Cambridge IGCSE Chemistry Coursebook 4th Edition

S ELECTROLYSIS OF BRINE Sodium hydroxide solution alkaline and corrosive Chlorine a poisonous, yellow-green gas used for making soaps used for making detergents PVC (poly(chloroethene)) textiles solvents for dry-cleaning (e.g. trichloroethane) paper paints and dyestuffs bleaches, weedkillers, pesticides Hydrogen hydrogen chloride and hydrochloric acid a colourless, flammable gas also used for used for making killing bacteria in the water supply margarine killing bacteria in swimming pools nylon hydrogen chloride and hydrochloric acid Figure 9.23 The chlor–alkali industry. Questions S 9.23 Give two important uses of chlorine. 9.24 What is brine? 9.25 Why is the electrolysis of brine such an important process? 9.26 What is the most efficient method of electrolysing brine? Figure 9.24 The water of our swimming pools is chlorine-treated to kill 9.6 Limestone harmful bacteria. Limestone is quarried (Figure 9.25) in large amounts Halogen Product and/or use world-wide. It has a wide range of uses (see page 14). fluorine In some of these, the limestone is used directly; in chlorine fluoride in toothpaste and drinking water others, it acts as a raw material for making other PTFE – non-stick coating for pans, etc. compounds. bromine iodine water treatment Direct uses of limestone bleach Powdered limestone is often used to neutralise acid PVC – plastic pipes, windows, clothing soils and lakes acidified by acid rain. It is cheaper fabric, etc. than using lime (calcium oxide), which has to be solvents for dry-cleaning and degreasing produced by heating limestone. In the blast furnace disinfectants and antiseptics for the extraction of iron, limestone is used to remove pesticides impurities found in the iron ore as slag (calcium silicate). flame retardants pesticides Cement is made by heating powdered limestone petrol additives with clay in a rotary kiln (Figure 9.26). This material photographic film is then powdered and mixed with gypsum (calcium sulfate, CaSO4·2H2O). When water is added to this antiseptics mixture, complex chemical changes take place, photographic film and paper giving a hard interlocked mass of crystals of Table 9.3 The halogens are involved in the making of a wide range of useful products. 242 Cambridge IGCSE Chemistry

hydrated calcium aluminate (Ca(AlO2)2) and for water purification. It is mixed with sand to give calcium silicate (CaSiO3). mortar. When mixed with water and then allowed to dry, mortar sets into a strongly bonded material to Concrete is a mixture of cement and aggregate hold bricks together: the calcium hydroxide reacts with (stone chippings and gravel), which give it body. carbon dioxide in the air to form calcium carbonate. The mixture is mixed with water and can be poured into wooden moulds. It is then allowed to harden. powdered powdered Reinforced concrete is made by allowing the concrete clay limestone to set around steel rods or mesh (Figure 9.27). mix Kiln rotates to mix the clay Sodium carbonate (Na2CO3) is an important and limestone. industrial chemical, which is manufactured from limestone. It is used in the manufacture of glass As the kiln rotary kiln made of steel, (Figure 9.28, overleaf), soaps, detergents, paper, dyes rotates, the about 100 metres long and other chemicals. mixture moves along. hot gases to heat The manufacture of lime (calcium oxide) the kiln Lime (quicklime) is calcium oxide and is produced by roasting limestone in a lime kiln (Figure 9.29, overleaf). Hot clay and The limestone is decomposed by heat: limestone react to make cement. calcium carbonate heat calcium oxide + carbon dioxide lumps of grind to cement powder CaCO3(s) heat CaO(s) + CO2(g) water sand Lime is used in agriculture to neutralise acid soils and to improve drainage in soils that contain a large crushed stone amount of clay. It is used with sodium carbonate or gravel and sand in making glass. Large amounts of lime are converted into slaked lime (hydrated lime), which is bags calcium hydroxide (Ca(OH)2). Equal amounts of lime of and water are mixed to produce this material. It is cement used to make bleaching powder, in making glass and concrete mixer ready to use Figure 9.26 Limestone is used to make cement. Figure 9.25 A limestone quarry in Maizeret, Belgium. Figure 9.27 Construction workers using a pump to lay concrete around a mesh of metal rods. The rods will strengthen this reinforced concrete. Chapter 9: Industrial inorganic chemistry 243

A mix hot recycled furnace glass hot glass (runny liquid) formed into shapes and cooled + sand + crushed limestone soda (sodium carbonate) old bottles windows bottle bank Figure 9.28 Making glass. ab air + carbon dioxide kiln rotates limestone in hot air in The temperature increases quicklime out as the limestone travels through the kiln. Figure 9.29 a A functioning rotary lime kiln in Belgium. Note the heated glow at the far end of the kiln. b A diagram of a rotary kiln. Study tip Questions Remember that the reaction involved in the 9.27 Why is limestone sometimes added to lakes? production of lime is an example of thermal 9.28 How is limestone used in iron production? decomposition. The calcium carbonate is not 9.29 Write an equation for the thermal reacting with anything else. It is breaking up into simpler substances because of the high temperature. decomposition of limestone. When you write the equation, do not try to include 9.30 Write the chemical formula of slaked anything else on the left-hand side of the equation. lime. 9.31 Give two important uses of lime. 244 Cambridge IGCSE Chemistry

A 9.7 The economics of the chemical The fertiliser industry in Burrup, Western Australia A industry This is the largest ammonia plant of its type in the world. It can produce 760 000 tonnes of liquid ammonia Siting a chemical plant a year. When building a chemical processing plant, many ◆ The raw materials for the production of ammonium things need to be taken into consideration. Some of the most important are: nitrate fertiliser are air and natural gas (Figure 9.31). ◆ availability of the raw materials needed by the This factory is situated next to the onshore base for an offshore gas field (Figure 9.32. overleaf). process ◆ The products are transported to customers from ◆ transport links for importing raw materials and the deep-water port by ship. They are transported directly to the port by pipeline. export of products ◆ availability of a workforce IRISH SEA Liverpool 3 ◆ nearness of customers for the products 4 Manchester ◆ environmental considerations. Widnes The chlor-alkali industry in the UK 2 The map (Figure 9.30) shows the location of the 1 Runcorn industry. ◆ The only raw material is salt, which is obtained 7 Northwich Macclesfield Stanlow 5 6 from the rock salt deposits in Cheshire. The major salt mine here is the Winsford mine. Chester Winsford 9 Holmes Chapel ◆ Transport links are plentiful, with motorways, 8 railways and seaports all close at hand. salt ◆ Greater Manchester with its large population deposits 11 Sandbach provides the workforce. ◆ There are many other chemical industries in Wrexham 10 12 the area which use the hydrogen, chlorine and Crewe sodium hydroxide produced in their own processes. Key motorway/major road main-line railway ◆ The process itself causes few environmental problems. The salt is extracted by solution mining. 1 chlorine 3 dyestuffs 7 petrochemicals Water is pumped down into the salt deposits, where sodium hydroxide plastics dyestuffs it dissolves salt to become brine as it rises back to solvents pharmaceuticals the surface. monomers polymers 8 pharmaceuticals plasticisers 2 inorganic chemicals 9 salt herbicides 4 pharmaceuticals organic chemicals 10 rubber chemicals detergents 5 alkali products pharmaceuticals 6 pharmaceuticals 11 salt agrochemicals inorganic chemicals dyestuffs 12 pharmaceuticals Figure 9.30 The location of the chlor–alkali industry in Cheshire, England. air nitrogen converter: 15% ammonia hydrogen 200 atmospheres natural gas pressure, 450 ˚C, + water iron catalyst 85% unreacted, recycled Figure 9.31 A summary of the steps involved in the Haber process. Chapter 9: Industrial inorganic chemistry 245

A Figure 9.32 An aerial view of the ammonia and fertiliser plants on the Figure 9.33 Orchids growing on a reclaimed quarry site. Burrup Peninsula, Western Australia. ◆ The process is largely automated and the workforce chemical plants to ensure it is clean, and chemical needed is not large. companies receive large fines if they fail to stick to the rules. For example, poisonous metal compounds ◆ Australian and worldwide customers are supplied are removed by adding sodium hydroxide, which by sea. removes the metals as a precipitate that can be filtered out. ◆ The Burrup peninsula is an area well known for ◆ Land pollution: Chemical processes often produce its Aboriginal art. The companies involved in the solid waste which used to be left in large ‘spoil heaps’. development contribute to its preservation. These often contained poisonous substances. Most countries in the world still have areas of land where The environmental costs of industry it is not safe to live because of poisons in the ground. Any factory has an effect on its surroundings. In the Solid waste products now have to be treated to make case of a chemical plant, the effects can be serious. There them harmless, and then have to be disposed of safely. have been a number of accidents in different parts of the ◆ Appearance of the surroundings: Quarrying and world, where something has gone wrong with disastrous mining can leave the land surrounding a chemical results. Most chemical plants, however, run successfully plant looking very much the worse for wear. without causing any great harm. The ways in which the Chemical companies now have a responsibility, environment could be affected are as follows. when they have finished using land, to restore it to the sort of environment it used to be (Figure 9.33). ◆ Air pollution: Acidic or poisonous gases are often produced in chemical processes. At one time these All of these responsibilities placed on chemical gases were released into the atmosphere, causing companies increase the cost of producing the health problems nearby and acid rain further afield. chemicals which they make, but they decrease the cost Nowadays, gases have to be treated before they are to the environment and make the world a more pleasant released into the atmosphere to make sure anything place to live in. harmful is removed. For example, acidic gases are removed by ‘scrubbing’ – passing them over Recycling an alkaline material with which they react. This Recycling used substances is good for three reasons: removes them from the waste gases before they are ◆ it conserves the raw materials which the substances released. were made from ◆ Water pollution: Liquid waste and cooling water ◆ often uses less energy to recycle something than from chemical plants were, at one time, released into rivers. Now this waste must be treated first. Water would be needed to make it from raw materials boards and river authorities test the water leaving 246 Cambridge IGCSE Chemistry

◆ avoids the need to bury the substances in landfill Plastics PET or PETE (plastics) sites, possibly causing pollution. PET and HDPE are the HDPE (plastics) commonest plastics used, and Many substances can be recycled. Some of those most both can be recycled. A plastic commonly recycled are aluminium, steel, glass, plastics bottle takes around 700 years and paper. to decompose in landfill, and recycling one bottle saves Aluminium (aluminium) enough energy to keep a light This metal is the ideal bulb lit for 6 hours. When candidate for recycling because recycling plastic, the different it costs so much to extract it types of plastic need to be kept from its ore. Using recycled separate. aluminium saves 95% of the energy needed to make new Paper (paper) aluminium. Recycling one Paper has been recycled for aluminium can saves enough many years. Recycling uses energy to run a television for more than 60% less energy 3 hours. Around 60% of the than making new paper from aluminium used in the UK and trees. In Europe, around 54.6% 50% worldwide is recycled. of paper is recycled and in the USA, around 53.4%. Steel (steel) Steel, too, is cheaper to recycle Recycling not only saves than to make from scratch energy and raw materials: it but the difference in cost is avoids the need for huge piles not as great as for aluminium. of rubbish in landfill sites. The However, steel is easy to highest point in the state of recycle because it is magnetic Ohio, USA, is Mount Rumpke and so can easily be separated at a height of 1000 ft. It is made from other rubbish. For this of rubbish. reason, quite a lot of steel is recycled. Questions Glass 9.32 Why is the recycling of aluminium the most If glass is left in landfill, it never profitable type of recycling? decomposes. Recycling it saves energy. Recycling one glass 9.33 Saving energy is one reason for recycling. What bottle saves enough energy to other reasons are there? run a light bulb for 4 hours. It is important to separate 9.34 Why can recycling plastics be difficult? different colours of glass. In the UK, clear glass is needed most, (glass) whereas in Europe, green glass is in greater demand. Chapter 9: Industrial inorganic chemistry 247

