in the order: Al3+>Ba2+>Na+ Similarly, in the coagulation of a positive sol, the flocculating power is in the order: [Fe(CN)6]4– > PO43– > SO42– > Cl– The minimum concentration of an electrolyte in millimoles per litre required to cause precipitation of a sol in two hours is called coagulating value. The smaller the quantity needed, the higher will be the coagulating power of an ion. Chemistry 144 2019-20

Coagulation of lyophilic sols There are two factors which are responsible for the stability of lyophilic sols. These factors are the charge and solvation of the colloidal particles. When these two factors are removed, a lyophilic sol can be coagulated. This is done (i) by adding an electrolyte and (ii) by adding a suitable solvent. When solvents such as alcohol and acetone are added to hydrophilic sols, the dehydration of dispersed phase occurs. Under this condition, a small quantity of electrolyte can bring about coagulation. Protection of colloids Lyophilic sols are more stable than lyophobic sols. This is due to the fact that lyophilic colloids are extensively solvated, i.e., colloidal particles are covered by a sheath of the liquid in which they are dispersed. Lyophilic colloids have a unique property of protecting lyophobic colloids. When a lyophilic sol is added to the lyophobic sol, the lyophilic particles form a layer around lyophobic particles and thus protect the latter from electrolytes. Lyophilic colloids used for this purpose are called protective colloids. 5.5 Emulsions These are liquid-liquid colloidal systems, i.e., the dispersion of finely divided droplets in another liquid. If a mixture of two immiscible or partially miscible liquids is shaken, a coarse dispersion of one liquid Water in the other is obtained which is called Oil emulsion. Generally, one of the two liquids is water. There are two types of emulsions. (i) Oil dispersed in water (O/W type) and (ii) Water dispersed in oil (W/O type). In the first system, water acts as Oil in water Water in oil dispersion medium. Examples of this type of emulsion are milk and vanishing cream. In Fig. 5.14: Types of emulsions milk, liquid fat is dispersed in water. In the second system, oil acts as dispersion medium. Common examples of this type are butter and cream. Emulsions of oil in water are unstable and sometimes they separate into two layers on standing. For stabilisation of an emulsion, a third component called emulsifying agent is usually added. The emulsifying agent forms an interfacial film between suspended particles and the medium. The principal emulsifying agents for O/W emulsions are proteins, gums, natural and synthetic soaps, etc., and for W/O, heavy metal salts of fatty acids, long chain alcohols, lampblack, etc. Emulsions can be diluted with any amount of the dispersion medium. On the other hand, the dispersed liquid when mixed, forms a separate layer. The droplets in emulsions are often negatively charged and can be precipitated by electrolytes. They also show Brownian movement and Tyndall effect. Emulsions can be broken into constituent liquids by heating, freezing, centrifuging, etc. 5.6 Colloids Most of the substances, we come across in our daily life, are colloids. The Around Us meals we eat, the clothes we wear, the wooden furniture we use, the houses we live in, the newspapers we read, are largely composed of colloids. 145 Surface Chemistry 2019-20

Following are the interesting and noteworthy examples of colloids: (i) Blue colour of the sky: Dust particles along with water suspended in air scatter blue light which reaches our eyes and the sky looks blue to us. (ii) Fog, mist and rain: When a large mass of air containing dust particles, is cooled below its dewpoint, the moisture from the air condenses on the surfaces of these particles forming fine droplets. These droplets being colloidal in nature continue to float in air in the form of mist or fog. Clouds are aerosols having small droplets of water suspended in air. On account of condensation in the upper atmosphere, the colloidal droplets of water grow bigger and bigger in size, till they come down in the form of rain. Sometimes, the rainfall occurs when two oppositely charged clouds meet. It is possible to cause artificial rain by throwing electrified sand or spraying a sol carrying charge opposite to the one on clouds from an aeroplane. (iii) Food articles: Milk, butter, halwa, ice creams, fruit juices, etc., are all colloids in one form or the other. (iv) Blood: It is a colloidal solution of an albuminoid substance. The styptic action of alum and ferric chloride solution is due to coagulation of blood forming a clot which stops further bleeding. (v) Soils: Fertile soils are colloidal in nature in which humus acts as a protective colloid. On account of colloidal nature, soils adsorb moisture and nourishing materials. (vi) Formation of delta: River water is a colloidal solution of clay. Sea water contains a number of electrolytes. When river water meets the sea water, the electrolytes present in sea water coagulate the colloidal solution of clay resulting in its deposition with the formation of delta. Applications of colloids Colloids are widely used in the industry. Following are some examples: (i) Electrical precipitation of smoke: Smoke is a colloidal solution of solid particles such as carbon, arsenic compounds, dust, etc., in air. The smoke, before it comes out from the chimney, is led through a chamber containing plates having a charge opposite to that carried by smoke particles. The particles on coming in contact with these plates lose their charge and get precipitated. The particles thus High voltage settle down on the floor of the chamber. The precipitator is called electrode Cottrell precipitator (Fig.5.15). (30000 volts or more) Gases (ii) Purification of drinking water: The water obtained from free natural sources often contains suspended impurities. from Alum is added to such water to coagulate the suspended carbon impurities and make water fit for drinking purposes. particles (iii) Medicines: Most of the medicines are colloidal in nature. For example, argyrol is a silver sol used as an eye lotion. Smoke Colloidal antimony is used in curing kalaazar. Colloidal gold is used for intramuscular injection. Milk of Precipitated magnesia, an emulsion, is used for stomach disorders. ash Colloidal medicines are more effective because they have large surface area and are therefore easily assimilated. Fig. 5.15: Cottrell smoke precipitator Chemistry 146 2019-20

(iv) Tanning: Animal hides are colloidal in nature. When a hide, which has positively charged particles, is soaked in tannin, which contains negatively charged colloidal particles, mutual coagulation takes place. This results in the hardening of leather. This process is termed as tanning. Chromium salts are also used in place of tannin. (v) Cleansing action of soaps and detergents: This has already been described in Section 5.4.3. (vi) Photographic plates and films: Photographic plates or films are prepared by coating an emulsion of the light sensitive silver bromide in gelatin over glass plates or celluloid films. (vii) Rubber industry: Latex is a colloidal solution of rubber particles which are negatively charged. Rubber is obtained by coagulation of latex. (viii) Industrial products: Paints, inks, synthetic plastics, rubber, graphite lubricants, cement, etc., are all colloidal solutions. Intext Questions 5.7 What modification can you suggest in the Hardy Schulze law? 5.8 Why is it essential to wash the precipitate with water before estimating it quantitatively? Summary Adsorption is the phenomenon of attracting and retaining the molecules of a substance on the surface of a solid resulting into a higher concentration on the surface than in the bulk. The substance adsorbed is known as adsorbate and the substance on which adsorption takes place is called adsorbent. In physisorption, adsorbate is held to the adsorbent by weak van der Waals forces, and in chemisorption, adsorbate is held to the adsorbent by strong chemical bond. Almost all solids adsorb gases. The extent of adsorption of a gas on a solid depends upon nature of gas, nature of solid, surface area of the solid, pressure of gas and temperature of gas. The relationship between the extent of adsorption (x/m) and pressure of the gas at constant temperature is known as adsorption isotherm. A catalyst is a substance which enhances the rate of a chemical reaction without itself getting used up in the reaction. The phenomenon using catalyst is known as catalysis. In homogeneous catalysis, the catalyst is in the same phase as are the reactants, and in heterogeneous catalysis the catalyst is in a different phase from that of the reactants. Colloidal solutions are intermediate between true solutions and suspensions. The size of the colloidal particles range from 1 to 1000 nm. A colloidal system consists of two phases - the dispersed phase and the dispersion medium. Colloidal systems are classified in three ways depending upon (i) physical states of the dispersed phase and dispersion medium (ii) nature of interaction between the dispersed phase and dispersion medium and (iii) nature of particles of dispersed phase. The colloidal systems show interesting optical, mechanical and electrical properties. The process of changing the colloidal particles in a sol into the insoluble precipitate by addition of some suitable electrolytes is known as coagulation. Emulsions are colloidal systems in which both dispersed phase and dispersion medium are liquids. These can be of: (i) oil in water type and (ii) water in oil type. The process of making emulsion is known as emulsification. To stabilise an emulsion, an emulsifying agent or emulsifier is added. Soaps and detergents are most frequently used as emulsifiers. Colloids find several applications in industry as well as in daily life. 147 Surface Chemistry 2019-20

Exercises 5.1 Distinguish between the meaning of the terms adsorption and absorption. Give one example of each. 5.2 5.3 What is the difference between physisorption and chemisorption? 5.4 5.5 Give reason why a finely divided substance is more effective as an adsorbent. 5.6 5.7 What are the factors which influence the adsorption of a gas on a solid? 5.8 5.9 What is an adsorption isotherm? Describe Freundlich adsorption isotherm. 5.10 What do you understand by activation of adsorbent? How is it achieved? 5.11 What role does adsorption play in heterogeneous catalysis? 5.12 Why is adsorption always exothermic ? 5.13 How are the colloidal solutions classified on the basis of physical states of 5.14 the dispersed phase and dispersion medium? 5.15 Discuss the effect of pressure and temperature on the adsorption of gases on solids. 5.16 5.17 What are lyophilic and lyophobic sols? Give one example of each type. Why 5.18 are hydrophobic sols easily coagulated ? 5.19 5.20 What is the difference between multimolecular and macromolecular colloids? 5.21 Give one example of each. How are associated colloids different from these 5.22 two types of colloids? 5.23 What are enzymes ? Write in brief the mechanism of enzyme catalysis. 5.24 5.25 How are colloids classified on the basis of 5.26 (i) physical states of components (ii) nature of dispersed phase and 5.27 (iii) interaction between dispersed phase and dispersion medium? Explain what is observed (i) when a beam of light is passed through a colloidal sol. (ii) an electrolyte, NaCl is added to hydrated ferric oxide sol. (iii) electric current is passed through a colloidal sol? What are emulsions? What are their different types? Give example of each type. How do emulsifires stabilise emulsion? Name two emulsifiers. Action of soap is due to emulsification and micelle formation. Comment. Give four examples of heterogeneous catalysis. What do you mean by activity and selectivity of catalysts? Describe some features of catalysis by zeolites. What is shape selective catalysis? Explain the following terms: (i) Electrophoresis (ii) Coagulation (iii) Dialysis (iv) Tyndall effect. Give four uses of emulsions. What are micelles? Give an example of a micellers system. Explain the terms with suitable examples: (i) Alcosol (ii) Aerosol (iii) Hydrosol. Comment on the statement that “colloid is not a substance but a state of substance”. Chemistry 148 2019-20

Unit 6 Objectives General Principles and After studying this Unit, you will be Processes of Isolation able to: of Elements • appreciate the contribution of Indian traditions in the Thermodynamics illustrates why only a certain reducing element metallurgical processes, and a minimum specific temperature are suitable for reduction of a metal oxide to the metal in an extraction. • explain the terms minerals, ores, concentration, benefaction, The history of civilisation is linked to the use of metals calcination, roasting, refining, etc.; in antiquity in many ways. Different periods of early human civilisations have been named after metals. • understand the principles of The skill of extraction of metals gave many metals oxidation and reduction as applied and brought about several changes in the human to the extraction procedures; society. It gave weapons, tools, ornaments, utensils, etc., and enriched the cultural life. The ‘Seven metals • apply the thermodynamic of antiquity’, as they are sometimes called, are gold, concepts like that of Gibbs energy copper, silver, lead, tin, iron and mercury. Although and entropy to the principles of modern metallurgy had exponential growth after extraction of Al, Cu, Zn and Fe; Industrial Revolution, it is interesting to note that many modern concepts in metallurgy have their roots • explain why reduction of certain in ancient practices that pre-dated the Industrial oxides like Cu2O is much easier Revolution. For over 7000 years, India has had a than that of Fe2O3; rich tradition of metallurgical skills. • explain why CO is a favourable The two important sources for the history of Indian reducing agent at certain metallurgy are archaeological excavations and literary temperatures while coke is better evidences. The first evidence of metal in Indian in some other cases; subcontinent comes from Mehrgarh in Baluchistan, where a small copper bead, dated to about 6000 • explain why specific reducing BCE was found. It is however thought to be native agents are used for the reduction copper, which has not been extracted from the ore. purposes. Spectrometric studies on copper ore samples obtained from the ancient mine pits at Khetri in Rajasthan and on metal samples cut from representative Harappan 149 General Principles and Processes of Isolation of Elements

artefacts recovered from Mitathal in Haryana and eight other sites distributed in Rajasthan, Gujarat, Madhya Pradesh and Maharashtra prove that copper metallurgy in India dates back to the Chalcolithic cultures in the subcontinent. Indian chalcolithic copper objects were in all probability made indigenously. The ore for extraction of metal for making the objects was obtained from chalcopyrite ore deposits in Aravalli Hills. Collection of archaeological texts from copper-plates and rock-inscriptions have been compiled and published by the Archaeological Survey of India during the past century. Royal records were engraved on copper plates (tamra-patra). Earliest known copper- plate has a Mauryan record that mentions famine relief efforts. It has one of the very few pre-Ashoka Brahmi inscriptions in India. Harappans also used gold and silver, as well as their joint alloy electrum. Variety of ornaments such as pendants, bangles, beads and rings have been found in ceramic or bronze pots. Early gold and silver ornaments have been found from Indus Valley sites such as Mohenjodaro (3000 BCE). These are on display in the National Museum, New Delhi. India has the distinction of having the deepest ancient gold mines in the world, in the Maski region of Karnataka. Carbon dating places them in mid 1st millennium BCE. Hymns of Rigveda give earliest indirect references to the alluvial placer gold deposits in India. The river Sindhu was an important source of gold in ancient times. It is interesting that the availability of alluvial placer gold in the river Sindhu has been reported in modern times also. It has been reported that there are great mines of gold in the region of Mansarovar and in Thokjalyug even now. The Pali text, Anguttara Nikaya narrates the process of the recovery of gold dust or particles from alluvial placer gold deposits. Although evidence of gold refining is available in Vedic texts, it is Kautilya’s Arthashastra, authored probably in 3rd or 4th century BCE, during Mauryan era, which has much data on prevailing chemical practices in a long section on mines and minerals including metal ores of gold, silver, copper, lead, tin and iron. Kautilya describes a variety of gold called rasviddha, which is naturally occurring gold solution. Kalidas also mentioned about such solutions. It is astonishing how people recognised such solutions. The native gold has different colours depending upon the nature and amount of impurity present in it. It is likely that the different colours of native gold were a major driving force for the development of gold refining. Recent excavations in central parts of Ganges Valley and Vindhya hills have shown that iron was produced there possibly as early as in 1800 BCE. In the recent excavations conducted by the Uttar Pradesh State Archaeological Department, iron furnaces, artefacts, tuyers and layers of slag have been found. Radiocarbon dating places them between BCE 1800 and 1000. The results of excavation indicate that the knowledge of iron smelting and manufacturing of iron artefacts was well known in Eastern Vindhyas and it was in use in the Central Ganga Plains, at least from the early 2nd millennium BCE. The quantity and types of iron artefacts and the level of technical advancements indicate that working of iron would have been introduced much earlier. Chemistry 150

The evidence indicates early use of iron in other areas of the country, which proves that India was indeed an independent centre for the development of the working of iron. Iron smelting and the use of iron was especially established in South Indian megalithic cultures. The forging of wrought iron seems to have been at peak in India in the Ist millennium CE. Greek accounts report the manufacture of steel in India by crucible process. In this process, iron, charcoal and glass were mixed together in a crucible and heated until the iron melted and absorbed the carbon. India was a major innovator in the production of advanced quality steel. Indian steel was called ‘the Wonder Material of the Orient’. A Roman historian, Quintus Curtius, records that one of the gifts Porus of Taxila (326 BCE) gave to Alexander the Great was some two-and-a-half tons of Wootz steel. Wootz steel is primarily iron containing a high proportion of carbon (1.0 – 1.9%). Wootz is the English version of the word ‘ukku’ which is used for steel in Karnataka and Andhra Pradesh. Literary accounts suggest that Indian Wootz steel from southern part of the Indian subcontinent was exported to Europe, China and Arab world. It became prominent in the Middle East where it was named as Damasus Steel. Michael Faraday tried to duplicate this steel by alloying iron with a variety of metals, including noble metals, but failed. When iron ore is reduced in solid state by using charcoal, porous iron blocks are formed. Therefore, reduced iron blocks are also called sponge iron blocks. Any useful product can be obtained from this material only after removing the porosity by hot forging. The iron so obtained is termed as wrought iron. An exciting example of wrought iron produced in ancient India is the world famous Iron Pillar. It was erected in its present position in Delhi in 5th century CE. The Sanskrit inscription engraved on it suggests that it was brought here from elsewhere during the Gupta Period. The average composition (weight%) of the components present in the wrought iron of the pillar, besides iron, are 0.15% C, 0.05% Si, 0.05% Mn, 0.25% P, 0.005% Ni, 0.03% Cu and 0.02% N. The most significant aspect of the pillar is that there is no sign of corrosion inspite of the fact that it has been exposed to the atmosphere for about 1,600 years. Radiocarbon dating of charcoal from iron slag revealed evidence of continuous smelting in Khasi Hills of Meghalaya. The slag layer, which is dated to 353 BCE – CE 128, indicates that Khasi Hill region is the earliest iron smelting site studied in the entire region of North East India. The remnants of former iron-ore excavation and iron manufacturing are visible even now in the landscape of Khasi Hills. British naturalists who visited Meghalaya in early 19th century described the iron industry that had developed in the upper part of the Khasi Hills. There is archaeological evidence of zinc production in Rajasthan mines at Zawar from the 6th or 5th BCE. India was the first country to master zinc distillation. Due to low boiling point, zinc tends to vapourise while its ore is smelted. Pure zinc could be produced after a sophisticated ‘downward’ distillation technique in which the vapour 151 General Principles and Processes of Isolation of Elements

6.1 Occurrence of was condensed in a lower container. This technique was also applied Metals to mercury. Indian metallurgists were masters in this technique. This has been described in Sanskrit texts of 14th century. Indians had knowledge about mercury. They used it for medicinal purpose. Development of mining and metallurgy declined during the British colonial era. By the 19th century, once flourished mines of Rajasthan were mostly abandoned and became almost extinct. In 1947 when India got independence, European literature on science had already found its way slowly into the country. Thus, in post independence era, the Government of India initiated the process of nation building through the establishment of various institutes of science and technology. In the following sections, we will learn about the modern methods of extraction of elements. A few elements like carbon, sulphur, gold and noble gases, occur in free state while others are found in combined forms in the earth’s crust. Elements vary in abundance. Among metals, aluminium is the most abundant. In fact, it is the third most abundant element in earth’s crust (8.3% approx. by weight). It is a major component of many igneous minerals including mica and clays. Many gemstones are impure forms of Al2O3. For example, gems ‘ruby’ and ‘sapphire’ have Cr and Co respectively as impurity. Iron is the second most abundant metal in the earth’s crust. It forms a variety of compounds and their various uses make it a very important element. It is one of the essential elements in biological systems as well. For obtaining a particular metal, first we look for minerals which are naturally occurring chemical substances in the earth’s crust and are obtained through mining. Out of many minerals in which a metal may be found, only a few are viable to be used as source of that metal. Such minerals are known as ores. The principal ores of aluminium, iron, copper and zinc are given in Table 6.1. Table 6.1: Principal Ores of Some Important Metals Metal Ores Composition Aluminium Iron Bauxite AlOx(OH)3-2x [where 0 < x < 1] Copper Kaolinite (a form of clay) [Al2 (OH)4 Si2O5] Zinc Haematite Fe2O3 Magnetite Fe3O4 Siderite FeCO3 Iron pyrites FeS2 Copper pyrites CuFeS2 Malachite CuCO3.Cu(OH)2 Cuprite Cu2O Copper glance Cu2S Zinc blende or Sphalerite ZnS Calamine ZnCO3 Zincite ZnO A particular element may occur in a variety of compounds. The process of isolation of element from its compound should be such that it is chemically feasible and commercially viable. Chemistry 152

For the purpose of extraction, bauxite is chosen for aluminium. For iron, usually the oxide ores which are abundant and do not produce polluting gases (like SO2 that is produced in case of iron pyrites) are taken. For copper and zinc, any of the ores listed in Table 6.1 may be used depending upon the availability and other relevant factors. The entire scientific and technological process used for isolation of the metal from its ore is known as metallurgy. The extraction and isolation of an element from its combined form involves various principles of chemistry. Still, some general principles are common to all the extraction processes of metals. An ore rarely contains only a desired substance. It is usually contaminated with earthly or undesired materials known as gangue. The extraction and isolation of metals from ores involves the following major steps: • Concentration of the ore, • Isolation of the metal from its concentrated ore, and • Purification of the metal. In the following Sections, we shall first describe the various steps for effective concentration of ores. After that principles of some of the common metallurgical processes will be discussed. Those principles will include the thermodynamic and electrochemical aspects involved in the effective reduction of the concentrated ore to the metal. 6.2 Concentration Removal of the unwanted materials (e.g., sand, clays, etc.) from the ore of Ores is known as concentration, dressing or benefaction. Before proceeding 6.2.1 Hydraulic for concentration, ores are graded and crushed to reasonable size. Washing Concentration of ores involves several steps and selection of these steps depends upon the differences in physical properties of the compound of 6.2.2 Magnetic the metal present and that of the gangue. The type of the metal, the Separation available facilities and the environmental factors are also taken into consideration. Some of the important procedures for concentration of ore are described below. This is based on the difference between specific gravities of the ore and the gangue particles. It is therefore a type of gravity separation. In one such process, an upward stream of running water is used to wash the powdered ore. The lighter gangue particles are washed away and the heavier ore particles are left behind. This is based on differences in magnetic properties of the ore components. If either the ore or the gangue is attracted towards magnetic field, then the separation is carried out by this method. For example iron ores are attracted towards magnet, hence, non–magnetic impurities can be separted from them using magnetic separation. The powdered ore is dropped over a conveyer belt which moves over a magnetic roller (Fig.6.1) Magnetic substance remains attracted Fig. 6.1: Magnetic separation towards the belt and falls close to it. (schematic) 153 General Principles and Processes of Isolation of Elements