Summary [1] [1] You should know: [2] ◆ the chemistry involved in the production of iron and steel [1] – the reduction of iron ore (hematite) in the blast furnace – the production of mild steel by the basic oxygen process S – the uses of different types of steel alloys – the problem of the rusting of iron and steel structures – barrier methods of preventing the rusting of iron and steel S – the use of sacrificial protection to prevent rusting ◆ how the extraction of other metals is linked to their reactivity S – the extraction of zinc S – the extraction of aluminium from its ore (bauxite) by electrolysis S – the protective oxide layer which prevents the corrosion of aluminium S ◆ the chemistry involved in the production of ammonia by the Haber process – the use of ammonia in the manufacture of fertilisers – the importance of NPK (nitrogen, phosphorus and potassium) fertilisers S ◆ the chemistry involved in the production of sulfuric acid – sources of the element sulfur – the uses of sulfur dioxide S – the uses of dilute and concentrated sulfuric acid ◆ how the electrolysis of brine forms useful products – the uses of chlorine, sodium hydroxide and hydrogen ◆ about the importance of limestone as a raw material – the production of lime in a lime kiln – the uses of lime and slaked lime – the use of limestone in iron production and to make cement ◆ the costs to the environment from pollution ◆ the advantages of recycling. End-of-chapter questions 1 Apart from saving money, why is it important to recycle as many substances as possible? 2 A farmer uses slaked lime (Ca(OH)2) and ammonium sulfate to increase the fertility of his fields. a What type of soil might the farmer use lime on? b Which essential element will ammonium sulfate add to the soil? c What reaction could take place between slaked lime and ammonium sulfate? Lime (CaO) is manufactured from limestone by heating it strongly. d What is the chemical formula of limestone? 248 Cambridge IGCSE Chemistry

e Write a symbol equation for the reaction which occurs when limestone is changed to lime. [2] f How is lime changed into slaked lime? [1] Ammonium sulfate is manufactured by reacting ammonia with sulfuric acid. Ammonia is manufactured by the Haber process and sulfuric acid by the Contact process. [4] S g Write an equation for the main reaction in the Haber process and give the conditions used. [4] h Write an equation for the main reaction in the Contact process and give the conditions used. 3 The diagram shows an experiment to investigate the rusting of some iron nails. C AB air air air iron nail iron nail iron nail coated with distilled drying agent water (calcium chloride) zinc distilled water a For each tube, A, B and C, predict whether the nails will rust. In each case give a reason. [3] b Iron from the blast furnace contains impurities such as carbon, phosphorus, silicon and sulfur. [3] Describe how the level of these impurities is decreased when steel is made from impure iron. [1] c State a use for stainless steel. [Cambridge IGCSE® Chemistry 0620/2, Question 7(a–c), November 2009] 4 Iron is extracted from its ore, hematite, in the blast furnace. waste gases firebrick lining raw materials coke, C iron ore, Fe2O3 limestone, CaCO3 air slag molten iron Describe the reactions involved in this extraction. Include in your description an equation for a redox reaction and one for an acid/base reaction. [5] [Cambridge IGCSE® Chemistry 0620/32, Question 4, June 2011] Chapter 9: Industrial inorganic chemistry 249

5 The diagram shows a basic oxygen converter. This is used to convert impure iron from the blast furnace into steel. During this process, some of the impurities in the iron are converted into a slag. a Label a copy of the diagram to show each of the following: [3] i where the oxygen enters ii the slag [1] iii the molten steel. [3] b In the converter, the oxygen oxidises sulfur, carbon and phosphorus to their oxides. [1] i Explain why sulfur dioxide and carbon dioxide are easily removed from the converter. ii Explain how calcium oxide is used to remove phosphorus(v) oxide from the converter. c Stainless steel is an alloy. i Which one of the diagrams, A, B, C or D, best represents an alloy? AB C D ii State one use of stainless steel. [1] [Cambridge IGCSE® Chemistry 0620/21, Question 7, June 2011] S6 a An important ore of zinc is zinc blende, ZnS. i How is zinc blende changed into zinc oxide? [1] ii Write a balanced equation for the reduction of zinc oxide to zinc by carbon. [2] b A major use of zinc is galvanising; steel objects are coated with a thin layer of zinc. This protects the steel from rusting even when the layer of zinc is broken. thin layer steel exposed to of zinc oxygen and water steel Explain, by mentioning ions and electrons, why the exposed steel does not rust. [3] [Cambridge IGCSE® Chemistry 0620/31, Question 3a, b, November 2009] 250 Cambridge IGCSE Chemistry

S 7 Aluminium is extracted by the electrolysis of aluminium oxide. positive electrode + – A B electrolyte (aluminium oxide dissolved in molten cryolite) C D a Hydrated aluminium oxide is heated to produce pure aluminium oxide. Al2O3·3H2O Al2O3 + 3H2O hydrated aluminium oxide What type of reaction is this? Choose from these possibilities: decomposition neutralisation oxidation reduction [1] b Explain why the electrolyte must be molten for electrolysis to occur. [1] c What is the purpose of the cryolite? [1] d Which letter in the diagram, A, B, C or D, represents the cathode? [1] e State the name of the products formed at the anode and cathode during this electrolysis. [2] f Why do the anodes have to be renewed periodically? [2] g Complete the equation for the formation of aluminium from aluminium ions. [1] Al3+ + ....... e− → Al [1] h State one use of aluminium. [Cambridge IGCSE® Chemistry 0620/2, Question 6, November 2009] Chapter 9: Industrial inorganic chemistry 251

10 Organic chemistry In this chapter, you will find out about: ◆ the unique properties of carbon S ◆ addition reactions ◆ hydrocarbons as compounds of carbon and ◆ the alcohols as a homologous series ◆ fermentation as a source of ethanol hydrogen only ◆ the alkanes and their properties S ◆ comparing the methods of ethanol production S ◆ isomerism ◆ the reactions of ethanol ◆ the halogen compounds of the alkanes ◆ the alkenes S ◆ carboxylic acids as a homologous series ◆ the reactivity of the C=C double bond in alkenes ◆ ethanoic acid as a weak acid S ◆ esterification. Carbon’s amazing versatility of pentacene. The bonding in this hydrocarbon Carbon is a non-metal in Group IV of the Periodic molecule has been observed electronically using Table. It forms covalent compounds. The uniqueness an atomic force microscope. This microscope is of carbon lies in the different ways in which it can able to probe structures at an atomic level. The form bonds. This shows itself even in the element images produced are the first to show the bonds in a itself. Carbon exists in several different forms. Two molecule. It is even possible to see the bonds between of the forms we have met earlier: diamond and the outer carbon atoms and the hydrogen atoms graphite (see page 83). A third form, the fullerenes attached to them. This image joins the other iconic (Figure 10.1) and carbon nanotubes, have been visualisations of the atomic world produced by this discovered relatively recently and their exploitation is microscope technology, including the ‘IBM logo’ (see one of the major features of the exciting new area of Chapter 2, Figure 2.22). research referred to as nanotechnology. The remarkable versatility and complexity of the The ring structures that carbon can form have structures that carbon is able to form is the very basis been highlighted recently in the revolutionary images of the different forms of life here on Earth. ab Figure 10.1 a A computer image of the structure of carbon-60 (C60) – the first fullerene to be discovered. The structure resembles the panelled structure of a modern soccer ball. b The revolutionary image of pentacene from the IBM research laboratory in Zurich (With kind permission of IBM Research - Zurich). 252 Cambridge IGCSE Chemistry

10.1 The unique properties There are three special features of covalent bonding of carbon involving carbon: ◆ Carbon atoms can join to each other to form long Amino acids, simple sugar molecules and even fats may be relatively simple molecules chains. Atoms of other elements can then attach but the construction of complex molecules, to the chain. such as long-chain carbohydrates and proteins, ◆ The carbon atoms in a chain can be linked by shows the versatility of carbon-containing single, double or triple covalent bonds. compounds. The peak of this complexity ◆ Carbon atoms can also arrange themselves in rings. must be DNA (deoxyribonucleic acid), the molecule that makes life possible Only carbon can achieve all these different bonding (Figure 10.2). arrangements to the extent that we see. Indeed, there are more compounds of carbon than of all the other Carbon is unique in the variety of molecules elements put together. Figure 10.3 gives some idea of it can form. The chemistry of these molecules is a how these bonding arrangements can produce different separate branch of the subject known as organic types of molecules. chemistry. Organic chemistry is the chemistry of carbon-containing compounds. GC CG GC G C G C T A A T T A AT G C C G G C GC CG Figure 10.2 Two ways of showing a section of the complex molecule DNA. Chapter 10: Organic chemistry 253

a Carbon can form four bonds, and carbon atoms can join to one d Carbon atoms can also join to form ring molecules, for example another to form long chains. glucose, as shown here. b In alkanes, only hydrogen atoms are joined to the side positions on HH the chains. Other atoms can be attached instead, forming other families of organic compounds. H C OH C C O HO H H HO C C OH C H H OH e Long-chain fat molecules can be formed, as well as numerous other molecules. c Double bonds can occur in simple molecules and in the long chains. Key Figure 10.3 Carbon is very versatile. carbon hydrogen oxygen Questions Key definition 10.1 What type of bonding do carbon atoms hydrocarbon – a compound that contains carbon normally participate in? and hydrogen only. 10.2 What is the valency of carbon? The hydrocarbons that we study at this level can be 10.3 What are the two different structural subdivided into two ‘families’. Some hydrocarbons are saturated. These molecules contain only single covalent forms of carbon? What is the name of bonds between carbon atoms. Since carbon has a valency the new form of carbon, discovered of 4, the bonds not used in making the chain are linked relatively recently? to hydrogen atoms (see Figure 10.3, overleaf). No further 10.4 What are the names of two different atoms can be added to molecules of these compounds. This carbon-containing molecules that are family of saturated hydrocarbons is known as the alkanes. important for living organisms? Key definition 10.2 Alkanes alkanes – saturated hydrocarbons. What is a hydrocarbon? Molecules of these compounds contain only Around six million compounds of carbon are already single bonds between the carbon atoms in known! Because there are so many, it is helpful to pick the chain and they have the general molecular out those compounds which have similar structures. formula CnH2n+2. One of the simplest types of organic compound is the hydrocarbons. 254 Cambridge IGCSE Chemistry

Table 10.1 gives the names and formulae of the first six Every organic compound has three different members of the series of alkanes. The simplest alkane formulae. The first of these is the empirical formula contains one carbon atom and is called methane. Note (see page 160). This formula is the simplest possible that the names of this series of hydrocarbons all end in whole-number ratio of the atoms in a compound; -ane. The first part of the name (the prefix) tells you the thus for methane it is CH4, but for ethane it is CH3. number of carbon atoms in the chain. These prefixes are used consistently in naming organic compounds. The second formula for any compound, and the most crucial, is the molecular formula. This represents The formulae given in Table 10.1 are the molecular the actual number of atoms present in the molecule; thus formulae of the compounds. Each molecule increases for methane it is CH4, for ethane it is C2H6, and so on. by a —CH2— group as the chain gets longer (see Figure 10.3). Indeed, the formulae of long-chain alkanes The final formula for any compound, and a highly can be written showing the number of — CH2— groups important one, is the structural formula of the in the chain. For example, octane (C8H18) can be molecule of the compound. This formula shows all written as CH3— (CH2)6— CH3. The formulae of these the atoms in the molecule and how they are bonded molecules all fit the general formula CnH2n+2 (where n is together. The structural formulae of the first six the number of carbon atoms present). alkanes are shown in Figure 10.4 (overleaf). In organic chemistry, the structure of a molecule is Study tip also very important. Figure 10.4 shows the structural formulae of the first six alkanes in the series. A It is important to think carefully when you are structural formula shows the bonds between the asked to give the formula of a compound. Make atoms. As the length of the hydrocarbon chain sure you realise whether you are being asked for increases, the strength of the weak forces between the the molecular formula or the structural formula, molecules (intermolecular forces or van der Waals’ and give the correct type. forces) is increased. This shows itself in the increasing boiling points of the members of the series (Table 10.1). When giving the structural formula of a The melting points and boiling points of the alkanes compound, make sure you show all the atoms increase gradually. Under normal conditions, the and all the bonds. first four members of the family are gases, and those between C5H12 and C16H34 (which in short are called Remember, too, to count the bonds around C5 to C16 alkanes) are liquids. The compounds in each carbon atom you draw; there can only be four. the alkane family with 17 or more carbon atoms It is worth practising drawing some of the regular are waxy solids. molecules that are asked about. Alkane Molecular Number Boiling b.p. increasing Physical formula of carbon point / °C state at room methane CnH2n+2 atoms temperature ethane −164 propane CH4 1 −87 gas butane −42 pentane C2H6 2 gas hexane 0 C3H8 3 +36 gas +69 C4H10 4 gas C5H12 5 liquid C6H14 6 liquid Table 10.1 Some details of the early members of the alkane series. Chapter 10: Organic chemistry 255

methane H butane H HHHH HC HCCCCH H HHHH ethane pentane HH HHHHH HCCH HCCCCCH HH HHHHH propane hexane HHH HHHHHH HCCCH HCCCCCCH HHH HHHHHH Figure 10.4 The structures of the first six alkanes. Burning alkanes One chemical property that all these alkanes have in common is that they burn very exothermically (Figure 10.5). They make good fuels. Controlling their availability and cost can have great political consequences. When they burn in a good supply of air, the products are carbon dioxide and water vapour: methane + oxygen → carbon dioxide + water Figure 10.5 A spectacular demonstration of methane burning on CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) the hand. ethane + oxygen → carbon dioxide + water portable camping stoves, blowtorches and gas lighters 2C2H6(g) + 7O2(g) → 4CO2(g) + 6H2O(g) (Figure 10.6). Methane forms the major part of natural gas. A homologous series Propane and butane burn with very hot flames and The family of alkanes has similar chemical properties. are sold as liquefied petroleum gas (LPG). They are Together they are an example of a homologous series of kept as liquids under pressure, but they vaporise easily compounds. when that pressure is released. In areas where there is no mains supply of natural gas, you may have seen propane tanks in gardens, which supply the fuel for heating systems. Cylinders of butane gas are used in portable gas fires in the home. Butane is also used in 256 Cambridge IGCSE Chemistry