6.2.3 Froth This method is used for removing gangue from sulphide ores. In this Floatation Method process, a suspension of the powdered ore is made with water. Collectors and froth stabilisers are added to it. Collectors (e.g., pine oils, fatty acids, xanthates, etc.) enhance non- wettability of the mineral particles and froth stabilisers (e.g., cresols, aniline) stabilise the froth. The mineral particles become wet by oils while the gangue particles by water. A rotating paddle agitates the mixture and draws air in it. As a result, froth is formed Fig. 6.2: Froth floatation process which carries the mineral particles. The froth is light and is skimmed off. It is then dried for recovery of the ore particles. Sometimes, it is possible to separate two sulphide ores by adjusting proportion of oil to water or by using ‘depressants’. For example, in the case of an ore containing ZnS and PbS, the depressant used is NaCN. It selectively prevents ZnS from coming to the froth but allows PbS to come with the froth. The Innovative Washerwoman One can do wonders if he or she has a scientific temperament and is attentive to observations. A washerwoman had an innovative mind too. While washing a miner’s overalls, she noticed that sand and similar dirt fell to the bottom of the washtub. What was peculiar, the copper bearing compounds that had come to the clothes from the mines, were caught in the soapsuds and so they came to the top. One of her clients, Mrs. Carrie Everson was a chemist. The washerwoman told her experience to Mrs. Everson. The latter thought that the idea could be used for separating copper compounds from rocky and earth materials on large scale. This way an invention came up. At that time only those ores were used for extraction of copper, which contained large amounts of the metal. Invention of the Froth Floatation Method made copper mining profitable even from the low- grade ores. World production of copper soared and the metal became cheaper. 6.2.4 Leaching Leaching is often used if the ore is soluble in some suitable solvent. Chemistry 154 Following examples illustrate the procedure: (a) Leaching of alumina from bauxite Bauxite is the principal ore of aluminium. It usually contains SiO2, iron oxides and titanium oxide (TiO2) as impurities. Concentration is carried out by heating the powdered ore with a concentrated solution of NaOH at 473 – 523 K and 35 – 36 bar pressure. This process is called digestion. This way, Al2O3 is extracted out as sodium aluminate.

The impurity, SiO2 too dissolves forming sodium silicate. Other impurities are left behind. Al2O3(s) + 2NaOH(aq) + 3H2O(l) → 2Na[Al(OH)4](aq) (6.1) The sodium aluminate present in solution is neutralised by passing CO2 gas and hydrated Al2O3 is precipitated. At this stage, small amount of freshly prepared sample of hydrated Al2O3 is added to the solution. This is called seeding. It induces the precipitation. 2Na[Al(OH)4](aq) + CO2(g) → Al2O3.xH2O(s) + 2NaHCO3 (aq) (6.2) Sodium silicate remains in the solution and hydrated alumina is filtered, dried and heated to give back pure Al2O3. Al2O3.xH2O(s) 1470 K Al2O3(s) + xH2O(g) (6.3) (b) Other examples In the metallurgy of silver and gold, the respective metal is leached with a dilute solution of NaCN or KCN in the presence of air, which supplies O2. The metal is obtained later by replacement reaction. 4M(s) + 8CN–(aq)+ 2H2O(aq) + O2(g) → 4[M(CN)2]– (aq) + (6.4) 4OH–(aq) (M= Ag or Au) 2 [M(CN)2 ]−(aq ) + Zn (s) → [Zn (CN)4 ]2− (aq) + 2M (s) (6.5) Intext Questions 6.1 Which of the ores mentioned in Table 6.1 can be concentrated by magnetic separation method? 6.2 What is the significance of leaching in the extraction of aluminium? 6.3 Extraction To extract metal from concentrated ore, it must be converted to a of Crude form which is suitable for reduction to metal. Usually sulphide ores Metal from are converted to oxide before reduction because oxides are easier to Concentrated reduce. Thus isolation of metals from concentrated ore involves two Ore major steps viz., (a) conversion to oxide, and (b) reduction of the oxide to metal. (a) Conversion to oxide (i) Calcination: Calcinaton involves heating. It removes the volatile matter which escapes leaving behind the metal oxide: Fe2O3.xH2O(s) D Fe2O3 (s) + xH2O(g) (6.6) ZnCO3 (s) D ZnO(s) + CO2(g) (6.7) CaCO3.MgCO3(s) D CaO(s) + MgO(s ) + 2CO2(g) (6.8) (ii) Roasting: In roasting, the ore is heated in a regular supply of air in a furnace at a temperature below the melting point of the metal. Some of the reactions involving sulphide ores are: 155 General Principles and Processes of Isolation of Elements

2ZnS + 3O2 → 2ZnO + 2SO2 (6.9) 2PbS + 3O2 → 2PbO + 2SO2 (6.10) 2Cu2S + 3O2 → 2Cu2O + 2SO2 (6.11) The sulphide ores of copper are heated in reverberatory furnace [Fig. 6.3]. If the ore contains iron, it is mixed with silica before heating. Iron oxide ‘slags of ’* as iron silicate and copper is produced in the form of copper matte which contains Cu2S and FeS. FeO + SiO2 → FeSiO3 (6.12) (slag) The SO2 produced is utilised for manufacturing H2SO4 . (b) Reduction of oxide to the metal Fig. 6.3: A section of a modern Reduction of the metal oxide usually reverberatory furnace involves heating it with a reducing agent, for example C, or CO or even another metal. The reducing agent (e.g., carbon) combines with the oxygen of the metal oxide. MxOy + yC → xM + y CO (6.13) Some metal oxides get reduced easily while others are very difficult to be reduced (reduction means electron gain by the metal ion). In any case, heating is required. 6.4 Some basic concepts of thermodynamics help us in understanding the Thermodynamic Principles of theory of metallurgical transformations. Gibbs energy is the most Metallurgy significant term. To understand the variation in the temperature required for thermal reductions and to predict which element will suit as the reducing agent for a given metal oxide (MxOy), Gibbs energy interpretations are made. The criterion for the feasibility of a thermal reduction is that at a given temperture Gibbs energy change of the reaction must be negative. The change in Gibbs energy, ∆G for any process at any specified temperature, is described by the equation: ∆G = ∆H – T∆S (6.14) where, ∆H is the enthalpy change and ∆S is the entropy change for the process. When the value of ∆G is negative in equation 6.14, only then the reaction will proceed. ∆G can become negative in the following situations: 1. If ∆S is positive, on increasing the temperature (T), the value of T∆S increases so that ∆H < T∆S. In this situation ∆G will become negative on increasing temperature. 2. If coupling of the two reactions, i.e. reduction and oxidation, results in negative value of ∆G for overall reaction, the final reaction becomes feasible. Such coupling is easily understood * During metallurgy, ‘flux’ is added which combines with ‘gangue’ to form ‘slag’. Slag separates more easily from the ore than the gangue. This way, removal of gangue becomes easier. Chemistry 156

through Gibbs energy (∆rGV) vs T plots for the formation of the oxides (Fig. 6.4). These plots are drawn for free energy changes that occur when one gram mole of oxygen is consumed. The graphical representation of Gibbs energy was first used by H.J.T. Ellingham. This provides a sound basis for considering the choice of reducing agent in the reduction of oxides. This is known as Ellingham Diagram. Such diagrams help us in predicting the feasibility of thermal reduction of an ore. Fig. 6.4: Gibbs energy (∆rG V) vs T plots (schematic) for the formation of some oxides per mole of oxygen consumed (Ellingham diagram) As we know, during reduction, the oxide of a metal decomposes and the reducing agent takes away the oxygen. The role of reducing V agent is to provide ∆rG negative and large enough to make the sum V of ∆rG of the two reactions, i.e, oxidation of the reducing agent and reduction of the metal oxide negative. MxO(s) → xM (solid or liq) + 1 O2 (g) (6.15) 2 If reduction is carried out by carbon the oxidation of the reducing agent (i.e., C) will be there: C(s) + 1 O2 (g) → CO(g) (6.16) 2 (6.17) There may also be complete oxidation of carbon to CO2. 1 C(s) + 1 O2 (g) → 1 CO2 ( g ) 2 2 2 157 General Principles and Processes of Isolation of Elements

On coupling (combing) reaction 6.15 and 6.16 we get: MxO(s) + C(s) → xM(s or l) + CO(g) (6.18) On coupling reaction 6.15 and 6.17 we have MxO(s) + 1 C(s) → xM(s or l) + 1 CO2(g) (6.19) 2 2 Similarly, if carbon monoxide is reducing agent, reactions 6.15 and 6.20 given below need to be coupled. CO(g) + 1 O2(g) → CO2(g) (6.20) 2 Over all reaction will be as follows: MxO(s) + CO(g) → xM(s or l) + CO2(g) (6.21) Ellingham Diagram (a) Ellingham diagram normally consists of plots of ∆fGV vs T for the formation of oxides of common metals and reducing agents i.e., for the reaction given below. 2xM(s) + O2(g) → 2MxO(s) In this reaction, gas is consumed in the formation of oxide hence, molecular randomness decreases in the formation of oxide which leades to a negative value of ∆S as a result sign of T∆S term in equation (6.14) becomes positive. Subsequently ∆fGV shifts towards higher side despite rising T. The result is positive slope in the curve for most of the reactions for the formation of MxO(s). (b) Each plot is a straight line and slopes upwards except when some change in phase (s→ l or l→ g) takes place. The temperature at which such change occurs, is indicated by an increase in the slope on positive side (e.g., in the Zn, ZnO plot, the melting is indicated by an abrupt change in the curve) [Fig. 6.4]. (c) When temperature is raised, a point is reached in the curve where it crosses ∆rGV=0 line. Below this temperature, ∆rGV for the formation of oxide is negative so MxO is stable. Above this point, free energy of formation of oxide is positive. The oxide, MxO will decompose on its own. (d) Similar diagrams are constructed for sulfides and halides also. From them it becomes clear that why reduction of MxS is difficult. Limitations of Ellingham Diagram 1. The graph simply indicates whether a reaction is possible or not, i.e., the tendency of reduction with a reducing agent is indicated. This is so because it is based only on the thermodynamic concepts. It does not explain the kinetics of the reduction process. It cannot answer questions like how fast reduction can proceed? However, it explains why the reactions are sluggish when every species is in solid state and smooth when the ore melts down. It is interesting to note here that ∆H (enthalpy change) and the ∆S (entropy change) values for any chemical reaction remain nearly constant even on varying temperature. So the only dominant variable in equation(6.14) becomes T. However, ∆S depends much on the physical state of the compound. Since entropy depends on disorder or randomness in the system, it will increase if a compound melts (s→ l) or vapourises (l→ g) since molecular randomness increases on changing the phase from solid to liquid or from liquid to gas. 2. The interpretation of ∆rGV is based on K (∆GV = – RT lnK). Thus it is presumed that the reactants and products are in equilibrium: MxO + Ared l xM + AredO This is not always true because the reactant/product may be solid. In commercial processes reactants and products are in contact for a short time. Chemistry 158

The reactions 6.18 and 6.21 describe the actual reduction of the metal oxide, MxO, that we want to accomplish. The ∆rG0 values for these reactions in general, can be obtained from the corresponding ∆f G0 values of oxides. As we have seen, heating (i.e., increasing T) favours a negative value of ∆rG0. Therefore, the temperature is chosen such that the sum of ∆rG0 in the two combined redox processes is negative. In ∆rG0 vs T plots (Ellingham diagram, Fig. 6.4), this is indicated by the point of intersection of the two curves, i.e, the curve for the formation of MxO and that for the formation of the oxide of the reducing substance. After that point, the ∆rG0 value becomes more negative for the combined process making the reduction of MxO possible. The difference in the two ∆rG0 values after that point determines whether reduction of the oxide of the element of the upper line is feasible by the element of which oxide formation is represented by the lower line. If the difference is large, the reduction is easier. Example 6.1 Suggest a condition under which magnesium could reduce alumina. Solution The two equations are: (a) 4 Al + O2 → 2 Al2O3 (b) 2Mg +O2 → 2MgO 3 3 At the point of intersection of the Al2O3 and MgO curves (marked “A” in diagram 6.4), the ∆rG0 becomes ZERO for the reaction: 2 Al2O3 +2Mg → 2MgO + 4 Al 3 3 Below that point magnesium can reduce alumina. Example 6.2 Although thermodynamically feasible, in practice, magnesium metal is not used for the reduction of alumina in the metallurgy of aluminium. Why ? Solution Temperatures below the point of intersection of Al2O3 and MgO curves, magnesium can reduce alumina. But the process will be uneconomical. Example 6.3 Why is the reduction of a metal oxide easier if the metal formed is in liquid state at the temperature of reduction? Solution The entropy is higher if the metal is in liquid state than when it is in solid state. The value of entropy change (∆S) of the reduction process is more on positive side when the metal formed is in liquid state and V the metal oxide being reduced is in solid state. Thus the value of ∆rG becomes more on negative side and the reduction becomes easier. 6.4.1 Applications (a) Extraction of iron from its oxides After concentration, mixture of oxide ores of iron (Fe2O3, Fe3O4) is subjected to calcination/roasting to remove water, to decompose carbonates and to oxidise sulphides. After that these are mixed with limestone and coke and fed into a Blast furnace from its top, in which the oxide is reduced to the metal. In the Blast furnace, 159 General Principles and Processes of Isolation of Elements

[Fig. 6.5] reduction of iron oxides takes place at different temperature ranges. A blast of hot air is blown from the bottom of the furnace by burning coke in the lower portion to give temperature upto about 2200K. The burning of coke, therefore, supplies most of the heat required in the process. The CO and heat move to the upper part of the furnace. In upper part, the temperature is lower and the iron oxides (Fe2O3 and Fe3O4) coming from the top are reduced in steps to FeO. These reactions can be summarised as follows: At 500 – 800 K (lower temperature range in the blast furnace), Fe2O3 is first reduced to Fe3O4 and then to FeO 3 Fe2O3 + CO → 2 Fe3O4 + CO2 (6.22) Fe3O4 + 4 CO → 3Fe + 4 CO2 (6.23) Fe2O3 + CO → 2FeO + CO2 (6.24) Limestone is also decomposed to CaO which removes silicate impurity of the ore as slag. The slag is in molten state and separates out from iron. At 900 – 1500 K (higher temperature range in the blast furnace): Fig. 6.5: Blast furnace C + CO2 → 2 CO (6.25) FeO + CO → Fe + CO2 (6.26) Thermodynamics helps us to understand how coke reduces the oxide and why this furnace is chosen. One of the main reduction steps in this process involves reaction 6.27 given below. FeO(s) + C(s) → Fe(s/l) + CO (g) (6.27) This reaction can be seen as a reaction in which two simpler reactions have coupled. In one the reduction of FeO is taking place and in the other, C is being oxidised to CO: FeO(s) → Fe(s) + 1 O2(g) (6.28) 2 C(s) + 1 O2 (g) → CO (g) (6.29) 2 When both the reactions take place to yield the equation (6.27), the net Gibbs energy change becomes: ∆rG0 (C, CO) + ∆rG0 (FeO, Fe) = ∆rG0 (6.30) Naturally, the resultant reaction will take place when the right hand side in equation 6.30 is negative. In ∆rG0 vs T plot representing the change Fe→ FeO in Fig. 6.6 goes upward and that representing the change C→ CO (C,CO) goes downward. They cross each other at about 1073K. At temperatures above 1073K (approx.), the C, CO line is below the Fe, FeO line < . So above 1073 K in the range of temprature 900–1500 K coke will reduce FeO and will itself be oxidised to CO. Let us try to understand this through Fig. 6.6 (approximate values of ∆rG0 are given). At about 1673K (1400oC) ∆rG0 value for the reaction: Chemistry 160

2FeO→ 2Fe+O2 is +341 kJmol-1 because it is reverse of Fe→ FeO change and for the reaction 2C+O2→ 2CO ∆rG0 is -447 kJmol-1. If we calculate ∆rG0 value for overall reaction (6.27 the value will be -53 kJmol-1). Therefore, reaction 6.27 becomes feasible. In a similar way the reduction of Fe3O4 and Fe2O3 by CO at relatively lower temperatures can be explained on the basis of lower lying points of intersection of their curves with the CO, CO2 curve. The iron obtained from Blast furnace contains about 4% carbon and many impurities in smaller amount (e.g., S, P, Si, Mn). This is known as pig iron. It can be moulded into variety of shapes. Cast iron is different from pig iron and is made by melting pig iron with scrap iron and coke using hot air blast. It has slightly lower carbon content (about 3%) and is extremely hard and brittle. Further Reductions Fig. 6.6: Gibbs energy Vs T plot (schematic) for Wrought iron or malleable iron is the purest form the formation of oxides of iron and of commercial iron and is prepared from cast iron carbon (Ellingham diagram) by oxidising impurities in a reverberatory furnace lined with haematite. The haematite oxidises carbon to carbon monoxide: Fe2O3 + 3 C → 2 Fe + 3 CO (6.31) Limestone is added as a flux and sulphur, silicon and phosphorus are oxidised and passed into the slag. The metal is removed and freed from the slag by passing through rollers. (b) Extraction of copper from cuprous oxide [copper(I) oxide] In the graph of ∆rG0 vs T for the formation of oxides (Fig. 6.4), the Cu2O line is almost at the top. So it is quite easy to reduce oxide ores of copper directly to the metal by heating with coke. The lines (C, CO) and (C, CO2) are at much lower positions in the graph particularly after 500 – 600K. However, many of the ores are sulphides and some may also contain iron. The sulphide ores are roasted/smelted to give oxides: 2Cu2S + 3O2 → 2Cu2O + 2SO2 (6.32) The oxide can then be easily reduced to metallic copper using coke: Cu2O + C → 2 Cu + CO (6.33) In actual process, the ore is heated in a reverberatory furnace after mixing with silica. In the furnace, iron oxide ‘slags of’ as iron slicate is formed. Copper is produced in the form of copper matte. This contains Cu2S and FeS. FeO + SiO2 → FeSiO3 (6.34) (Slag) Copper matte is then charged into silica lined convertor. Some silica is also added and hot air blast is blown to convert the remaining 161 General Principles and Processes of Isolation of Elements

FeS, FeO and Cu2S/Cu2O to the metallic copper. Following reactions take place: 2FeS + 3O2 → 2FeO + 2SO2 (6.35) FeO + SiO2 → FeSiO3 (6.36) 2Cu2S + 3O2 → 2Cu2O + 2SO2 (6.37) 2Cu2O + Cu2S → 6Cu + SO2 (6.38) The solidified copper obtained has blistered appearance due to the evolution of SO2 and so it is called blister copper. (c) Extraction of zinc from zinc oxide The reduction of zinc oxide is done using coke. The temperature in this case is higher than that in the case of copper. For the purpose of heating, the oxide is made into brickettes with coke and clay. ZnO + C coke,1673K Zn + CO (6.39) The metal is distilled off and collected by rapid chilling. Intext Questions 6.3 The reaction, Cr2O3+2Al → Al2O3+2Cr (∆rG0= – 421kJ) is thermodynamically feasible as is apparent from the Gibbs energy value. Why does it not take place at room temperature? 6.4 Is it true that under certain conditions, Mg can reduce Al2O3 and Al can reduce MgO? What are those conditions? 6.5 We have seen how principles of thermodyamics are applied to Electrochemical pyrometallurgy. Similar principles are effective in the reductions of metal Principles of ions in solution or molten state. Here they are reduced by electrolysis or Metallurgy by adding some reducing element. Chemistry 162 In the reduction of a molten metal salt, electrolysis is done. Such methods are based on electrochemical principles which could be understood through the equation, ∆G0 = – nE0F (6.40) here n is the number of electrons and E0 is the electrode potential of the redox couple formed in the system. More reactive metals have large negative values of the electrode potential. So their reduction is difficult. If the difference of two E0 values corresponds to a positive E0 and consequently negative ∆G0 in equation 6.40, then the less reactive metal will come out of the solution and the more reactive metal will go into the solution, e.g., Cu2+ (aq) + Fe(s) → Cu(s) + Fe2+ (aq) (6.41) In simple electrolysis, the Mn+ ions are discharged at negative electrodes (cathodes) and deposited there. Precautions are taken considering the reactivity of the metal produced and suitable materials are used as electrodes. Sometimes a flux is added for making the molten mass more conducting.

Aluminium In the metallurgy of aluminium, purified Al2O3 is mixed with Na3AlF6 or CaF2 which lowers the melting point of the mixture and brings conductivity. The fused matrix is electrolysed. Steel vessel with lining of carbon acts as cathode and graphite anode is used. The overall reaction may be written as: Fig. 6.7: Electrolytic cell for the 2Al2O3 + 3C → 4Al + 3CO2 (6.42) extraction of aluminium This process of electrolysis is widely known as Hall-Heroult process. Thus electrolysis of the molten mass is carried out in an electrolytic cell using carbon electrodes. The oxygen liberated at anode reacts with the carbon of anode producing CO and CO2. This way for each kg of aluminium produced, about 0.5 kg of carbon anode is burnt away. The electrolytic reactions are: Cathode: Al3+ (melt) + 3e– → Al(l) (6.43) Anode: C(s) + O2– (melt) → CO(g) + 2e– (6.44) (6.45) C(s) + 2O2– (melt) → CO2 (g) + 4e– Copper from Low Grade Ores and Scraps Copper is extracted by hydrometallurgy from low grade ores. It is leached out using acid or bacteria. The solution containing Cu2+ is treated with scrap iron or H2 (equations 6.40; 6.46). Cu2+(aq) + H2(g) → Cu(s) + 2H+ (aq) (6.46) Example 6.4 At a site, low grade copper ores are available and zinc and iron scraps are also available. Which of the two scraps would be more suitable for reducing the leached copper ore and why? Solution Zinc being above iron in the electrochemical series (more reactive metal is zinc), the reduction will be faster in case zinc scraps are used. But zinc is costlier metal than iron so using iron scraps will be advisable and advantageous. 6.6 Oxidation Besides reductions, some extractions are based on oxidation particularly Reduction for non-metals. A very common example of extraction based on oxidation is the extraction of chlorine from brine (chlorine is abundant in sea water as common salt) . 2Cl–(aq) + 2H2O(l) → 2OH–(aq) + H2(g) + Cl2(g) (6.47) The ∆G0 for this reaction is + 422 kJ. When it is converted to E0 (using ∆G0 = – nE0F), we get E0 = – 2.2 V. Naturally, it will require an external emf that is greater than 2.2 V. But the electrolysis requires an excess potential to overcome some other hindering reactions (Unit–3, Section 3.5.1). Thus, Cl2 is obtained by electrolysis giving out H2 and aqueous NaOH as by-products. Electrolysis of molten NaCl is also carried out. But in that case, Na metal is produced and not NaOH. 163 General Principles and Processes of Isolation of Elements