Key definition 10.3 Alkenes homologous series – a family of organic Unsaturated hydrocarbons compounds that: The ability of carbon atoms to form double bonds gives ◆ have the same general formula rise to the alkenes. The alkenes are another family of ◆ have similar chemical properties hydrocarbons or homologous series. ◆ show a gradual increase in physical properties Key definition such as melting point and boiling point. alkenes – unsaturated hydrocarbons. Molecules Questions of these compounds contain a C=C double bond somewhere in the chain and they have the general 10.5 Write down the names and formulae of the formula CnH2n. first six alkanes. Alkenes have the general formula CnH2n (where n is 10.6 Draw the structural formulae of methane and the number of carbon atoms). Such molecules are butane. unsaturated because it is possible to break this double bond and add extra atoms to the molecule. 10.7 Plot a graph of the boiling points of the first six alkanes against the number of carbon The simplest alkene must contain two carbon atoms atoms in the molecule. Comment on the (needed for one C=C double bond) and is called shape of the graph. ethene (Figure 10.7). Table 10.2 shows the molecular 10.8 What is the formula of the first alkane that is: H H a a liquid at room temperature and pressure? b a solid at room temperature? C two shared pairs C of electrons 10.9 Write a word equation for the complete H combustion of ethane. (a double bond) 10.10 What is the major natural source of methane? H 10.11 Draw a diagram of the arrangement of the electrons in the bonding of methane, showing just the outer (valency) electrons. ethene H H C C H H propene H HH H C CC HH butene H HHH H C C CC HH H Figure 10.6 A butane Figure 10.7 The structures of the first three alkenes, and the bonding in ethene. portable camping stove. Chapter 10: Organic chemistry 257

Alkene Molecular Number of Boiling point / °C b.p. increasing Physical state at formula CnH2n carbon atoms room temperature ethene C2H4 −104 gas propene C3H6 2 −47 gas butene C4H8 3 −6 gas pentene 4 +30 C5H10 liquid 5 Table 10.2 Details of the first four alkenes. formulae of the first alkenes. The boiling points of type of reaction, where a double bond breaks and adds S these compounds again show a gradual increase as two new atoms, is known as an addition reaction. An the molecules get larger. Figure 10.7 also shows the alkane would give no reaction with bromine water; the structures of the first three alkenes. solution would stay orange-brown (Figure 10.9). In an addition reaction, two substances add together to form Alkenes are similar to other hydrocarbons when a single product. burnt. They give carbon dioxide and water vapour as long as the air supply is sufficient: A similar colour reaction occurs between alkenes and an acidified dilute solution of potassium ethene + oxygen → carbon dioxide + water manganate(vii). This solution is purple, and it turns C2H4(g) + 3O2(g) → 2CO2(g) + 2H2O(g) colourless when shaken with an unsaturated compound. Again, an alkane would produce no change. The presence of the C=C double bond in an alkene molecule makes alkenes much more reactive than + alkanes (alkanes contain only C—C single bonds). Other atoms can add on to alkene molecules when the Figure 10.8 The addition of bromine to ethene. double bond breaks open. This difference produces a ab simple test for unsaturation. alkane It is the presence of the C=C double bond in an added alkene molecule that gives the homologous series its characteristic properties. For this reason the C=C double bond is known as the functional group of the alkenes. We will meet other functional groups, such as that for the alcohols (—OH), later in the chapter. All the members of a particular homologous series contain the same functional group. Chemical tests for unsaturation alkene If an alkene, such as ethene, is shaken with a solution of added bromine in water, the bromine loses its colour. Bromine has reacted with ethene, producing a colourless compound: ethene + bromine → 1,2-dibromoethane C2H4(g) + Br2(aq) → C2H4Br2(l) orange-brown solution colourless S The double bond in ethene breaks open and forms Figure 10.9 The test for unsaturation: bromine water with a an alkane and new bonds to the bromine atoms (Figure 10.8). This b an alkene added and then shaken. The bromine water is decolorised by the alkene. There is no reaction with the alkane. 258 Cambridge IGCSE Chemistry

Activity 10.1 10.4 Hydrocarbon structure Testing alkanes and alkenes and isomerism Skills Naming organic compounds The alkanes are a ‘family’ (or homologous series) of saturated AO3.1 Demonstrate knowledge of how to safely use hydrocarbons. Their names all end in -ane; Figure 10.10 techniques, apparatus and materials (including shows a model of tetradecane (C14H30). The names of the first following a sequence of instructions where appropriate) six alkanes were given in Figure 10.4 (page 256). AO3.3 Make and record observations, measurements and The prefixes to the names of the alkanes are standard estimates and indicate the number of carbon atoms in the chain (see Table 10.1, page 255). So a compound in any AO3.4 Interpret and evaluate experimental observations homologous series with just one carbon atom will and data always have a name beginning with meth-, one with two carbon atoms eth-, and so on. Hence the names of the This activity compares the reaction with bromine alcohols and carboxylic acids are as shown in Tables 10.4 water of several liquid alkanes and alkenes. The and 10.6 (pages 265 and 270). When a halogen atom test for unsaturation is demonstrated. Small is introduced into a chain, the name of the compound samples of the liquids are also ignited and the contains a prefix indicating which halogen is present. appearance of the flames compared. The different homologous series all have particular A worksheet is included on the CD-ROM. endings to their names (Table 10.3). Questions Many different organic compounds are formed when a hydrogen in the original alkane ‘backbone’ is 10.12 Write down the names and molecular replaced by another group. The product formed when formulae of the first four alkenes. ethene reacts with bromine in solution (Figure 10.8) illustrates the system of naming organic compounds. 10.13 Draw the structures of ethene and propene. 10.14 What is the common empirical formula of 1 HH 2 H 1C C 2 H the first four alkenes? 10.15 What do you observe if ethene is bubbled Br Br through bromine water? ◆ The product has two carbon atoms joined by a single 10.16 Write a word equation for the reaction bond. So it is named after ethane. between ethene and bromine water. ◆ The molecule contains two bromine atoms. It is 10.17 Draw a diagram showing the arrangement of called dibromoethane. electrons in the bonding of ethene. Show just the outer (valency) electrons. Figure 10.10 A model of a straight-chain alkane (C14H30). Chapter 10: Organic chemistry 259

Homologous Name colourless gas S series ending burns in air to form CO2 and H2O Example liquefies at −12 °C alkane -ane alkene -ene propane In butane, all four carbon atoms are arranged in one alcohol -ol propene ‘straight’ main chain. However, the atoms do not have carboxylic acid -oic acid propanol to be arranged in this way. The fourth carbon atom can propanoic acid go off from the main chain to give the ‘Y-shaped’ or branched structure of 2-methylpropane. Compounds Table 10.3 The naming of the different homologous series. such as these are known as isomers. The properties of these particular isomers are quite similar; the difference ◆ The bromine atoms are not both attached to the same shows itself mainly in their melting points and boiling carbon atom. One bromine atom is bonded to each points. Hydrocarbons containing branched chains have carbon atom. The carbon atoms are numbered 1 and 2. lower melting points and lower boiling points than The full name of the compound is 1,2-dibromoethane. straight-chain compounds with the same number of carbon atoms. Branched-chain alkanes exist where a hydrocarbon side- chain has replaced a hydrogen to produce a more complex All the alkane molecules with four or more carbon molecule. In order to show where this side-chain is atoms possess isomers. For example, there are three isomers attached, we number the carbon atoms in the chain. This with the formula C5H12. The alkenes with four or more means that we can indicate where the side-chain is, in the carbon atoms can show a different kind of isomerism. In name. The numbering always starts at one end of the chain. this, the position of the C=C double bond is moved along The counting starts at the end which keeps the number of the chain. There are two molecules with a ‘straight’ chain of the side-chain position as low as possible. You will see some four carbon atoms and the molecular formula C4H8: examples of this numbering in the next section. S Isomerism but-1-ene The system of naming compounds emphasises the importance of structure. Molecules with the same H HHH Here the molecular formula can have different structures. The double bond same number of atoms can be connected together in 1 234 starts at the different ways. This is known as isomerism. first carbon. There are two different compounds with the C C C CH molecular formula C4H10: H HH butane HHHH HC C C C H but-2-ene HHHH Here the double bond burns in air to form CO2 and H2O H H HH starts at the liquefies at 0 °C second carbon. 1 2 34 2-methylpropane HC C C CH H H H methyl group The structures are different. Again, the carbon atoms are numbered. The number added to the formula indicates HCH the position of the double bond. In but-1-ene the double bond is between carbon atoms 1 and 2, whereas H 2 H This carbon in but-2-ene it is between carbon atoms 2 and 3. atom is the 1 C 3 second in the chain. HC CH HH H 260 Cambridge IGCSE Chemistry

S Key definition A isomers – compounds that have the same Alkynes are a third family of hydrocarbons. In molecular formula but different structural alkynes, the molecules contain a C≡C triple bond. formulae. The simplest member is ethyne (C2H2). This highly reactive gas used to be known as acetylene. It is used in oxy-acetylene welding torches. We do not study the alkynes any further at this level. S Activity 10.2 Models are built to answer questions as to how Modelling the structures of many isomers there are for certain given molecular hydrocarbon isomers formulae and to visualise the structural differences between certain isomers. The basis of different types Skills of isomerism is modelled. AO3.3 Make and record observations, measurements The approach can be extended by looking at and estimates computer-generated images of hydrocarbons at certain websites such as ‘Molecule of the Month’ AO3.4 Interpret and evaluate experimental observations (www.chm.bris.ac.uk/motm/motm.htm). and data A worksheet is included on the CD-ROM. The purpose of this activity is to use molecular models to explore the differences in structure between isomers. Questions 10.18 Give the names of the first member of each of 10.23 Draw the structure of 1,2-dibromoethane. S these homologous series: 10.24 Structures A to H are the structural formulae a the alkenes b the alcohols c the carboxylic acids. of some organic compounds. a Give the letters that represent: S 10.19 Define the term isomer. 10.20 Draw the structures of but-1-ene and but-2-ene. i two alkanes 10.21 Draw the structures of the two isomers ii two compounds which are not having the formula C4H10. 10.22 Draw two isomers of an alkane with five hydrocarbons carbon atoms. iii the molecule that is ethene. b What is the name of H? A HH B C H DH HC CH HC CH H HC C CH HH H CC HH EH F HH G H H C Br H C C OH HHHH HHH H HH HC C C CH HC C CH HHHH HH H CH H Chapter 10: Organic chemistry 261

10.5 Chemical reactions of Study tip the alkanes Exam questions very frequently ask about The alkanes are rather unreactive compounds. They are the combustion products of organic fuels and saturated, so they cannot take part in addition reactions. hydrocarbons, in both the presence of sufficient They are unaffected by acids or alkalis. However, they and insufficient air. It is useful to make sure you can take part in substitution reactions, particularly with know the word equations. the halogens. Questions on the combustion of hydrocarbons Combustion are also quite often asked in the form of equations We have seen earlier that, when a hydrocarbon burns that you have to complete by balancing them. in a good supply of air or oxygen, the two products are carbon dioxide and water. The word equation for the burning of butane, for instance, is: butane + oxygen → carbon dioxide + water Figure 10.11 An electronic carbon monoxide detector. 2C4H10(g) + 13O2(g) → 8CO2(g) + 10H2O(g) The same products are obtained whichever alkane is burnt, so long as there is a sufficient oxygen supply. However, if the air supply is limited, then the poisonous gas carbon monoxide can also be formed. Carbon monoxide is the product of incomplete combustion of a hydrocarbon. For example: methane + oxygen → carbon monoxide + water 2CH4(g) + 3O2(g) → 2CO(g) + 4H2O(g) Carbon monoxide (CO) is toxic because it interferes Figure 10.12 The ‘safety’ flame of the Bunsen burner. The air supply to the with the transport of oxygen around our bodies flame is restricted. by our red blood cells. Every year a number of people die accidentally from carbon monoxide poisoning because of poorly serviced gas fires in their homes. If the flues to the fire are blocked, insufficient air is supplied to the fire and carbon monoxide is produced. Simple carbon monoxide detectors can be bought in supermarkets, or electronic detectors fitted in homes (Figure 10.11). Incomplete combustion can also produce fine particles of carbon itself. These have not even reacted to produce carbon monoxide. It is these fine carbon particles (or soot) which can glow yellow in the heat of a flame. They give a candle flame or the ‘safety’ flame of a Bunsen burner its characteristic yellow colour (Figure 10.12). 262 Cambridge IGCSE Chemistry