As studied earlier, extraction of gold and silver involves leaching the metal with CN–. This is also an oxidation reaction (Ag → Ag+ or Au → Au+). The metal is later recovered by displacement method. 4Au(s) + 8CN–(aq) + 2H2O(aq) + O2(g) → 4[Au(CN)2]–(aq) + 4OH–(aq) (6.48) 2[Au(CN)2]–(aq) + Zn(s) → 2Au(s) + [Zn(CN)4]2– (aq) (6.49) In this reaction zinc acts as a reducing agent. 6.7 Refining A metal extracted by any method is usually contaminated with some impurity. For obtaining metals of high purity, several techniques are Chemistry 164 used depending upon the differences in properties of the metal and the impurity. Some of them are listed below. (a) Distillation (b) Liquation (c) Electrolysis (d) Zone refining (e) Vapour phase refining (f ) Chromatographic methods These are described in detail here. (a) Distillation This is very useful for low boiling metals like zinc and mercury. The impure metal is evaporated to obtain the pure metal as distillate. (b) Liquation In this method a low melting metal like tin can be made to flow on a sloping surface. In this way it is separated from higher melting impurities. (c) Electrolytic refining In this method, the impure metal is made to act as anode. A strip of the same metal in pure form is used as cathode. They are put in a suitable electrolytic bath containing soluble salt of the same metal. The more basic metal remains in the solution and the less basic ones go to the anode mud. This process is also explained using the concept of electrode potential, over potential, and Gibbs energy which you have seen in previous sections. The reactions are: Anode: M → Mn+ + ne– Cathode: Mn+ + ne– → M (6.50) Copper is refined using an electrolytic method. Anodes are of impure copper and pure copper strips are taken as cathode. The electrolyte is acidified solution of copper sulphate and the net result of electrolysis is the transfer of copper in pure form from the anode to the cathode: Anode: Cu → Cu2+ + 2 e– Cathode: Cu2+ + 2e– → Cu (6.51) Impurities from the blister copper deposit as anode mud which contains antimony, selenium, tellurium, silver, gold and platinum; recovery of these elements may meet the cost of refining. Zinc may also be refined this way.

(d) Zone refining This method is based on the principle that the impurities are more soluble in the melt than in the solid state of the metal. A mobile heater surrounding the rod of impure metal is fixed at its one end (Fig. 6.8). The molten zone moves along with the heater which is moved forward. As the heater moves forward, the pure metal crystallises out of the melt left behind and the impurities pass on into the adjacent new molten zone created by movement of heaters. The process is repeated several times and the heater is moved in the same direction again and again. Impurities get concentrated at one end. This end is cut off. This method is very useful for producing semiconductor and other metals of Fig. 6.8: Zone refining process very high purity, e.g., germanium, silicon, boron, gallium and indium. (e) Vapour phase refining In this method, the metal is converted into its volatile compound which is collected and decomposed to give pure metal. So, the two requirements are: (i) the metal should form a volatile compound with an available reagent, (ii) the volatile compound should be easily decomposable, so that the recovery is easy. Following examples will illustrate this technique. Mond Process for Refining Nickel: In this process, nickel is heated in a stream of carbon monoxide forming a volatile complex named as nickel tetracarbonyl. This compex is decomposed at higher temperature to obtain pure metal. Ni + 4CO 330 – 350 K Ni(CO)4 (6.52) Ni(CO)4 450 – 470 K Ni + 4CO (6.53) van Arkel Method for Refining Zirconium or Titanium: This method is very useful for removing all the oxygen and nitrogen present in the form of impurity in certain metals like Zr and Ti. The crude metal is heated in an evacuated vessel with iodine. The metal iodide being more covalent, volatilises: Zr + 2I2 → ZrI4 (6.54) The metal iodide is decomposed on a tungsten filament, electrically heated to about 1800K. The pure metal deposits on the filament. ZrI4 → Zr + 2I2 (6.55) (f) Chromatographic methods You have learnt about chromatographic technique of purification of substances in Class XI (Unit–12). 165 General Principles and Processes of Isolation of Elements

6.8 Uses of Column chromatography is very useful for purification of the Aluminium, elements which are available in minute quantities and the impurities Copper, Zinc are not very different in chemical properties from the element to be and Iron purified. Aluminium foils are used as wrappers for food materials. The fine dust of the metal is used in paints and lacquers. Aluminium, being highly reactive, is also used in the extraction of chromium and manganese from their oxides. Wires of aluminium are used as electricity conductors. Alloys containing aluminium, being light, are very useful. Copper is used for making wires used in electrical industry and for water and steam pipes. It is also used in several alloys that are rather tougher than the metal itself, e.g., brass (with zinc), bronze (with tin) and coinage alloy (with nickel). Zinc is used for galvanising iron. It is also used in large quantities in batteries. It is constituent of many alloys, e.g., brass, (Cu 60%, Zn 40%) and german silver (Cu 25-30%, Zn 25-30%, Ni 40–50%). Zinc dust is used as a reducing agent in the manufacture of dye-stuffs, paints, etc. Cast iron, which is the most important form of iron, is used for casting stoves, railway sleepers, gutter pipes , toys, etc. It is used in the manufacture of wrought iron and steel. Wrought iron is used in making anchors, wires, bolts, chains and agricultural implements. Steel finds a number of uses. Alloy steel is obtained when other metals are added to it. Nickel steel is used for making cables, automobiles and aeroplane parts, pendulum, measuring tapes. Chrome steel is used for cutting tools and crushing machines, and stainless steel is used for cycles, automobiles, utensils, pens, etc. Summary Although modern metallurgy had exponential growth after Industrial Revolution, many modern concepts in metallurgy have their roots in ancient practices that predated the Industrial Revolution. For over 7000 years, India has had high tradition of metallurigical skills. Ancient Indian metallurgists have made major contributions which deserve their place in metallurgical history of the world. In the case of zinc and high–carbon steel, ancient India contributed significantly for the developemnt of base for the modern metallurgical advancements which induced metallurgical study leading to Industrial Revolution. Metals are required for a variety of purposes. For this, we need their extraction from the minerals in which they are present and from which their extraction is commercially feasible.These minerals are known as ores. Ores of the metal are associated with many impurities. Removal of these impurities to certain extent is achieved in concentration steps. The concentrated ore is then treated chemically for obtaining the metal. Usually the metal compounds (e.g., oxides, sulphides) are reduced to the metal. The reducing agents used are carbon, CO or even some metals. Chemistry 166

In these reduction processes, the thermodynamic and electrochemical concepts are given due consideration. The metal oxide reacts with a reducing agent; the oxide is reduced to the metal and the reducing agent is oxidised. In the two reactions, the net Gibbs energy change is negative, which becomes more negative on raising the temperature. Conversion of the physical states from solid to liquid or to gas, and formation of gaseous states favours decrease in the Gibbs energy for the entire system. This concept is graphically displayed in plots of ∆G0 vs T (Ellingham diagram) for such oxidation/reduction reactions at different temperatures. The concept of electrode potential is useful in the isolation of metals (e.g., Al, Ag, Au) where the sum of the two redox couples is positive so that the Gibbs energy change is negative. The metals obtained by usual methods still contain minor impurities. Getting pure metals requires refining. Refining process depends upon the differences in properties of the metal and the impurities. Extraction of aluminium is usually carried out from its bauxite ore by leaching it with NaOH. Sodium aluminate, thus formed, is separated and then neutralised to give back the hydrated oxide, which is then electrolysed using cryolite as a flux. Extraction of iron is done by reduction of its oxide ore in blast furnace. Copper is extracted by smelting and heating in a reverberatory furnace. Extraction of zinc from zinc oxides is done using coke. Several methods are employed in refining the metal. Metals, in general, are very widely used and have contributed significantly in the development of a variety of industries. A Summary of the Occurrence and Extraction of some Metals is Presented in the following Table Metal Occurrence Common method Remarks of extraction Aluminium 1. Bauxite, Al2O3. x H2O Electrolysis of For the extraction, a 2. Cryolite, Na3AlF6 Al2O3 dissolved in good source of molten Na3AlF6 electricity is required. Iron 1. Haematite, Fe2O3 2. Magnetite, Fe3O4 Reduction of the Temperature oxide with CO approaching 2170 K Copper 1. Copper pyrites, CuFeS2 and coke in Blast is required. 2. Copper glance, Cu2S furnace 3. Malachite, It is self reduction in a Roasting of specially designed CuCO3.Cu(OH)2 sulphide converter. The 4. Cuprite, Cu2O partially and reduction takes place reduction easily. Sulphuric acid leaching is also used in hydrometallurgy for low grade ores. Zinc 1. Zinc blende or Roasting followed The metal may be Sphalerite, ZnS by reduction with purified by fractional 2. Calamine, ZnCO3 3. Zincite, ZnO coke distillation. 167 General Principles and Processes of Isolation of Elements

Exercises 6.1 Copper can be extracted by hydrometallurgy but not zinc. Explain. 6.2 What is the role of depressant in froth floatation process? 6.3 Why is the extraction of copper from pyrites more difficult than that from its 6.4 oxide ore through reduction? 6.5 6.6 Explain: (i) Zone refining (ii) Column chromatography. Out of C and CO, which is a better reducing agent at 673 K ? 6.7 Name the common elements present in the anode mud in electrolytic refining of copper. Why are they so present ? 6.8 Write down the reactions taking place in different zones in the blast furnace 6.9 during the extraction of iron. 6.10 Write chemical reactions taking place in the extraction of zinc from zinc blende. State the role of silica in the metallurgy of copper. 6.11 Which method of refining may be more suitable if element is obtained in minute 6.12 quantity? 6.13 Which method of refining will you suggest for an element in which impurities 6.14 present have chemical properties close to the properties of that elements? 6.15 6.16 Describe a method for refining nickel. 6.17 How can you separate alumina from silica in a bauxite ore associated with 6.18 silica? Give equations, if any. 6.19 Giving examples, differentiate between ‘roasting’ and ‘calcination’. 6.20 How is ‘cast iron’ different from ‘pig iron”? 6.21 Differentiate between “minerals” and “ores”. 6.22 Why copper matte is put in silica lined converter? 6.23 What is the role of cryolite in the metallurgy of aluminium? How is leaching carried out in case of low grade copper ores? 6.24 Why is zinc not extracted from zinc oxide through reduction using CO? The value of ∆fG0 for formation of Cr2 O3 is – 540 kJmol−1and that of Al2 O3 is 6.25 – 827 kJmol−1. Is the reduction of Cr2 O3 possible with Al ? 6.26 Out of C and CO, which is a better reducing agent for ZnO ? The choice of a reducing agent in a particular case depends on thermodynamic 6.27 factor. How far do you agree with this statement? Support your opinion with two examples. Name the processes from which chlorine is obtained as a by-product. What will happen if an aqueous solution of NaCl is subjected to electrolysis? What is the role of graphite rod in the electrometallurgy of aluminium? Outline the principles of refining of metals by the following methods: (i) Zone refining (ii) Electrolytic refining (iii) Vapour phase refining Predict conditions under which Al might be expected to reduce MgO. (Hint: See Intext question 6.4) Chemistry 168