S Substitution reactions with the halogens Trichloromethane (CHCl3), or chloroform, was S The substitution reaction with chlorine is interesting an early anaesthetic. However, the dose which can because it is a photochemical reaction: kill a patient is not much higher than the amount needed to anaesthetise a patient! So it was very methane + chlorine easy to make mistakes. Something else was needed. s⎯un⎯lig→ht chloromethane + hydrogen chloride Investigations were carried out on the anaesthetic effect of other substituted alkanes. In 1956, halothane was CH4(g) + Cl2(g) s⎯un⎯lig→ht CH3Cl(g) + HCl(g) discovered. It is a more useful anaesthetic. Its formula is CF3CHBrCl, and its structure is: Methane and chlorine react in the presence of sunlight. Ultraviolet light splits chlorine molecules Br F into separate energised atoms. These atoms then react with methane. So the overall result is that HC C F a chlorine atom replaces (substitutes for) a hydrogen atom in a methane molecule to give chloromethane Cl F (CH3Cl). The reaction can continue further as more halothane hydrogen atoms are substituted. Compounds such as dichloromethane (CH2Cl2), trichloromethane Substituted alkanes are also good organic solvents. (CHCl3) and tetrachloromethane (CCl4) are formed 1,1,1-trichloroethane is one solvent that is used in this way. frequently, in dry-cleaning, for example. Questions 10.31 What source of energy is required for the S substitution reaction between methane and 10.25 Write a word equation for the incomplete chlorine to take place? combustion of methane. 10.32 Bromine reacts with alkanes in a similar way 10.26 What is the formula of carbon monoxide? to chlorine. Hydrogen bromide is made in 10.27 What causes a candle flame to be yellow? the substitution reaction between propane 10.28 Why is carbon monoxide toxic? and bromine: S 10.29 What are the name and formula of the first propane + bromine → bromopropane + hydrogen bromide substitution product of the reaction between a Draw the structure of propane. methane and chlorine? b Draw the structure of a form of 10.30 The hydrocarbon propane is an important bromopropane. constituent of the fuel liquid petroleum gas c The reaction between propane and (LPG). For the burning of propane in an bromine is a photochemical reaction. excess of air, give: Suggest what is meant by photochemical. a a word equation b a balanced symbol equation. S 10.6 Chemical reactions of the Bromination S alkenes This reaction is used as the chemical test for an unsaturated hydrocarbon (see Figure 10.9, page 258). Alkenes are much more reactive than alkanes. Under Bromine water is decolorised when shaken with an suitable conditions, molecules such as bromine, alkene. The reaction will also work with the bromine hydrogen and water (steam) will add across the C=C dissolved in an organic solvent such as hexane. double bond. Chapter 10: Organic chemistry 263

S Hydrogenation This is why margarines are left partially unsaturated: S The addition of hydrogen across a C=C double bond is not all the C=C double bonds are hydrogenated. Olive known as hydrogenation. Ethene reacts with hydrogen oil is distinctive in having a high content of oleic acid, if the heated gases are passed together over a catalyst. which is a monounsaturated fatty acid. Margarine can The unsaturated ethane is the product: be made from olive oil without any hydrogenation. ethene + hydrogen ⎯150–⎯300→°C ethane Hydration nickel Another important addition reaction is the one used in the manufacture of ethanol. Ethanol is an important C2H4(g) + H2(g) ⎯150⎯–300→°C C2H6(g) industrial chemical and solvent. It is formed when a nickel mixture of steam and ethene is passed over a catalyst of immobilised phosphoric(v) acid (the acid is adsorbed Hydrogenation reactions similar to the reaction with on silica pellets) at a temperature of 300 °C and a ethene are used in the manufacture of margarine from pressure of 60 atmospheres: vegetable oils. 300 °C, 60 atmospheres ethanol The vegetable oils of interest include corn oil and sunflower oil. They are edible oils and contain long-chain ethene + steam ⎯⎯⎯⎯→ organic acids (fatty acids). The hydrocarbon chains of phosphoric acid these acids contain one or more C=C double bonds; they are unsaturated molecules (Figure 10.13). Oils such as C2H4(g) + H2O(g) → C2H5OH(g) sunflower oil are rich in polyunsaturated molecules. This means that the melting point is relatively low and the oil This reaction produces the ethanol of high purity remains liquid at normal temperatures (and even with needed in industrial organic chemistry. refrigeration). By hydrogenating some, but not all, of the C=C double bonds, the liquid vegetable oil can be made Questions into a solid but spreadable fat (margarine). 10.33 What are the molecular and structural Animal fats tend to be more saturated than formulae of 1,2-dibromoethane? vegetable oils and fats. The animal fats in cream can be made into butter. Many doctors now believe that 10.34 Write the word and chemical equations for unsaturated fats are healthier than saturated ones. the hydrogenation of ethene. O 10.35 What is the catalyst used in hydrogenation OH reactions? Figure 10.13 Sunflower oil and its products are rich in fats containing 10.36 Unsaturated hydrocarbons take part in unsaturated molecules (note the C=C double bonds in the chain). addition reactions. a Write a word equation for the reaction between propene and hydrogen. b Write a symbol equation for the reaction between butene and steam. 10.37 a Chloroethane is one of the chemicals manufactured from ethene. i Name the compound that reacts with ethene in an addition reaction to give chloroethane. ii Draw the structural formula of chloroethane. b Chloroethane can also be made by a substitution reaction. What are the reagents and reaction conditions for this reaction? 264 Cambridge IGCSE Chemistry

10.7 Alcohols methanol H OH H HC Ethanol is one of the best-known organic compounds. It is just one of a whole family of compounds – the HCOH H alcohols. The alcohols are a homologous series of H compounds that contain —OH as the functional group (Figure 10.14). A functional group is a group of ethanol propan-1-ol atoms in a structure that determines the characteristic HH HHH reactions of a compound. HCCOH HCCCOH Table 10.4 shows the molecular formulae of the early HH HHH members of the series. The simplest alcohol contains one carbon atom and is called methanol. Note that butan-1-ol butan-2-ol H the names all have the same ending (-ol). The general HHHH formula of the alcohols is CnH2n+1OH, and they can be HHOH referred to as the alkanols. The structural formulae of the first four alcohols are as shown in Figure 10.15. The HCCC COH HCCC CH early alcohols are all neutral, colourless liquids that do not conduct electricity. HHHH HHHH Key definition Figure 10.15 Alcohols are a homologous series – these are the structures of the first four members. Two isomers of butanol are shown. alcohols – a series of organic compounds containing the functional group –OH and with Making ethanol the general formula CnH2n+1OH Hydration of ethene The industrial method of making ethanol involves the HH addition reaction that we saw at the end of Section 10.6. In this, ethene and steam are compressed to 60 HCCOH atmospheres and passed over a catalyst (immobilised phosphoric(v) acid) at 300 °C: HH All alcohols ethene + steam 3⎯00°⎯C, 6⎯0 at⎯mos⎯phe→res ethanol have the phosphoric acid –OH group. C2H4(g) + H2O(g) → C2H5OH(g) Figure 10.14 The structure of ethanol. Alcohol Molecular Boiling Ethanol is an important solvent and a raw material for formula point / making other organic chemicals. Many everyday items CnH2n+1OH °C use ethanol as a solvent. These include paints, glues, perfumes, aftershave, etc. 65 methanol CH3OH 78 ethanol C2H5OH Fermentation 97 Ethanol and carbon dioxide are the natural waste propan-1-ol C3H7OH products of yeasts when they ferment sugar. Sugar is 117 present in all fruit and grains, and in the sap and nectar butan-1-ol C4H9OH of all plants. Yeasts are found everywhere. The ancient 137 Babylonians and Egyptians found that, if they crushed pentan-1-ol C5H11OH b.p. increasing grapes or germinated grain, the paste would bubble Table 10.4 Some alcohols. Chapter 10: Organic chemistry 265

Activity 10.3 Fermentation can be carried out in the laboratory The fermentation of glucose using the apparatus in Figure 10.16. The air-lock allows using yeast gas to escape from the vessel but prevents airborne bacteria entering. Skills Study tip AO3.1 Demonstrate knowledge of how to safely use techniques, apparatus and materials (including Fermentation is an anaerobic process. It takes following a sequence of instructions where appropriate) place under conditions where there is no air or oxygen available. AO3.3 Make and record observations, measurements and estimates Therefore, there is no oxygen (O2) present in the equation for the reaction taking place. Beer and wine are produced by fermenting glucose with yeast. Yeast contains enzymes that Carbon dioxide is the gas produced in catalyse the breakdown of glucose to ethanol the reaction. and carbon dioxide. In this experiment, a glucose solution is left to ferment. The products Comparing the methods of ethanol production S of fermentation are then tested. The solutions The two different methods of producing ethanol have their generated by the class may be retained for a respective advantages and disadvantages. The method demonstration of distillation (see Activity 2.3). chosen will depend on the availability of resources and the main purpose for producing the ethanol. A comparison of A worksheet is included on the CD-ROM. the methods is summarised in Table 10.5. and produce an intoxicating drink. Pasteur discovered The ethanol produced by fermentation comes from that yeasts are single-cell, living fungi. They ferment a renewable resource. When used as a fuel, the ethanol sugar to gain energy – by anaerobic respiration. As produced in this way is potentially ‘carbon neutral’. The ethanol is toxic to yeast, fermentation is self-limiting. carbon dioxide released during fermentation and by Once the ethanol concentration has reached about 14%, burning the fuel is balanced by that absorbed from the or the sugar runs out, the multiplying yeast die and atmosphere by the crop, usually sugar cane, as it grows. fermentation ends. The best temperature for carrying out the process is 37 °C. The reaction is catalysed by air-lock enzymes in the yeast: containing water glucose ⎯yea→st ethanol + carbon dioxide C6H12O6(aq) ⎯en⎯zym→es 2C2H5OH(aq) + 2CO2(g) Alcoholic drinks such as beer and wine are made on a glucose solution large scale in vast quantities in copper or steel fermentation with yeast vats. Beer is made from barley, with hops and other ingredients added to produce distinctive flavours. Wine Figure 10.16 A laboratory fermentation vessel. is made by fermenting grape juice. Beer contains about 4% by volume of ethanol, whereas wine contains between 8% and 14%. Stronger, more alcoholic, drinks are made in one of two ways. Fortified wines, such as sherry and port, have pure ethanol added to them. Spirits, such as whisky, brandy and vodka, are made by distillation (see page 30). 266 Cambridge IGCSE Chemistry

Questions a Name the gas produced during the S fermentation shown above. 10.38 Name the first three members of the alcohol homologous series. b This gas escapes through the piece of apparatus labelled A. What is the main 10.39 Write the word and chemical equations for purpose of this piece of apparatus? the hydration of ethene by steam. c What must be added to a sugar solution to 10.40 What are the essentials needed for the make it ferment? production of ethanol by fermentation? d At about what temperature does 10.41 Ethanol can be made by the addition of water fermentation take place at its fastest rate? to ethene. Ethanol can also be made by the fermentation of sugars using the apparatus e Explain your choice of temperature shown here. given in d. A 10.42 Methanol and ethanol are members of a homologous series. bubbles of gas a Draw the molecular structures of methanol and ethanol. fermenting liquid b Explain what the term homologous series means. 10.43 What are the names and structures of the two isomers of propanol? S Ethanol by the 10.8 The reactions of ethanol hydration of ethene Ethanol by fermentation Ethanol as a fuel Ethanol burns with a clear flame, giving out quite a lot originates from a non- made from readily renewable of heat: renewable resource – resources petroleum ethanol + oxygen → carbon dioxide + water C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(g) small-scale equipment capable of withstanding relatively simple, large vessels On a small scale, ethanol can be used as methylated spirit pressure (ethanol mixed with methanol or other compounds) in spirit lamps and stoves. However, ethanol is such a useful a continuous process a batch process – need to fuel that some countries have developed it as a fuel for cars. start process again each time Brazil, whose climate is suitable for growing sugar cane, a fast reaction rate a relatively slow process started producing ethanol fuel in 1973. It has one of the largest ethanol fuel programmes in the world (Figure 10.17). yields highly pure ethanol must be purified The ethanol is produced by fermenting crop residues, and as ethanol by subsequent distillation – such is considered a biofuel. Ethanol and other biofuels are though fermented product used in motor vehicles as an alternative to fuel obtained from can be used as it is for some oil deposits. Ethanol produced by fermentation of sugar from purposes sugar cane has been used as an alternative fuel to gasoline (petrol), or mixed with gasoline to produce ‘gasohol’. It a sophisticated, a simple, straightforward complex method method Chapter 10: Organic chemistry 267 Table 10.5 A comparison of the methods of ethanol production.