Answers to Some Intext Questions 6.1 Ores in which one of the components (either the impurity or the actual ore) is magnetic can be concentrated, e.g., ores containing iron (haematite, magnetite, siderite and iron pyrites). 6.2 Leaching is significant as it helps in removing the impurities like SiO2, Fe2O3, etc. from the bauxite ore. 6.3 Certain amount of activation energy is essential even for such reactions which are thermodynamically feasible, therefore heating is required. 6.4 Yes, below 1350°C Mg can reduce Al2O3 and above 1350°C, Al can reduce MgO. This can be inferred from ∆GV Vs T plots (Fig. 6.4). 169 General Principles and Processes of Isolation of Elements

Unit 7 Objectives The p -Block After studying this Unit, you will be Elements able to Diversity in chemistry is the hallmark of p–block elements manifested • appreciate general trends in the in their ability to react with the elements of s–, d– and f–blocks as chemistry of elements of groups well as with their own. 15,16,17 and 18; In Class XI, you have learnt that the p-block elements • learn the preparation, properties are placed in groups 13 to 18 of the periodic table. and uses of dinitrogen and Their valence shell electronic configuration is ns2np1–6 phosphorus and some of their (except He which has 1s2 configuration). The properties important compounds; of p-block elements like that of others are greatly influenced by atomic sizes, ionisation enthalpy, electron • describe the preparation, gain enthalpy and electronegativity. The absence of d- properties and uses of dioxygen orbitals in second period and presence of d or d and f and ozone and chemistry of some orbitals in heavier elements (starting from third period simple oxides; onwards) have significant effects on the properties of elements. In addition, the presence of all the three types • know allotropic forms of sulphur, of elements; metals, metalloids and non-metals bring chemistry of its important diversification in chemistry of these elements. compounds and the structures of its oxoacids; Having learnt the chemistry of elements of Groups 13 and 14 of the p-block of periodic table in Class XI, • describe the preparation, you will learn the chemistry of the elements of properties and uses of chlorine subsequent groups in this Unit. and hydrochloric acid; • know the chemistry of interhalogens and structures of oxoacids of halogens; • enumerate the uses of noble gases; • appreciate the importance of these elements and their compounds in our day to day life. 7.1 Group 15 Group 15 includes nitrogen, phosphorus, arsenic, antimony, bismuth Elements and moscovium. As we go down the group, there is a shift from non- metallic to metallic through metalloidic character. Nitrogen and 7.1.1 Occurrence phosphorus are non-metals, arsenic and antimony metalloids, bismuth and moscovium are typical metals. Molecular nitrogen comprises 78% by volume of the atmosphere. In the earth’s crust, it occurs as sodium nitrate, NaNO3 (called Chile saltpetre) and potassium nitrate (Indian saltpetre). It is found in the form of proteins in plants and animals. Phosphorus occurs in minerals Chemistry 170 2019-20

of the apatite family, Ca9(PO4)6. CaX2 (X = F, Cl or OH) (e.g., fluorapatite Ca9 (PO4)6. CaF2) which are the main components of phosphate rocks. Phosphorus is an essential constituent of animal and plant matter. It is present in bones as well as in living cells. Phosphoproteins are present in milk and eggs. Arsenic, antimony and bismuth are found mainly as sulphide minerals. Moscovium is a synthetic radioactive element. Its symbol is Mc, atomic number 115, atomic mass 289 and electronic configuration [Rn] 5f 146d107s27p3. Due to very short half life and availability in very little amount, its chemistry is yet to be established. Here, except for moscovium, important atomic and physical properties of other elements of this group along with their electronic configurations are given in Table 7.1. Table 7.1: Atomic and Physical Properties of Group 15 Elements Property N P As Sb Bi Atomic number 7 15 33 51 83 Atomic mass/g mol–1 14.01 30.97 208.98 [He]2s22p3 [Ne]3s23p3 74.92 121.75 [Xe]4f145d106s26p3 1402 1012 703 Electronic configuration 2856 1903 [Ar]3d104s24p3 [Kr]4d105s25p3 1610 4577 2910 2466 Ionisation enthalpy I 3.0 2.1 947 834 1.9 (∆iH/(kJ mol–1) II 70 110 148 III 171b 212b 1798 1595 103c 63* 317d 544 77.2* 554d 2736 2443 1837 0.879g 1.823 9.808 Electronegativity 2.0 1.9 Covalent radius/pma 121 141 Ionic radius/pm 222b 76c 1089e 904 Melting point/K 888f 1860 5.778h 6.697 Boiling point/K Density/[g cm–3(298 K)] a EIII single bond (E = element); b E 3–; c E 3+; d White phosphorus; e Grey α-form at 38.6 atm; f Sublimation temperature; g At 63 K; hGrey α-form; * Molecular N2. Trends of some of the atomic, physical and chemical properties of the group are discussed below. 7.1.2 Electronic The valence shell electronic configuration of these elements is ns2np3. Configuration The s orbital in these elements is completely filled and p orbitals are half-filled, making their electronic configuration extra stable. 7.1.3 Atomic and Covalent and ionic (in a particular state) radii increase in size Ionic Radii down the group. There is a considerable increase in covalent radius from N to P. However, from As to Bi only a small increase in covalent radius is observed. This is due to the presence of completely filled d and/or f orbitals in heavier members. 7.1.4 Ionisation Ionisation enthalpy decreases down the group due to gradual increase Enthalpy in atomic size. Because of the extra stable half-filled p orbitals electronic configuration and smaller size, the ionisation enthalpy of the group 15 elements is much greater than that of group 14 elements in the corresponding periods. The order of successive ionisation enthalpies, as expected is ∆iH1 < ∆iH2 < ∆iH3 (Table 7.1). 171 The p-Block Elements 2019-20

7.1.5 The electronegativity value, in general, decreases down the group with Electronegativity increasing atomic size. However, amongst the heavier elements, the 7.1.6 Physical difference is not that much pronounced. Properties All the elements of this group are polyatomic. Dinitrogen is a diatomic gas while all others are solids. Metallic character increases down the group. 7.1.7 Chemical Nitrogen and phosphorus are non-metals, arsenic and antimony metalloids Properties and bismuth is a metal. This is due to decrease in ionisation enthalpy and increase in atomic size. The boiling points, in general, increase from top to bottom in the group but the melting point increases upto arsenic and then decreases upto bismuth. Except nitrogen, all the elements show allotropy. Oxidation states and trends in chemical reactivity The common oxidation states of these elements are –3, +3 and +5. The tendency to exhibit –3 oxidation state decreases down the group due to increase in size and metallic character. In fact last member of the group, bismuth hardly forms any compound in –3 oxidation state. The stability of +5 oxidation state decreases down the group. The only well characterised Bi (V) compound is BiF5. The stability of +5 oxidation state decreases and that of +3 state increases (due to inert pair effect) down the group. Besides +5 oxidation state, nitrogen exhibits + 1, + 2, + 4 oxidation states also when it reacts with oxygen. However, it does not form compounds in +5 oxidation state with halogens as nitrogen does not have d-orbitals to accommodate electrons from other elements to form bonds. Phosphorus also shows +1 and +4 oxidation states in some oxoacids. In the case of nitrogen, all oxidation states from +1 to +4 tend to disproportionate in acid solution. For example, 3HNO2 → HNO3 + H2O + 2NO Similarly, in case of phosphorus nearly all intermediate oxidation states disproportionate into +5 and –3 both in alkali and acid. However +3 oxidation state in case of arsenic, antimony and bismuth becomes increasingly stable with respect to disproportionation. Nitrogen is restricted to a maximum covalency of 4 since only four (one s and three p) orbitals are available for bonding. The heavier elements have vacant d orbitals in the outermost shell which can be used for bonding (covalency) and hence, expand their covalence as in PF6–. Anomalous properties of nitrogen Nitrogen differs from the rest of the members of this group due to its small size, high electronegativity, high ionisation enthalpy and non-availability of d orbitals. Nitrogen has unique ability to form pπ -pπ multiple bonds with itself and with other elements having small size and high electronegativity (e.g., C, O). Heavier elements of this group do not form pπ -pπ bonds as their atomic orbitals are so large and diffuse that they cannot have effective overlapping. Thus, nitrogen exists as a diatomic molecule with a triple bond (one s and two p) between the two atoms. Consequently, its bond enthalpy (941.4 kJ mol–1) is very high. On the contrary, phosphorus, arsenic and antimony form single bonds as P–P, As–As and Sb–Sb while bismuth forms metallic bonds in elemental state. However, the single Chemistry 172 2019-20

N–N bond is weaker than the single P–P bond because of high interelectronic repulsion of the non-bonding electrons, owing to the small bond length. As a result the catenation tendency is weaker in nitrogen. Another factor which affects the chemistry of nitrogen is the absence of d orbitals in its valence shell. Besides restricting its covalency to four, nitrogen cannot form dπ –pπ bond as the heavier elements can e.g., R3P = O or R3P = CH2 (R = alkyl group). Phosphorus and arsenic can form dπ –dπ bond also with transition metals when their compounds like P(C2H5)3 and As(C6H5)3 act as ligands. (i) Reactivity towards hydrogen: All the elements of Group 15 form hydrides of the type EH3 where E = N, P, As, Sb or Bi. Some of the properties of these hydrides are shown in Table 7.2. The hydrides show regular gradation in their properties. The stability of hydrides decreases from NH3 to BiH3 which can be observed from their bond dissociation enthalpy. Consequently, the reducing character of the hydrides increases. Ammonia is only a mild reducing agent while BiH3 is the strongest reducing agent amongst all the hydrides. Basicity also decreases in the order NH3 > PH3 > AsH3 > SbH3 > BiH3. Due to high electronegativity and small size of nitrogen, NH3 exhibits hydrogen bonding in solid as well as liquid state. Because of this, it has higher melting and boiling points than that of PH3. Table 7.2: Properties of Hydrides of Group 15 Elements Property NH3 PH3 AsH3 SbH3 BiH3 Melting point/K 195.2 139.5 156.7 185 – Boiling point/K 238.5 185.5 210.6 254.6 290 (E–H) Distance/pm 101.7 141.9 151.9 170.7 – HEH angle (°) 107.8 93.6 91.8 91.3 – ∆f HV/kJ mol–1 –46.1 13.4 66.4 145.1 278 ∆dissHV(E–H)/kJ mol–1 389 322 297 255 – (ii) Reactivity towards oxygen: All these elements form two types of oxides: E2O3 and E2O5. The oxide in the higher oxidation state of the element is more acidic than that of lower oxidation state. Their acidic character decreases down the group. The oxides of the type E2O3 of nitrogen and phosphorus are purely acidic, that of arsenic and antimony amphoteric and those of bismuth predominantly basic. (iii) Reactivity towards halogens: These elements react to form two series of halides: EX3 and EX5. Nitrogen does not form pentahalide due to non-availability of the d orbitals in its valence shell. Pentahalides are more covalent than trihalides. This is due to the fact that in pentahalides +5 oxidation state exists while in the case of trihalides +3 oxidation state exists. Since elements in +5 oxidation 173 The p-Block Elements 2019-20

state will have more polarising power than in +3 oxidation state, the covalent character of bonds is more in pentahalides. All the trihalides of these elements except those of nitrogen are stable. In case of nitrogen, only NF3 is known to be stable. Trihalides except BiF3 are predominantly covalent in nature. (iv) Reactivity towards metals: All these elements react with metals to form their binary compounds exhibiting –3 oxidation state, such as, Ca3N2 (calcium nitride) Ca3P2 (calcium phosphide), Na3As (sodium arsenide), Zn3Sb2 (zinc antimonide) and Mg3Bi2 (magnesium bismuthide). Though nitrogen exhibits +5 oxidation state, it does not form Example 7.1 pentahalide. Give reason. Solution Nitrogen with n = 2, has s and p orbitals only. It does not have d orbitals to expand its covalence beyond four. That is why it does not form pentahalide. PH3 has lower boiling point than NH3. Why? Example 7.2 Solution Unlike NH3, PH3 molecules are not associated through hydrogen bonding in liquid state. That is why the boiling point of PH3 is lower than NH3. Intext Questions 7.1 Why are pentahalides of P, As, Sb and Bi more covalent than their trihalides? 7.2 Why is BiH3 the strongest reducing agent amongst all the hydrides of Group 15 elements ? 7.2 Dinitrogen Preparation Chemistry 174 Dinitrogen is produced commercially by the liquefaction and fractional distillation of air. Liquid dinitrogen (b.p. 77.2 K) distils out first leaving behind liquid oxygen (b.p. 90 K). In the laboratory, dinitrogen is prepared by treating an aqueous solution of ammonium chloride with sodium nitrite. NH4CI(aq) + NaNO2(aq) → N2(g) + 2H2O(l) + NaCl (aq) Small amounts of NO and HNO3 are also formed in this reaction; these impurities can be removed by passing the gas through aqueous sulphuric acid containing potassium dichromate. It can also be obtained by the thermal decomposition of ammonium dichromate. (NH4)2Cr2O7 Heat → N2 + 4H2O + Cr2O3 Very pure nitrogen can be obtained by the thermal decomposition of sodium or barium azide. Ba(N3)2 → Ba + 3N2 2019-20

Properties Dinitrogen is a colourless, odourless, tasteless and non-toxic gas. Nitrogen atom has two stable isotopes: 14N and 15N. It has a very low solubility in water (23.2 cm3 per litre of water at 273 K and 1 bar pressure) and low freezing and boiling points (Table 7.1). Dinitrogen is rather inert at room temperature because of the high bond enthalpy of N ≡ N bond. Reactivity, however, increases rapidly with rise in temperature. At higher temperatures, it directly combines with some metals to form predominantly ionic nitrides and with non-metals, covalent nitrides. A few typical reactions are: 6Li + N2 Heat → 2Li3N 3Mg + N2 Heat → Mg3N2 It combines with hydrogen at about 773 K in the presence of a catalyst (Haber’s Process) to form ammonia: N2(g) + 3H2(g) 773 k 2NH3(g); ∆f H = –46.1 kJmol–1 Dinitrogen combines with dioxygen only at very high temperature (at about 2000 K) to form nitric oxide, NO. N2 + O2(g) Heat 2NO(g) Uses: The main use of dinitrogen is in the manufacture of ammonia and other industrial chemicals containing nitrogen, (e.g., calcium cyanamide). It also finds use where an inert atmosphere is required (e.g., in iron and steel industry, inert diluent for reactive chemicals). Liquid dinitrogen is used as a refrigerant to preserve biological materials, food items and in cryosurgery. Example 7.3 Write the reaction of thermal decomposition of sodium azide. Solution Thermal decomposition of sodium azide gives dinitrogen gas. 2NaN3 → 2Na + 3N2 Intext Question 7.3 Why is N2 less reactive at room temperature? 7.3 Ammonia Preparation Ammonia is present in small quantities in air and soil where it is formed by the decay of nitrogenous organic matter e.g., urea. NH2CONH2 + 2H2O → (NH4 )2CO3 2NH3 + H2O + CO2 On a small scale ammonia is obtained from ammonium salts which decompose when treated with caustic soda or calcium hydroxide. 2NH4Cl + Ca(OH)2 → 2NH3 + 2H2O + CaCl2 (NH4)2 SO4 + 2NaOH → 2NH3 + 2H2O + Na2SO4 175 The p-Block Elements 2019-20

On a large scale, ammonia is manufactured by Haber’s process. N2(g) + 3H2(g) Ö 2NH3(g); ∆f H0 = – 46.1 kJ mol–1 In accordance with Le Chatelier’s principle, high pressure would favour the formation of ammonia. The optimum conditions for the production of ammonia are a pressure of 200 × 105 Pa (about 200 atm), a temperature of ~ 700 K and the use of a catalyst such as iron oxide with small amounts of K2O and Al2O3 to increase the rate of attainment of equilibrium. The flow chart for the production of ammonia is shown in Fig. 7.1. Earlier, iron was used as a catalyst with molybdenum as a promoter. Fig. 7.1 Flow chart for the manufacture of ammonia Properties N Ammonia is a colourless gas with a pungent odour. Its freezing and H HH boiling points are 198.4 and 239.7 K respectively. In the solid and liquid states, it is associated through hydrogen bonds as in the case Chemistry 176 of water and that accounts for its higher melting and boiling points than expected on the basis of its molecular mass. The ammonia molecule is trigonal pyramidal with the nitrogen atom at the apex. It has three bond pairs and one lone pair of electrons as shown in the structure. Ammonia gas is highly soluble in water. Its aqueous solution is weakly basic due to the formation of OH– ions. NH3(g) + H2O(l) l NH4+ (aq) + OH– (aq) It forms ammonium salts with acids, e.g., NH4Cl, (NH4)2 SO4, etc. As a weak base, it precipitates the hydroxides (hydrated oxides in case of some metals) of many metals from their salt solutions. For example, ZnSO4 ( aq ) + 2NH4OH ( aq ) → Zn (OH)2 (s ) + ( NH4 ) SO4 ( aq ) 2 ( white ppt ) FeCl3 (aq) + NH4OH (aq) → Fe2O3.x H2O (s) + NH4Cl (aq) (brown ppt) 2019-20

The presence of a lone pair of electrons on the nitrogen atom of the ammonia molecule makes it a Lewis base. It donates the electron pair and forms linkage with metal ions and the formation of such complex compounds finds applications in detection of metal ions such as Cu2+, Ag+: Cu2+ (aq) + 4 NH3(aq) Ö [Cu(NH3)4]2+(aq) (blue) (deep blue) Ag+ (aq ) + Cl− (aq ) → AgCl (s) (colourless) (white ppt) AgCl (s) + 2NH3 (aq ) →  Ag ( NH3 )  Cl (aq ) 2 (white ppt) (colourless) Uses: Ammonia is used to produce various nitrogenous fertilisers (ammonium nitrate, urea, ammonium phosphate and ammonium sulphate) and in the manufacture of some inorganic nitrogen compounds, the most important one being nitric acid. Liquid ammonia is also used as a refrigerant. Example 7.4 Why does NH3 act as a Lewis base ? Solution Nitrogen atom in NH3 has one lone pair of electrons which is available for donation. Therefore, it acts as a Lewis base. Intext Questions 7.4 Mention the conditions required to maximise the yield of ammonia. 7.5 How does ammonia react with a solution of Cu2+? 7.4 Oxides of Nitrogen forms a number of oxides in different oxidation states. The Nitrogen names, formulas, preparation and physical appearance of these oxides are given in Table 7.3. Table 7.3: Oxides of Nitrogen Name Formula Oxidation Common Physical state of methods of appearance and nitrogen preparation chemical nature Dinitrogen oxide N2O +1 NH4NO3 Heat → colourless gas, [Nitrogen(I) oxide] NO N2O + 2H2O neutral Nitrogen monoxide + 2 2NaNO2 + 2FeSO4 + 3H2SO4 colourless gas, [Nitrogen(II) oxide] → Fe2 (SO4 ) + 2NaHSO4 neutral 3 + 2H2O + 2NO 177 The p-Block Elements 2019-20

Dinitrogen trioxide N2O3 + 3 2NO + N2O4250K → 2N2O3 blue solid, [Nitrogen(III) oxide] acidic Nitrogen dioxide NO2 +4 ( )2Pb NO3 2 673K→ brown gas, [Nitrogen(IV) oxide] acidic 4NO2 + 2PbO+ O2 Dinitrogen tetroxide N2O4 +4 2NO2 Cool N2O4 colourless solid/ [Nitrogen(IV) oxide] Heat liquid, acidic Dinitrogen pentoxide N2O5 +5 4HNO3 + P4O10 colourless solid, [Nitrogen(V) oxide] → 4HPO3 + 2N2O5 acidic Lewis dot main resonance structures and bond parameters of oxides are given in Table 7.4. Table 7.4: Structures of Oxides of Nitrogen Chemistry 178 2019-20

Why does NO2 dimerise ? Example 7.5 Solution NO2 contains odd number of valence electrons. It behaves as a typical odd molecule. On dimerisation, it is converted to stable N2O4 molecule with even number of electrons. Intext Question 7.6 What is the covalence of nitrogen in N2O5 ? 7.5 Nitric Acid Nitrogen forms oxoacids such as H2N2O2 (hyponitrous acid), HNO2 (nitrous acid) and HNO3 (nitric acid). Amongst them HNO3 is the most important. Preparation In the laboratory, nitric acid is prepared by heating KNO3 or NaNO3 and concentrated H2SO4 in a glass retort. NaNO3 + H2SO4 → NaHSO4 + HNO3 On a large scale it is prepared mainly by Ostwald’s process. This method is based upon catalytic oxidation of NH3 by atmospheric oxygen. 4NH3 (g) + 5O2 (g) ( ) ( )Pt /R5h00gaKu,g9e catalyst → 4NO g + 6H2O g bar (from air) Nitric oxide thus formed combines with oxygen giving NO2. 2NO (g) + O2 (g) 2NO2 (g) Nitrogen dioxide so formed, dissolves in water to give HNO3. 3NO2 (g) + H2O (l) → 2HNO3 (aq) + NO (g) NO thus formed is recycled and the aqueous HNO3 can be concentrated by distillation upto ~ 68% by mass. Further concentration to 98% can be achieved by dehydration with concentrated H2SO4. Properties It is a colourless liquid (f.p. 231.4 K and b.p. 355.6 K). Laboratory grade nitric acid contains ~ 68% of the HNO3 by mass and has a specific gravity of 1.504. In the gaseous state, HNO3 exists as a planar molecule with the structure as shown. In aqueous solution, nitric acid behaves as a strong acid giving hydronium and nitrate ions. HNO3(aq) + H2O(l) → H3O+(aq) + NO3– (aq) Concentrated nitric acid is a strong oxidising agent and attacks most metals except noble metals such as gold and platinum. The 179 The p-Block Elements 2019-20