A rocksil and broken pot ethanol heat glass rod Bunsen valve rubber tube (prevents with slit sucking back) Figure 10.18 The dehydration of ethanol in the laboratory. Figure 10.17 An ethanol and petrol station in Sao Paulo, Brazil. Dehydration Ethanol can be dehydrated to produce ethene. This is is a renewable resource and has the potential to reduce one way of preparing ethene in the laboratory. Ethanol petroleum imports. ‘Gasohol’ now accounts for 10% of the vapour is passed over a heated catalyst. The catalyst can gasoline sales in the USA. ‘Gasohol’ and other ‘oxygenated be aluminium oxide or broken pieces of porous pot. fuels’ have the advantage of reducing the emissions of Ethene is not soluble in water, so it can be collected as carbon monoxide from cars. It is thought that biofuels can shown in Figure 10.18. reduce environmental damage if developed and controlled properly, but overuse of biofuels could be harmful if it Esterification S leads to more deforestation to grow biofuel crops. Alcohols react with organic acids (see Section 10.9) to form sweet-smelling oily liquids known as esters. For S Study tip example: Exam questions often ask you to balance the ethanoic acid + ethanol → ethyl ethanoate + water equations for the combustion reactions of either CH3COOH(l) + C2H5OH(l) hydrocarbons or alcohols. → CH3COOC2H5(l) + H2O(l) Make sure you balance the oxygen (O) atoms in the equation. Remember, with alcohols, that there Concentrated sulfuric acid is added as a catalyst for this is an oxygen atom in the alcohol molecule itself. esterification reaction. Oxidation Alcohol and health Vinegar is a weak solution of ethanoic acid (previously Ethanol is the only alcohol that is safe to drink. It must only called acetic acid). It is produced commercially be drunk in moderation, if at all. Methanol is very toxic and from wine by biochemical oxidation using bacteria even in small amounts can cause blindness and death. (Acetobacter). Wine can also become ‘vinegary’ if it is left open to the air. The same oxidation can be achieved Ethanol mixes totally with water, which takes it quickly by powerful oxidising agents such as warm everywhere in the body that water goes. The amount acidified potassium manganate(vii): of alcohol that a person may drink varies with age, sex, weight and drinking history. ethanol + oxygen → ethanoic acid + water Heavy drinking can cause a healthy liver to become from oxidising agent fatty and enlarged. Eventually scarring (cirrhosis) can cause liver failure and death. Prolonged heavy drinking can C2H5OH + 2[O] → CH3COOH + H2O eventually damage the muscle tissue of the heart. It may also lead to some long-term damage to the brain. Alcohol The colour of the potassium manganate(vii) solution is a depressive drug and can be addictive. Drinking heavily turns from purple to colourless. on a particular occasion produces drunkenness, during which speech becomes slurred, vision is blurred and reaction times are slowed. Some cultures forbid its use. 268 Cambridge IGCSE Chemistry

Questions burning ethanol 10.44 Sugar cane grows quickly in tropical areas. Sugar can be fermented to make ethanol. Either i Name these products. S ethanol or mixtures of petrol and ethanol ii Suggest why it is not possible to set fire (gasohol) can be used as the fuel for cars. a Ethanol consists of organic molecules. to the contents of a bottle of wine. i What type of compound is ethanol? 10.46 Write the word and chemical equations for ii Ethanol has the formula C2H5OH. Draw its structure. the oxidation of ethanol to ethanoic acid (use b Gasohol boils over a temperature range of [O] for the oxidising agent). 40–150 °C in the laboratory. Ethanol has 10.47 What alkene is produced when propan-1-ol a boiling point of 78 °C. Draw a labelled is dehydrated? diagram to show how a sample of ethanol 10.48 Name the ester produced when ethanol reacts may be obtained from gasohol. with ethanoic acid. Write the word equation for the reaction. What is the catalyst for the reaction? 10.45 Ethanol is a fuel. In laboratories that do not have a gas supply, it may be used in a spirit burner. a The diagram shows a spirit burner being used to heat a beaker of water. A black solid is formed on the bottom of the beaker. i Name the black substance formed on the beaker. ii Suggest why the black substance is formed when the ethanol is burnt. b Both methanol and ethanol burn in an excess of air to form the same products. S 10.9 Organic acids and esters Study tip Carboxylic acids When naming a carboxylic acid, remember that The carboxylic acids are another homologous series the carbon atom of the acid group is part of the of organic compounds. All these acids have the chain. It is counted as the first carbon in the chain. functional group —COOH attached to a hydrocarbon chain. Table 10.6 (overleaf) shows the molecular That is why CH3COOH is the formula of ethanoic formulae of the first two members of the series. The acid: there are two carbon atoms in the molecule. compounds have the general formula CnH2n+1COOH (or CnH2n+1CO2H). Figure 10.19 shows the structural Ethanoic acid as a weak acid formulae of the first four acids in the series. Whereas a strong acid such as hydrochloric acid is completely split into ions, ethanoic acid only partially The first two acids in the series are liquids at room dissociates into ions in water. A dynamic equilibrium temperature, although ethanoic acid will solidify if is set up in the solution. The solution does contain the temperature falls only slightly. The acids dissolve an excess of hydrogen ions (H+) over hydroxide ions in water to produce solutions that are weakly acidic. (OH−), so the solution is weakly acidic (see page 143): Methanoic acid is present in nettle stings and ant stings, while ethanoic acid (once called acetic acid) is well ethanoic acid ethanoate ions + hydrogen ions known as the acid in vinegar. CH3COOH(aq) CH3COO−(aq) + H+(aq) Chapter 10: Organic chemistry 269

S Carboxylic Molecular formula Melting Boiling point S acid / °C CnH2n+1COOH point / °C methanoic acid 101 HCOOH 9 ethanoic acid CH3COOH 17 118 m.p. and b.p. increasing Table 10.6 The first two carboxylic acids. methanoic acid Commercial descalers are often based on weak acids, methanoic acid for instance, or on moderately strong O acids such as sulfamic acid. HC Activity 10.4 OH The acidic reactions of ethanoic acid All carboxylic acids have the Skills –COOH group. AO3.1 Demonstrate knowledge of how to safely use ethanoic acid H techniques, apparatus and materials (including following a sequence of instructions where H appropriate) O AO3.2 Plan experiments and investigations HCC AO3.3 Make and record observations, measurements O and estimates H AO3.4 Interpret and evaluate experimental observations propanoic acid H butanoic acid O H and data HHH C HH This activity tests ethanoic acid with Universal O HCCC O Indicator solution, magnesium, sodium HHH hydroxide solution and sodium carbonate HCCC solution. These reactions are then compared O with those of a hydrochloric acid solution of the same concentration. The comparison shows that HH ethanoic acid is a weak acid. Figure 10.19 The structures of methanoic, ethanoic, propanoic and A worksheet is included on the CD-ROM. butanoic acids. Esterification A solution of the acid will show the characteristic reactions Ethanoic acid will react with ethanol, in the presence of an acid. For example, it will react with bases to form salts: of a few drops of concentrated sulfuric acid, to produce ethyl ethanoate. The concentrated sulfuric acid is a ethanoic acid + sodium hydroxide catalyst for the reaction: → sodium ethanoate + water ethanoic acid + ethanol ⎯⎯→conc. H2SO4 ethyl ethanoate + water CH3COOH(l) + C2H5OH(l) → CH3COOC2H5(l) + H2O(l) CH3COOH(aq) + NaOH(aq) → CH3COONa(aq) + H2O(l) Vinegar can be used as a ‘descaler’ in hard water areas. The ethanoic acid in vinegar reacts with limescale (calcium carbonate), producing carbon dioxide and dissolving the scale: calcium carbonate + ethanoic acid → calcium ethanoate + water + carbon dioxide CaCO3(s) + 2CH3COOH(aq) → (CH3COO)2Ca(aq) + H2O(l) + CO2(g) 270 Cambridge IGCSE Chemistry

S This type of reaction is known as esterification. The linkage is also found in complex molecules such as S structure of ethyl ethanoate is shown below: natural fats and oils, and in man-made fibres such as Terylene (see page 288). HH HC CO H Ester Smell or flavour ethyl 2-methylbutanoate apple HH CCH 3-methylbutyl ethanoate pear 1-methylbutyl ethanoate banana OH butyl butanoate pineapple octyl ethanoate orange ethyl ethanoate methylpropyl methanoate raspberry pentyl butanoate strawberry Ethyl ethanoate is just one example of an ester. This family of compounds have strong and pleasant smells. Table 10.7 The smells of esters. Many of these compounds occur naturally. They are responsible for the flavours in fruits and for the scents of flowers (Table 10.7). We use them as food flavourings and in perfumes. The ester group or S S Activity 10.5 In this activity, the reactions between a range of Making esters from alcohols alcohols and acids are carried out on a test-tube and acids scale, to produce small quantities of a variety of esters quickly. The characteristic odours of the Skills different esters are then tested. AO3.1 Demonstrate knowledge of how to safely use A worksheet is included on the CD-ROM. techniques, apparatus and materials (including following a sequence of instructions where appropriate) AO3.3 Make and record observations, measurements and estimates AO3.4 Interpret and evaluate experimental observations and data Chapter 10: Organic chemistry 271

Questions S 10.49 Ethanol is a product of many fermentation 10.51 The diagram below shows a method used in S reactions. The molecular formula of ethanol France to change wine into vinegar. Living is C2H5OH. organisms (Acetobacter) bringing about a a Draw the structural formula for ethanol. chemical change and producing a marketable b When ethanol is heated with an excess product is an example of traditional of acidified potassium dichromate, it is biotechnology. converted to ethanoic acid: C2H5OH → CH3COOH wine slowly trickles in ethanol ethanoic acid What type of chemical reaction is this? filled with large vat c Some synthetic flavourings are made by wood shavings covered with temperature reacting an alcohol with a carboxylic acid: bacteria called rises to 30 °C alcohol + carboxylic acid → ester + water ‘mother of What other substance and conditions are vinegar’ wine vinegar needed to carry out this reaction? out d The structure of the ester propyl butanoate is shown here. HH HO HHH HC C CC O C C CH a What type of reaction has occurred in the vat? b Name a chemical reagent, other than HH H HHH oxygen, that can change ethanol into Draw the structural formula of the carboxylic ethanoic acid. acid from which this ester is made. c Name a technique that could be used to 10.50 The flavour and smell of foods are partly due separate ethanoic acid from the other to esters. An ester can be made from ethanol liquids in vinegar. and ethanoic acid. d Describe a chemical test that would a Name this ester. distinguish between wine and vinegar. b Write a word equation for the reaction between ethanol and ethanoic acid. Summary You should know: ◆ that carbon forms a vast range of compounds and that the study of their properties is known as organic chemistry ◆ that hydrocarbons are the simplest of the many types of organic compound ◆ about the different ‘families’ (or homologous series) of hydrocarbons: the alkanes are saturated hydrocarbons, while the alkenes are a second series of unsaturated hydrocarbons ◆ how the alkanes are important fuels and that the simplest, methane, is the main component of natural gas ◆ that the simple test for unsaturated hydrocarbons is the fact that they decolorise bromine water ◆ that there are many more different series of organic compounds, each with a different functional group attached to a hydrocarbon backbone 272 Cambridge IGCSE Chemistry

◆ that the alcohols are a separate series of compounds, the most important of which is ethanol ◆ how ethanol can be manufactured industrially by the hydration of ethene or by large-scale fermentation ◆ that ethanol has major uses as a fuel and as a solvent ◆ that hydrocarbons and alcohols burn in excess air to produce carbon dioxide and water vapour S ◆ how different organic compounds can have the same molecular formula but different structural formulae – they can be isomers of each other S ◆ how alkanes undergo substitution reactions with the halogens, but alkenes are more reactive and take part in addition reactions with hydrogen, the halogens and steam S ◆ that oxidation of alcohols produces a further series of compounds, the carboxylic acids S ◆ the fact that these acids are weak acids, only partly ionised in water S ◆ how alcohols and carboxylic acids react to produce esters in reactions known as esterification. End-of-chapter questions 1 There are very many more compounds of the element carbon than there are of any other element. Why are these compounds particularly important to us? 2 These three compounds, A, B and C, belong to three different homologous series. C2H4 C2H6 C2H5OH A B C a What is meant by the term homologous series? [1] b To which homologous series does each compound belong? [3] c Give a chemical test which could distinguish between compound A and compound B. Describe the [3] test and give the result for compounds A and B. [2] d How could compound A be chemically converted to compound C? [1] e What is the name of the process which forms compound C from sugar? S 3 But-1-ene is a typical alkene. It has the structural formula shown below. CH3—CH2—CH=CH2 The structural formula of cyclobutane is given below. H H H H C C C C H H H H a These two hydrocarbons are isomers. [2] i Define the term isomer. [1] ii Draw the structural formula of another isomer of but-1-ene. iii Describe a test which would distinguish between but-1-ene and cyclobutane. Name [3] the reagent used and give the result for both isomers. Chapter 10: Organic chemistry 273