products of oxidation depend upon the concentration of the acid, temperature and the nature of the material undergoing oxidation. 3Cu + 8 HNO3(dilute) →3Cu(NO3)2 + 2NO + 4H2O Cu + 4HNO3(conc.) →Cu(NO3)2 + 2NO2 + 2H2O Zinc reacts with dilute nitric acid to give N2O and with concentrated acid to give NO2. 4Zn + 10HNO3(dilute) →4 Zn (NO3)2 + 5H2O + N2O Zn + 4HNO3(conc.) →Zn (NO3)2 + 2H2O + 2NO2 Some metals (e.g., Cr, Al) do not dissolve in concentrated nitric acid because of the formation of a passive film of oxide on the surface. Concentrated nitric acid also oxidises non–metals and their compounds. Iodine is oxidised to iodic acid, carbon to carbon dioxide, sulphur to H2SO4, and phosphorus to phosphoric acid. I2 + 10HNO3 → 2HIO3 + 10NO2 + 4H2O C + 4HNO3 →CO2 + 2H2O + 4NO2 S8 + 48HNO3 →8H2SO4 + 48NO2 + 16H2O P4 + 20HNO3 →4H3PO4 + 20NO2 + 4H2O Brown Ring Test: The familiar brown ring test for nitrates depends on the ability of Fe2+ to reduce nitrates to nitric oxide, which reacts with Fe2+ to form a brown coloured complex. The test is usually carried out by adding dilute ferrous sulphate solution to an aqueous solution containing nitrate ion, and then carefully adding concentrated sulphuric acid along the sides of the test tube. A brown ring at the interface between the solution and sulphuric acid layers indicates the presence of nitrate ion in solution. NO3- + 3Fe2+ + 4H+ →NO + 3Fe3+ + 2H2O [Fe (H2O)6 ]2+ + NO →[Fe (H2O)5 (NO)]2+ + H2O (brown) Uses: The major use of nitric acid is in the manufacture of ammonium nitrate for fertilisers and other nitrates for use in explosives and pyrotechnics. It is also used for the preparation of nitroglycerin, trinitrotoluene and other organic nitro compounds. Other major uses are in the pickling of stainless steel, etching of metals and as an oxidiser in rocket fuels. 7.6 Phosphorus — Phosphorus is found in many allotropic forms, the important ones Allotropic being white, red and black. Forms White phosphorus is a translucent white waxy solid. It is poisonous, insoluble in water but soluble in carbon disulphide and glows in dark (chemiluminescence). It dissolves in boiling NaOH solution in an inert atmosphere giving PH3. P4 + 3NaOH + 3H2O → PH3 + 3NaH2PO2 (sodium hypophosphite) Chemistry 180 2019-20

P White phosphorus is less stable and therefore, more reactive than the other solid phases under normal conditions because of angular 60° P strain in the P4 molecule where the angles are only 60°. It readily catches fire in air to give dense white fumes of P4O10. P P4 + 5O2 → P4O10 P It consists of discrete tetrahedral P4 molecule as shown in Fig. 7.2. Fig. 7.2 White phosphorus Red phosphorus is obtained by heating white phosphorus at 573K in an inert atmosphere for several days. When red phosphorus is heated under high pressure, a series of phases of black phosphorus is formed. Red phosphorus possesses iron grey lustre. It is odourless, non- poisonous and insoluble in water as well as in carbon disulphide. Chemically, red phosphorus is much less reactive than white phosphorus. It does not glow in the dark. PP P It is polymeric, consisting of chains of P4 tetrahedra linked together in the manner as shown PP PP PP in Fig. 7.3. P P P Black phosphorus has two forms α-black phosphorus and β-black phosphorus. α-Black phosphorus is formed when red phosphorus is Fig.7.3: Red phosphorus heated in a sealed tube at 803K. It can be sublimed in air and has opaque monoclinic or rhombohedral crystals. It does not oxidise in air. β-Black phosphorus is prepared by heating white phosphorus at 473 K under high pressure. It does not burn in air upto 673 K. 7.7 Phosphine Preparation Phosphine is prepared by the reaction of calcium phosphide with water or dilute HCl. Ca3P2 + 6H2O →3Ca(OH)2 + 2PH3 Ca3P2 + 6HCl →3CaCl2 + 2PH3 In the laboratory, it is prepared by heating white phosphorus with concentrated NaOH solution in an inert atmosphere of CO2. P4 + 3NaOH + 3H2O → PH3 + 3NaH2PO2 (sodium hypophosphite) When pure, it is non inflammable but becomes inflammable owing to the presence of P2H4 or P4 vapours. To purify it from the impurities, it is absorbed in HI to form phosphonium iodide (PH4I) which on treating with KOH gives off phosphine. PH4I + KOH → KI + H2O + PH3 Properties It is a colourless gas with rotten fish smell and is highly poisonous. It explodes in contact with traces of oxidising agents like HNO3, Cl2 and Br2 vapours. It is slightly soluble in water. The solution of PH3 in water decomposes in presence of light giving red phosphorus and H2. When absorbed in 181 The p-Block Elements 2019-20

copper sulphate or mercuric chloride solution, the corresponding phosphides are obtained. 3CuSO4 + 2PH3 → Cu3P2 + 3H2SO4 3HgCl2 + 2PH3 → Hg3P2 + 6HCl Phosphine is weakly basic and like ammonia, gives phosphonium compounds with acids e.g., PH3 + HBr → PH4Br Uses: The spontaneous combustion of phosphine is technically used in Holme’s signals. Containers containing calcium carbide and calcium phosphide are pierced and thrown in the sea when the gases evolved burn and serve as a signal. It is also used in smoke screens. In what way can it be proved that PH3 is basic in nature? Example 7.6 PH3 reacts with acids like HI to form PH4I which shows that it is Solution basic in nature. PH3 + HI → PH4I Due to lone pair on phosphorus atom, PH3 is acting as a Lewis base in the above reaction. Intext Questions 7.7 (a) Bond angle in PH4+ is higher than that in PH3. Why? (b) What is formed when PH3 reacts with an acid? 7.8 What happens when white phosphorus is heated with concentrated NaOH solution in an inert atmosphere of CO2 ? 7 . 8 Phosphorus Halides Phosphorus forms two types of halides, PX3 (X = F, Cl, Br, I) and PX5 (X = F, Cl, Br). 7.8.1 Phosphorus Preparation Trichloride It is obtained by passing dry chlorine over heated white phosphorus. P4 + 6Cl2 → 4PCl3 It is also obtained by the action of thionyl chloride with white phosphorus. P4 + 8SOCl2 → 4PCl3 + 4SO2 + 2S2Cl2 Properties P It is a colourless oily liquid and hydrolyses in the presence of moisture. PCl3 + 3H2O →H3PO3 + 3HCl Cl Cl Cl It reacts with organic compounds containing –OH group such as 7.8.2 Phosphorus CH3COOH, C2H5OH. Pentachloride 3CH3COOH + PCl3 → 3CH3COCl + H3PO3 3C2H5OH + PCl3 → 3C2H5Cl + H3PO3 It has a pyramidal shape as shown, in which phosphorus is sp3 hybridised. Chemistry 182 2019-20

Preparation Phosphorus pentachloride is prepared by the reaction of white phosphorus with excess of dry chlorine. P4 + 10Cl2 → 4PCl5 It can also be prepared by the action of SO2Cl2 on phosphorus. P4 + 10SO2Cl2 → 4PCl5 + 10SO2 Properties PCl5 is a yellowish white powder and in moist air, it hydrolyses to POCl3 and finally gets converted to phosphoric acid. PCl5 + H2O → POCl3 + 2HCl POCl3 + 3H2O → H3PO4 + 3HCl When heated, it sublimes but decomposes on stronger heating. PCl5 Heat → PCl3 + Cl2 It reacts with organic compounds containing –OH group converting them to chloro derivatives. C2H5OH + PCl5 → C2H5Cl + POCl3 + HCl CH3COOH + PCl5 → CH3COCl + POCl3 +HCl 240 pm Finely divided metals on heating with PCl5 give corresponding Cl chlorides. Cl 2Ag + PCl5 → 2AgCl + PCl3 Sn + 2PCl5 → SnCl4 + 2PCl3 P 202 pm It is used in the synthesis of some organic compounds, e.g., C2H5Cl, CH3COCl. In gaseous and liquid phases, it has a trigonal Cl Cl bipyramidal structure as shown. The three equatorial P–Cl bonds are equivalent, while the two axial bonds are longer than equatorial bonds. This is due to the fact that Cl the axial bond pairs suffer more repulsion as compared to equatorial bond pairs. Example 7.7 Why does PCl3 fume in moisture ? Solution PCl3 hydrolyses in the presence of moisture giving fumes of HCl. PCl3 + 3H2O →H3PO3 + 3HCl Example 7.8 Are all the five bonds in PCl5 molecule equivalent? Justify your answer. Solution PCl5 has a trigonal bipyramidal structure and the three equatorial P-Cl bonds are equivalent, while the two axial bonds are different and longer than equatorial bonds. 183 The p-Block Elements 2019-20

Intext Questions 7.9 What happens when PCl5 is heated? 7.10 Write a balanced equation for the reaction of PCl5 with water. 7.9 Oxoacids of Phosphorus forms a number of oxoacids. The important oxoacids of Phosphorus phosphorus with their formulas, methods of preparation and the presence of some characteristic bonds in their structures are given in Table 7.5. Table 7.5: Oxoacids of Phosphorus Name Formula Oxidation Characteristic Preparation state of bonds and their phosphorus number Hypophosphorous H3PO2 +1 One P – OH white P4 + alkali (Phosphinic) Two P – H One P = O Orthophosphorous H3PO3 +3 Two P – OH P2O3 + H2O (Phosphonic) One P – H One P = O Pyrophosphorous H4P2O5 +3 Two P – OH PCl3 + H3PO3 Two P – H Two P = O Hypophosphoric H4P2O6 +4 Four P – OH red P4 + alkali Two P = O One P – P Orthophosphoric H3PO4 +5 Three P – OH P4O10+H2O One P = O Pyrophosphoric H4P2O7 +5 Four P – OH heat phosphoric Two P = O acid One P – O – P Metaphosphoric* (HPO3)n +5 Three P – OH phosphorus acid Three P = O Three P – O – P + Br2, heat in a sealed tube * Exists in polymeric forms only. Characteristic bonds of (HPO3)3 have been given in the Table. The compositions of the oxoacids are interrelated in terms of loss or gain of H2O molecule or O-atom. The structures of some important oxoacids are given next. In oxoacids phosphorus is tetrahedrally surrounded by other atoms. All these acids contain at least one P=O bond and one P–OH bond. The oxoacids in which phosphorus has lower oxidation state (less than +5) contain, in addition to P=O and P–OH bonds, either P–P (e.g., in H4P2O6) or P–H (e.g., in H3PO2) bonds but not both. These acids in +3 oxidation state of phosphorus tend to disproportionate to higher and lower oxidation states. For example, orthophophorous acid (or phosphorous acid) on heating disproportionates to give orthophosphoric acid (or phosphoric acid) and phosphine. 4H3PO3 → 3H3PO4 + PH3 Chemistry 184 2019-20