S b Describe how alkenes, such as but-1-ene, can be made from alkanes. [2] c Name the product formed when but-1-ene reacts with each of the following: i bromine [1] ii hydrogen [1] iii steam. [1] [Cambridge IGCSE® Chemistry 0620/32, Question 4, June 2010] 4 Butane is an alkane. It has the following structural formula. HHHH HCC C CH HHHH a The equation for the complete combustion of butane is given below. Insert the two missing volumes. 2C4H10(g) + 13O2(g) → 8CO2(g) + 10H2O(g) .......... .......... 40 volume of gas / cm3 [2] b Butane reacts with chlorine to form two isomers of chlorobutane. i What type of reaction is this? [1] ii Explain the term isomer. [2] iii Draw the structural formulae of these two chlorobutanes. [2] c One of the chlorobutanes reacts with sodium hydroxide to form butan-1-ol. Butan-1-ol can be oxidised to a carboxylic acid. i State a reagent, other than oxygen, which will oxidise butan-1-ol to a carboxylic acid. [1] ii Name the carboxylic acid formed. [1] iii Butan-1-ol reacts with ethanoic acid to form an ester. Name this ester and give its structural formula, showing all the individual bonds. [3] [Cambridge IGCSE® Chemistry 0620/31, Question 6, June 2012] 5 Propenoic acid is an unsaturated carboxylic acid. The structural formula of propenoic acid is given below. H COOH CC HH a i Describe how you could show that propenoic acid is an unsaturated compound. Give details of the test and the result. [2] ii Without using an indicator, describe how you could show that a compound is an acid. Give details of the test and the result. [2] b Propenoic acid reacts with ethanol to form an ester. Deduce the name of this ester. Draw its structural formula, showing all bonds. [3] c An organic compound has a molecular formula C6H8O4. It is an unsaturated carboxylic acid. One mole of the compound reacts with two moles of sodium hydroxide. i Explain the phrase molecular formula. [2] ii One mole of this carboxylic acid reacts with two moles of sodium hydroxide. How many moles of –COOH groups are there in one mole of this compound? [1] iii What is the formula of another functional group in this compound? [1] iv Deduce a structural formula of this compound. [1] [Cambridge IGCSE® Chemistry 0620/33, Question 5, November 2012] 274 Cambridge IGCSE Chemistry

11 Petrochemicals and polymers In this chapter, you will find out about: ◆ fossil fuels S◆ addition polymerisation ◆ the formation and fractional distillation of S◆ condensation polymerisation S◆ the disposal and recycling of plastic waste petroleum (crude oil) S◆ biological condensation polymers ◆ catalytic cracking – proteins S ◆ alternative transport fuels – carbohydrates. S ◆ biogas Sheets and tubes of carbon – new technology It allows electrons to flow very efficiently across its Industry based on organic chemistry has given us surfaces. Its electrical conductivity is high and it may a vast range of new fuels and materials that have eventually replace silicon in computer chips. revolutionised our lives. As we explore the virtuosity of carbon as an element, novel materials continue to Graphene sheets are also the starting point for be discovered. In 2010, the Nobel Prize was awarded making carbon nanotubes (Figure 11.1b). The discovery to Andre Geim and Konstantin Novoselov for their of nanotubes has suggested, among other things, new work on graphene sheets (Figure 11.1a). methods of medical drug delivery and conducting materials. Chemical polymerisation reactions have been Graphene is a flat sheet of carbon atoms arranged carried out in miniature within nanotubes, with the in a hexagonal pattern – atomic-scale carbon carbon tubes being used as linear reaction vessels. ‘chicken wire’. Stacks of graphene sheets make up the graphite in the pencils you use every day. However, This chapter will focus on some of the ways we the individual sheets are providing many new ideas exploit the rich chemistry of carbon. The future holds for their use. Graphene is very strong and flexible. the prospect of even more incredible adaptations and technological developments. Figure 11.1 Computer graphics of the structures of: a a honeycombed sheet of graphene; b a stack of carbon nanotubes. Chapter 11: Petrochemicals and polymers 275

11.1 Petroleum Fossil fuels were formed in the Earth’s crust from material that was once living. Coal comes from fossil plant material. Petroleum (or crude oil) and natural gas are formed from the bodies of marine microorganisms. The formation of these fuels took place over geological periods of time (many millions of years). These fuels are therefore a non-renewable and finite resource. There are three major fossil fuels: Figure 11.2 An oil rig in the Caspian Sea. ◆ coal ◆ petroleum (crude oil) Recently, novel approaches for the exploitation of ◆ natural gas. shale gas- and oilfields have added new and abundant availability of fossil fuel resources. The ‘fracking’ The formation of petroleum techniques involved in shale gas extraction are proving Petroleum (or crude oil) is one of the Earth’s major controversial, however, particularly in countries with natural resources, the result of a process that began large population density. up to 400 million years ago. When prehistoric marine creatures died, they sank to the seabed and Fractional distillation were covered by mud. The change into petroleum Petroleum is a mixture of many different and natural gas was brought about by high pressure, hydrocarbon molecules. Most of the petroleum that high temperature and bacteria acting over millions is extracted from the ground is used to make fuel, but of years. The original organic material broke down around 10% is used as a feedstock, or raw material, into hydrocarbons. Compression of the mud above in the chemical industry. Before it can be used, the the hydrocarbon mixture transformed it into shale. various hydrocarbon molecules are separated by Then geological movements and pressure changed refining. This is done by fractional distillation at an this shale into harder rocks, squeezing out the oil oil refinery. and gas. The oil and gas moved upwards through the porous rocks, moving from high-pressure to low- At a refinery, petroleum is separated into different pressure conditions. Sometimes they reached the fractions – groups of hydrocarbons that have different surface, but often they became trapped by a layer of boiling points. These different boiling points are non-porous rock. roughly related to the number of carbon atoms in the hydrocarbons (Table 11.1). Reservoirs of oil and gas were created. These reservoirs are not lakes of oil or pockets of gas. Instead, Separation of the hydrocarbons takes place the oil or gas is spread throughout the pores in coarse by fractional distillation using a fractionating rocks such as sandstone or limestone, in much the same column (or tower). At the start of the refining way as water is held in a sponge. process, petroleum is preheated to a temperature of 350–400 °C and pumped in at the base of the tower. Oilfields and gasfields are detected by a series As it boils, the vapour passes up the tower. It passes of geological and seismic surveys. Promising areas through a series of bubble caps, and cools as it rises are then drilled to gain more geological information further up the column. The different fractions cool or, if oil or gas is found, to see how extensive the and condense at different temperatures, and therefore oilfield or gasfield is. Once a field is established, at different heights in the column. The fractions production oil rigs can be set up, on land or at sea condensing at the different levels are collected on (Figure 11.2). trays. Fractions from the top of the tower are called 276 Cambridge IGCSE Chemistry

‘light’ and those from the bottom ‘heavy’. Each obtained by further distillation. Figure 11.3 shows fraction contains a number of different hydrocarbons. the separation into different fractions and some of The individual single hydrocarbons can then be their uses. Fraction Approximate number of carbon atoms Approximate in hydrocarbons boiling range / °C refinery gas 1–4 C1–C4 below 25 b.p. and viscosity gasoline/petrol(a) 4–12 C4–C12 40–100 increasing naphtha 7–14 C7–C14 90–150 kerosene/paraffin(a) 12–16 C9–C16 150–240 diesel oil/gas oil 14–18 C14–C18 220–300 fuel oil 19–25 C19–C25 250–320 lubricating oil 20–40 C20–C40 300–350 bitumen over 70 > C70 above 350 (a)Different terms are used in the UK and the USA. Note that ‘crude oil’ (UK) is the same as ‘petroleum’ (USA), ‘petrol’ (UK) is the same as ‘gasoline’ (USA), and ‘paraffin’ (UK) is the same as ‘kerosene’ (USA). Table 11.1 Various petroleum(a) fractions. FRACTIONATING refinery gas TOWER bottled gas for heating and cooking petroleum 40°C (crude oil) gasoline used as fuel in cars (petrol) 110°C naphtha used to make chemicals 180°C kerosene/paraffin used as a fuel in jet engines and as heating oil 260°C diesel oil used as a fuel in diesel engines fuel oil HEATER lubricating oil Figure 11.3 Fractional distillation of petroleum in a refinery. used in waxes and polishes 340°C bitumen used to make bitumen for surfacing roads Chapter 11: Petrochemicals and polymers 277

Study tip Study tip Take care over names for the different fractions in Make sure you know the order of the fractions as different parts of the world. Note that the syllabus they are produced from the fractionating tower, uses ‘petroleum’ as the name of the crude oil and a use for each of the fractions. drilled from the ground. Activity 11.1 to condense the low boiling point fraction. The Fractional distillation of petroleum bulb of the thermometer should be level with, or Skills just below, the side arm. Heat the bottom of the side- AO3.1 Demonstrate knowledge of how to safely use techniques, apparatus and materials (including arm test tube gently, with the lowest following a sequence of instructions where appropriate) Bunsen flame. Watch the thermometer carefully. AO3.3 Make and record observations, measurements and estimates 3 When the temperature reaches 100°C, replace the AO3.4 Interpret and evaluate experimental observations collection tube with another empty one. The beaker of and data water is no longer necessary and can be removed. Wear eye protection. The petroleum is highly flammable and harmful. 4 Collect three further fractions, to give the This experiment simulates the industrial fractional fractions as follows: distillation of petroleum (crude oil) in the laboratory. A Room temperature to 100 °C B 100–150 °C C 150–200 °C D 200–250 °C 5 A black residue remains in the side-arm test tube. Test the four fractions for viscosity (see how easily they pour), colour, smell and flammability. ◆ To test the smell, gently waft the smell towards you with your hand. thermometer ◆ To test for flammability, pour a small quantity clamp on to a hard-glass watch glass and light the ceramic fibre fraction with a burning splint. + 6 Keep one set of fractions and see that they combine petroleum substitute to form a mixture very like the original sample. heat A worksheet is included on the CD-ROM. cold water Questions 1 Place about a 2cm3 depth of ceramic fibre in the A1 What differences did you observe in the viscosity of bottom of a side-arm test tube. Add about 2cm3 of the fractions? What molecular property would you petroleum substitute to this, using a teat pipette. suggest causes this difference? 2 Set up the apparatus as shown in the diagram. The A2 What differences were there in flammability beaker of cold water around the collecting tube helps between the fractions? Catalytic cracking opposite is true for heavier fractions such as kerosene The demand for the various fractions from the refinery (paraffin) and diesel. Larger molecules from these does not necessarily match with their supply from the heavier fractions can be broken into smaller, more oil (Figure 11.4). For lighter fractions such as gasoline valuable, molecules. This process is called catalytic (petrol), the demand is greater than the supply. The cracking (‘cat cracking’). 278 Cambridge IGCSE Chemistry

Cracking takes place in a huge reactor (Figure 11.5). Both products are useful. The shortened alkanes can be In this reactor, particles of catalyst (made of powdered blended with the gasoline fraction to enrich the petrol. minerals such as silica, alumina and zeolites) are The alkenes are useful as raw materials for making mixed with the hydrocarbon fraction at a temperature several important products. Figure 11.6 shows the around 500 °C. The cracked vapours containing smaller various uses for the ethene produced. molecules are separated by distillation. Propene polymerises to poly(propene) (trade The shortened hydrocarbon molecules are produced name ‘polypropylene’), while butene polymerises to by the following type of reaction: produce synthetic rubber. The cracking reaction can be carried out in the laboratory using paraffin oil heat (Figure 11.7). decane octane + ethene catalyst C10H22 C8H18 + C2H4 HHHHHHHHHH HC C C C C C C C C CH HHHHHHHHHH HHHHHHHH HH HC C C C C C C CH+C C HHHHHHHH HH This is just one of the possible reactions when decane is cracked. The molecules may not all break in the same place. The alkene fragment is not always ethene: propene and but-1-ene may also be produced. All cracking reactions give: ◆ an alkane with a shorter chain than the original, and a short-chain alkene ◆ or two or more alkenes and hydrogen. Figure 11.5 A cracking plant in an oil refinery. Higher demand Lower demand Figure 11.4 There is not the same economic demand for all the fractions from petroleum. Chapter 11: Petrochemicals and polymers 279

Activity 11.2 paraffin soaked broken porcelain Cracking hydrocarbons into absorbent or aluminium wool oxide granules Skills delivery AO3.1 Demonstrate knowledge of how to safely use tube techniques, apparatus and materials (including following a sequence of instructions where strong appropriate) warm heat AO3.3 Make and record observations, measurements and cold water bath crystallising dish estimates Figure 11.7 The cracking of a long-chain alkane in the laboratory. AO3.4 Interpret and evaluate experimental observations and data A The exact composition of petroleum varies depending Figure 11.8 Computer image of an internal combustion engine cylinder. The on where it comes from, but most oil contains more piston (lower right) is moved up and down by the combustion (burning) of fuel. of the larger molecules than the smaller ones. The This image shows the fuel/air mixture being ignited by the spark plug. smaller ones, however, are more useful and therefore more economically important. To increase the profit Blending gasoline that can be made from a barrel of oil, the larger Some of the products from cracking are added to the hydrocarbons are broken down into smaller ones. gasoline fraction to improve the quality of the petrol. This activity involves a small-scale version of this As many as 12 different components (containing over conversion, which is performed in industry every day. 300 different hydrocarbons and additives) may be used in a blend of petrol for the motorist. Different blends A worksheet is included on the CD-ROM. are made for winter and summer use. An important consideration is how easily the fuel vapour ignites. If Details of a scaled-up version of this experiment the fuel ignites too easily, then the engine will not run are given in the Notes on Activities for teachers/ smoothly – ‘knocking’ will occur. However, if the fuel is technicians. too difficult to ignite, then the engine will be difficult to start, especially on cold mornings. High-quality petrol ethene contains many branched-chain hydrocarbons, made in from cracking a process known as re-forming, so that the fuel does not ignite too soon (Figure 11.8). The ignition temperature react with polymerise react with react with of a petrol/[solidus] air mixture is around 550 °C. steam chlorine benzene ‘Lead’ (actually tetraethyl-lead) was added to ethanol poly(ethene) chloroethene phenylethene gasoline to prevent ‘knocking’. But this caused high (polythene) (vinyl chloride) (styrene) polymerise polymerise poly(chloroethene) poly(phenylethene) (PVC) (polystyrene) Figure 11.6 Important products can be made from the ethene produced by catalytic cracking. 280 Cambridge IGCSE Chemistry

A levels of lead in the air, particularly in large cities. This sulfur dioxide. The carbon monoxide (CO), unburnt A led to concern over the link with brain damage in hydrocarbons (HC) and oxides of nitrogen (NOx) in young children. Unleaded fuel is now almost universally exhaust fumes do continue to cause concern. The levels available and has to be used in modern cars fitted with of emission of these compounds are reduced by fitting a catalytic converters (the lead would poison the catalyst catalytic converter to the exhaust. and so prevent it working). The removal of sulfur from gasoline fractions is now Gasoline vapour also escapes into the air at petrol very efficient. Car exhaust emissions contain very little stations. Modern pumps now have hoods on the nozzles to cut down the escape of fumes. Activity 11.3 The apparatus shown could be used to Comparing fuels discover how much heat a fuel produces. A small quantity of the fuel is placed on the ‘ceramic wool’ Skills and ignited. The temperature change of the water in the boiling tube is then recorded. AO3.1 Demonstrate knowledge of how to safely use techniques, apparatus and materials (including 1 You are asked to compare paraffin, a following a sequence of instructions where appropriate) fossil fuel obtained from petroleum (crude oil), and ethanol, a renewable fuel which can AO3.2 Plan experiments and investigations be made by fermenting sugar from plants. You AO3.3 Make and record observations, measurements and will need to use a relatively small amount of fuel (around 1 cm3). estimates AO3.4 Interpret and evaluate experimental observations 2 When you have planned your investigation, you should carry it out and record all your and data observations and measurements. AO3.5 Evaluate methods and suggest possible improvements 3 You should then write a reasoned conclusion that Wear eye protection. states which fuel you think is better, together with your reasons. There is concern that fossil fuels will run out. There is also concern that burning fossil fuels is causing A worksheet is included on the CD-ROM. global warming because of all the carbon dioxide which is released into the atmosphere. The Notes on Activities for teachers/technicians contain details of how the experiment can be used You are asked to design an experiment to as an assessment of skills AO3.2 and AO3.5. compare two fuels: one a fossil fuel and the other a renewable fuel. thermometer clamp and stand boiling tube ceramic wool heatproof mat Chapter 11: Petrochemicals and polymers 281

Questions group of alumino-silicates of sodium, potassium, A calcium and barium.) Methane is first converted into 11.1 Put the following fractions in order of methanol. The methanol produced is then turned into increasing boiling point: kerosene, diesel, hydrocarbons using the ZSM-5 catalyst: petrol, refinery gas, bitumen, naphtha. methanol → hydrocarbons + water 11.2 Name three fossil fuels. 11.3 Explain what is meant by ‘cracking’, and write nCH3OH → (CH2)n + nH2O word and balanced symbol equations to show ZSM-5 is an artificial zeolite composed of aluminium, how ethene can be formed from decane by this silicon and oxygen. It was first made by two chemists method. working for the US Mobil Oil company. 11.4 State a use for the following fractions from the distillation of petroleum (crude oil): bitumen, LPG and CNG fuel oil, diesel, kerosene. Liquid petroleum gas (LPG or ‘autogas’) is composed of propane and butane. Compressed natural gas 11.2 Alternative fuels and energy (CNG) is 90% methane. These products already have sources a significant market in some countries. For example, all the taxis in Japan use LPG. In India, many of the Alternative transport fuels motorised rickshaws in the major cities run on CNG There is growing interest and a great deal of (Figure 11.9). development work taking place on alternative transport fuels in order to reduce dependence on fossil fuels and Biofuels find ‘greener’ forms of transport. There have been significant developments in the use of fuels based on ethanol (see page 267) and vegetable Diesel oils such as rapeseed or sunflower oil. The potential High-speed diesel engines in cars, buses and trucks for adding plant oils to diesel fuel is being investigated use a fuel (DERV – diesel engine road vehicle) worldwide. The future use of these fuels will depend that contains hydrocarbon molecules consisting on economic factors. Some countries grow oil- of between 6 and 20 carbon atoms (in short this is producing crops but do not have their own reserves of written as C6–C20 molecules). Slower-speed diesel petroleum. Recently there has been controversy about engines for ships, etc., use a slightly heavier fuel. the use of land to grow crops for these fuels and the Diesel engines are compression ignition engines (the fuel ignites spontaneously without a spark). Diesel Figure 11.9 In India, many of the motorised rickshaws (‘tuk-tuks’) run engines are more efficient than petrol engines and on CNG. produce much less carbon monoxide. However, because their working temperature is higher, they produce more oxides of nitrogen. The major problems are smoke, and the particles it contains, and odour. Gasoline from methanol New Zealand has large reserves of natural gas (mainly methane) but very little petroleum. The problem of producing petrol has been transformed by a catalyst known as zeolite ZSM-5. (A zeolite is one of a large 282 Cambridge IGCSE Chemistry

A fact that this diverts farmland from food production. organic matter under geological conditions to produce A The consequent decrease in food availability and natural gas. Methane accumulates in coalmines, where increase in prices have been sources of concern. These it can cause explosions. Marsh gas, which bubbles up have led to other sources of ethanol being explored, through the stagnant water of marshes, swamps and rice including, for instance, using sawdust from sawmills paddy fields, is also methane. Methane produced in this as a starting point for production (Figure 11.10). way contributes to the greenhouse effect. Vegetable oils from crops such as rape and sunflowers are also being developed as fuels. Recycled cooking oil Methane is produced from organic waste (biomass) is being used as a component of biodiesel. Biodiesel when it decays in the absence of air. This can be contains esters, which are made from vegetable oils such exploited as a source of energy. In countries such as as rapeseed or animal fats. The continual growth of new India and China, biomass digesters are important oil-producing plants, which absorb carbon dioxide from sources of fuel for rural villages (Figure 11.12). The the air through photosynthesis, means that biodiesel methane is useful for heating and cooking, and the solid contributes less to global warming than fossil fuels residue is used as a fertiliser. do. Motor manufacturers have all developed cars with engines adapted to run on these new fuels. Industrialised countries produce large amounts of waste, much of which is deposited in landfill sites. Others Biogas forms as the rubbish decays (Figure 11.13). We discussed the use of ethanol- and hydrogen-powered This gas can be used as a fuel for local industry. On cars on pages 89 and 267. Electric and solar-powered cars Merseyside in the UK, biogas is used to heat the ovens are also being investigated as alternatives to gasoline, and in a Cadbury’s biscuit factory. many manufacturers now have hybrid cars that run on a combination of power systems (Figure 11.11). Figure 11.11 The Toyota Prius is a hybrid gas/electric car with low emissions. The battery charges while running on petrol and an on-board Alternative energy sources computer switches when petrol is low. Alternative energy sources are very much under discussion. Nuclear energy, wind farms and solar energy are some of the areas being explored. In terms of chemical systems, those based around our use of waste offer an opportunity for development, particularly at local level. Biogas Methane gas is formed naturally in a number of different circumstances. Anaerobic bacteria helped to decompose Figure 11.10 The sawdust from sawmills can be used as a starting point in Figure 11.12 A small-scale biogas generator uses animal waste to produce making ethanol for biofuels. methane for a village’s needs. Chapter 11: Petrochemicals and polymers 283

Aa Figure 11.14 Making a chain of beads is similar to joining the monomers together to make a polymer. macromolecules are copolymers, made of two or more S different types of monomer. For example, nylon is made b from two monomers, and biological proteins are made from 20 different monomers, the amino acids. Figure 11.13 a Deep in the waste of landfill sites, methane gas accumulates and must be burnt off or it could become dangerous – a landfill flare-off. The alkene fragments from the catalytic cracking of b Alternatively, a power-generating plant can be set up at a landfill site to petroleum fractions produced the starting monomers generate power for the local area. for the first plastics. Alkenes such as ethene contain a C=C double bond. These molecules can take part in S 11.3 Addition polymerisation addition reactions (see page 258) where the double bond is broken and other atoms attach to the carbons. All living things contain polymers. Proteins, The double bond in ethene enables many molecules of carbohydrates, wood and natural rubber are all ethene to join to each other to form a large molecule, polymers. What nature first invented, chemists have poly(ethene) (Figure 11.15). This is an addition learned to copy, alter and use successfully. Synthetic polymer. When first made by ICI, it was a revolutionary polymers, often called plastics, are to be found new material called ‘Alkathene’. It is now commonly everywhere in modern technological societies, made called by the trade name ‘polythene’. into bulky objects, films and fibres. They have properties to suit particular needs, ranging from car and aircraft Various conditions can be used to produce different components to packaging and clothing. types of poly(ethene). Generally a high pressure, a temperature at or above room temperature and a Polymers are large organic macromolecules. catalyst are needed. The reaction can be summarised by They are made up of small repeating units known the equation: as monomers (Figure 11.14) joined together by polymerisation. These units are repeated any number ethene high pressure poly(ethene) of times from about a hundred to more than a million. heat, catalyst S Some are homopolymers, containing just one monomer. Poly(ethene), poly(propene) and poly(chloroethene) HH HH are three examples of homopolymers. Other high pressure CC H Hn n CC heat, catalyst HH where n is a very large number. Study tip When drawing the structure of poly(ethene) and other polymers, do not forget to put the n outside the bracket. 284 Cambridge IGCSE Chemistry

S HH H HH HH HH H S monomer: CC CC CC CC CC ethene H HH HH HH HH H double bonds HH HH HH HH HH break open CC CC CC CC CC HH HH HH HH HH polymer: HHHHHHHHHH poly(ethene) CCCCCCCCCC HHHHHHHHHH Figure 11.15 The polymerisation of ethene produces poly(ethene), whose structure is shown. Poly(ethene) was found to be a chemically resistant Chemists also experimented with other substituted material that was very tough and durable, and a very alkenes to produce plastics with particular properties in good electrical insulator. mind. Poly(chloroethene) (known by the trade name of polyvinyl chloride or PVC) and poly(tetrafluoroethene) Other alkene molecules can also produce addition (known by the trade name of polytetrafluoroethylene, polymers. Propene will polymerise to produce ‘Teflon’ or PTFE) are two such polymers: poly(propene): propene poly(propene) chloroethene → poly(chloroethene) HH HH (vinyl chloride) (PVC) n CC CC H CH 3 n H H HH H CH 3 nC C CC H Cl n This long-chain molecule is similar in structure to H Cl poly(ethene) but with a methyl (⎯CH3) group attached to every other carbon atom in the chain (Figure 11.16a). It is tetrafluoroethene → poly(tetrafluoroethene) commonly referred to by its trade name ‘polypropylene’. (PTFE) Study tip FF FF The diagram of the structure of poly(propene) is n CC CC quite easy to get wrong. It is important to realise that the ⎯CH3 group is a side-group here – it FF F Fn does not become part of the chain. The chain is formed by the carbon atoms that are joined by the Their structures are shown in Figures 11.16b and c. C=C bond in the monomer. Poly(chloroethene) (PVC) was found to be stronger and harder than poly(ethene) and therefore good for Chapter 11: Petrochemicals and polymers 285

Sa making pipes for plumbing. PTFE proved to have S b some unusual properties: it was very stable at high temperatures and formed a very slippery surface. The properties of some addition polymers are given in Table 11.2. Such synthetic polymers have proved to be very versatile. Many, for example poly(propene), are easy to shape by melting and moulding. Poly(propene) is therefore used to make sturdy plastic objects such as crates. However, it can also be drawn out into long fibres for making ropes. c Some of the properties of addition polymers: ◆ All polymers are long-chain molecules made by Figure 11.16 The structures of a poly(propene), PP, b poly(chloroethene), PVC, and c poly(tetrafluoroethene), PTFE. joining together a large number of monomer molecules. ◆ Addition polymerisation involves monomer molecules that contain a C=C double bond. ◆ Addition polymers are homopolymers, made from a single monomer. ◆ During addition, the double bonds open up and the molecules join to themselves to make a molecule with a very long chain. Polymer (and trade name(s)) Monomer Properties Examples of use tough, durable plastic bags, bowls, poly(ethene) ethene bottles, packaging (polyethylene, polythene, PE) CH2=CH2 tough, durable crates and boxes, plastic poly(propene) (polypropylene, PP) rope propene strong, hard (not as insulation, pipes and poly(chloroethene) CH3CH=CH2 flexible as polythene) guttering (polyvinyl chloride, PVC) non-stick surface, non-stick frying pans, poly(tetrafluoroethene) chloroethene withstands high non-stick taps and joints (polytetrafluoroethylene, Teflon, CH2=CHCl temperatures PTFE) light, poor conductor insulation, packaging tetrafluoroethene of heat (foam) CF2=CF2 poly(phenylethene) phenylethene (styrene) (polystyrene, PS) C6H5CH=CH2 Table 11.2 Examples of some widely used addition polymers. 286 Cambridge IGCSE Chemistry

Activity 11.4 11.4 Condensation polymerisation S Comparing different plastics Nylon Skills In the early 1930s, DuPont were conducting research into artificial fibres. Knowledge of silk and wool gave AO3.1 Demonstrate knowledge of how to safely use clues as to how protein molecules are built. Wallace techniques, apparatus and materials (including Carothers imitated the linkage in proteins and following a sequence of instructions where produced the first synthetic fibre, ‘nylon’. Nylon is a appropriate) solid when first formed, but it can then be melted and forced through small holes. The long filaments cool, and AO3.3 Make and record observations, measurements the fibres produced are stretched to align the polymer and estimates molecules and then dried. The fibres can be woven into fabric to make shirts, ties, sheets, etc., or turned into AO3.4 Interpret and evaluate experimental observations ropes or racquet strings. However, nylon is not just and data made into fibres. It has proved to be a very versatile material and can be moulded into strong plastic items ICT skills such as gearwheels. Different plastics have been devised for different Nylon is a copolymer of two different monomers, purposes. This activity explores both the a diamine and a dicarboxylic acid. Each monomer physical property of density and the chemical consists of a chain of carbon atoms (which are shown properties of acid/alkali and solvent resistance of in the following diagrams simplified as blocks). At a range of different materials. The results of the both ends of the monomers are functional groups. An investigation can be linked with the uses to which amine group (⎯NH2) on the first monomer reacts the plastics are put and to a PowerPoint or poster with a carboxylic acid group (⎯COOH) on the second presentation on the impact of plastics on the monomer to make a link between the two molecules. environment. Each time a link is made, a water molecule is lost: A worksheet, with a self-assessment checklist, is included on the CD-ROM. first monomer second monomer (a diamine) (a dicarboxylic acid) H HO O + N N+ C C + H Questions H H HO O N 11.5 Give the molecular and structural formulae of forms a water H ethene. molecule 11.6 State what is meant by addition polymerisation an amide polymer (a polyamide) and give an equation for the formation of (peptide) link poly(ethene) from ethene. OO OO 11.7 Draw the structure of the repeating unit in the following polymers: C CN NC CN a poly(propene) b poly(chloroethene) (PVC). H H H 11.8 What is the monomer used for making Teflon? + H2O + H2O + H2O 11.9 Give a use for the following polymers: As a result, this type of polymer is known as a a poly(propene) b poly(vinyl chloride) condensation polymer. Because an amide link c poly(tetrafluoroethene). (or peptide link) is formed during polymerisation, nylon is known as a polyamide. A version of nylon polymerisation can be carried out in the laboratory (Figure 11.17, overleaf). Chapter 11: Petrochemicals and polymers 287

S Study tip S Questions on the structure of nylon, and other Activity 11.5 condensation polymers, are difficult. You will not The nylon rope trick be asked the detailed structure of the monomers. You can represent the central structure of each Skills monomer as a block, as shown in the diagrams here. AO3.3 Make and record observations, measurements and estimates You should know the structure (and name) of the links between the monomers in the chain, This demonstration shows the production of nylon though – in this case the peptide link. at the interface between two reactant layers. A solution of decanedioyl dichloride in cyclohexane It is worth practising these diagrams to make is carefully floated on an aqueous solution of sure you are very familiar with them and can 1,6-diaminohexane. Nylon forms at the interface and recall them readily. can be pulled out as fast as it is produced, forming a long thread – the ‘nylon rope’ (see Figure 11.17). a rod is rotated A worksheet is included on the CD-ROM. Nylon fibre is Polyesters pulled out. Condensation polymerisation can also be used to make first monomer, other polymers with properties different from those of dissolved in an nylon. Polyesters are condensation copolymers made organic solvent from two monomers. One monomer has an alcohol Polymerisation group (⎯OH) at each end. The other monomer has a occurs at the carboxylic acid group ⎯COOH) at each end. When interface between the monomers react, an ester link is formed, with water the two liquids. being lost each time: second monomer, dissolved in water first monomer second monomer has alcohol functional has carboxylic acid functional b groups groups Figure 11.17 Nylon is a polyamide and can be made in the laboratory. O O 288 Cambridge IGCSE Chemistry C +O O+ C + H H HO OH forms a water molecule an ester link polymer (polyester) O O OO O CO CC O OC + H2O + H2O + H2O One such polyester has the trade name ‘Terylene’. Like nylon, Terylene can be turned into fibres and woven into clothing. Terylene clothing is generally softer than that made from nylon (Figure 11.18). The ester linkage that joins the monomer units in the man-made fibre Terylene can be broken down by acid or alkaline hydrolysis. So it is not good news if spots of alkali fall on your shirts or sweaters!

S moulded and dyed bright colours. The problem arises S because most plastics are not biodegradable – there are no natural microorganisms that can break them down. Some modern plastics are suitable for re-use. Soft-drinks bottles can be made from a plastic with the trade name ‘polyethylene terephthalate’ (PET) (Figure 11.19). These bottles are sturdy and have Figure 11.18 Shirts are often made out of a fabric made of Terylene and cotton. Comparing synthetic addition and condensation polymers Some immensely useful synthetic polymers have been made by the two types of polymerisation. Both methods take small molecules and make long repeating chains from them. However, there are differences between the two methods. These are summarised in Table 11.3. The re-use, recycling and disposal of plastic waste Figure 11.19 Soft-drinks bottles are often made from polyethylene Plastic rubbish is a common but unwelcome sight terephthalate, PET. around the world. Over the past 30 years, plastics have taken over as replacement materials in many applications. This is not surprising because they are light, cheap and corrosion-resistant, and they can be easily monomers used Addition polymerisatiom Condensation polymerisation reaction taking place usually many molecules of a single monomer nature of product monomer is unsaturated, usually contains a molecules of two monomers usually used C=C bond an addition reaction – monomers join monomers contain reactive functional together by opening the groups at ends of molecule C=C double bond only a single product – the polymer condensation reaction with loss of a small molecule (usually water) each time a non-biodegradable monomer joins the chain resistant to acids two products – the polymer plus water (or some other small molecule) biodegradable hydrolysed by acids Table 11.3 A comparison of the processes of making synthetic polymers. Chapter 11: Petrochemicals and polymers 289

S several advantages for this particular use. complex DNA that makes life itself possible to the more S In some countries, schemes for the re-use of these straightforward proteins and carbohydrates that keep bottles are operated. However, such a re-use policy is living things ‘running’. not suitable for most plastics. So what do we do with our waste plastic? We must either recycle it Proteins or dispose of it. Recycling is more economical and Proteins are what cells are made of. All the tissues satisfactory than the alternative, of depositing plastic and organs of our bodies are made up of protein. waste in landfill sites. But there are problems with In addition, enzymes, which are responsible for recycling because most plastic waste is a mixture of controlling the body’s chemical reactions, are different types. proteins. DNA makes life possible and allows living Identification numbers and symbols (Figure 11.20) things to reproduce, but without proteins there are in use to identify different plastics for recycling. would be no structure or chemistry to keep the Methods of sorting plastic waste by optical scanners, or living things going. manually, are being introduced. Once sorted, alternative treatments are available for recycling the different types Proteins are built from amino acid monomers. of plastic waste. There are 20 different amino acids used, and they each Incineration can be used to burn plastic waste, contain two functional groups ⎯NH2 and ⎯COOH. though care must be taken not to release toxic fumes Glycine and alanine are two of the simplest amino acids. into the air. Incineration of PVC, for instance, can When they react together, an amide linkage (or peptide release acidic fumes of hydrogen chloride. Pyrolysis – linkage) is formed to produce a dipeptide (two amino the burning of the plastic waste at around 600 °C in acids joined together): the absence of air – is an alternative to incineration. Most of the products from pyrolysis can be used as HHO HHO fuels or separated by fractional distillation. They NCC + NCC can then be made into monomers for making more plastics. Research is also being carried out to produce H H OH H CH 3 OH plastics that are biodegradable or photodegradable (can be broken down by the action of light). A glycine alanine poly(ethenol) plastic has been developed that is soluble in hot water. forms a water molecule Biological polymers All living organisms rely on polymers for their an amide existence. These polymers range from the very (peptide) link 1234 H HO HO PET HDPE V LDPE NCCNCC + H2O 567 HH H CH 3 OH PP PS Other a dipeptide Figure 11.20 Different identification symbols help with the sorting When this is repeated many times using the different of plastics for recycling. The symbols stand for the following plastics: 1 = amino acids, a polymer is formed. Short polymers polyethylene terephthalate (PET); 2 = high-density poly(ethene); 3 = vinyl (up to 15 amino acids) are known as peptides. Chains polymers such as PVC; 4 = low-density poly(ethene); 5 = poly(propene); with between 15 and 100 amino acids are known as 6 = poly(styrene); 7 = others, such as multi-layer plastics. polypeptides. Chains involving more than 100 amino acids are called proteins. Protein analysis Proteins can be hydrolysed (broken down) to amino acids by heating in concentrated hydrochloric acid. 290 Cambridge IGCSE Chemistry

S (6 mol/dm3). This is the reverse of the condensation Carbohydrates S process that formed them: The sugar we use to sweeten our tea or coffee is sucrose (C12H22O11). This is just one example of a H OH O carbohydrate; glucose (C6H12O6) is another. +N C+N C+ Carbohydrates are an important source of H OH H OH energy in our bodies, and in all living organisms. amino acids A carbohydrate is a compound containing carbon, hydrogen and oxygen only. The ratio of hydrogen to hydrolysis condensation oxygen is always 2 : 1 (as in water). (+ H2O) (– H2O) All long-chain carbohydrates (polysaccharides) are OO long-chain condensation polymers of sugar molecules (monosaccharides). Starch, for example, is a polysaccharide N CN C found in plants. Condensation polymerisation of sugar monomers produces such long-chain carbohydrates. HH many sugar monomers a protein (e.g. glucose molecules) The mixture of amino acids can then be separated by HO O HH O O HH O OH chromatography (see page 31). Amino acids are colourless substances, so a locating agent is used. The locating agent elimination of water reacts with the amino acids to produce coloured spots. between molecules Activity 11.6 a polysaccharide Chromatography of amino acids (e.g. starch) Skills OOO AO3.1 Demonstrate knowledge of how to safely use Starch and glycogen are two different polysaccharides techniques, apparatus and materials (including of glucose, a monosaccharide. They store the glucose in following a sequence of instructions where appropriate) an insoluble form in plants and animals, respectively. When energy is needed, cells break down the starch or AO3.2 Plan experiments and investigations glycogen back to glucose. The glucose is then oxidised AO3.3 Make and record observations, measurements by respiration. Cellulose is a third polymer of glucose. It forms the fibrous structure of plant cell walls. These and estimates three polymers differ in the way in which the glucose AO3.4 Interpret and evaluate experimental observations monomer units are linked together. and data Polysaccharides can be broken down in the laboratory AO3.5 Evaluate methods and suggest possible improvements by warming with hydrochloric acid (acid hydrolysis). The sugars present in the hydrolysis mixture can then be The artificial sweetener aspartame contains two analysed by chromatography (see page 31). A locating amino acids: aspartic acid and phenylalanine. In this agent must be used to detect the spots, because sugars are activity, aspartame is hydrolysed by heating with colourless. An interesting comparison is to analyse the hydrochloric acid and the hydrolysed product is then products of the acid and enzyme digestions of starch. Acid analysed by paper chromatography using amino acid hydrolysis breaks down starch to give glucose. However, standards to demonstrate their identity. The amino the enzyme amylase present in human saliva only breaks acids are located using a UV lamp or ninhydrin spray. starch down to give maltose. This is a molecule made of two glucose units joined together. The difference can be A worksheet, with a self-assessment checklist, is seen on chromatography of the products. included on the CD-ROM. Chapter 11: Petrochemicals and polymers 291