Tactics of Passing Chemistry

Learning chemistry made easier Sample question 13 State Graham`s law of diffusion. The Wrong Answers Rate of gas A Mass of gas B Time of gas B Rate of gas B Mass of gas A Time of gas A This is an expression of Graham`s law. The correct Answer The rate of diffusion of a gas in inversely proportional to the square root of its mass/density at constant temperature and pressure. Sample question 14 State Hess` law. The correct Answer The enthalpy change in converting reactants into product is the same regardless of the route or path used. Sample question 15 Where P1 - Initial pressure Express the general gas equation V1 - Initial volume Answer T1 - Initial temperature P1V1 P2V2 P2 - Final pressure T1 T2 V2 - Final volume T2 - Final temperature This is an expression, it is not a statement. Application of the laws is also very important. The examiners may ask a question which requires the application of the law. 39

Tactics of Chemistry Tactical practice C Learn to state the following laws and principles in secondary chemistry syllabus. Ensure they are all stated appropriately. State the following Theories, laws and principles 1. Kinetic theory of matter 2. Boyle`s law 3. Charles` law 4. Combined gas law 5. Graham`s law of diffusion 6. Gay Lussac`s law 7. Avogadro`s law 8. Hess` law 9. Collision theory 10. Le Chatelier`s principle 11. Faraday`s law of electrolysis 12. Radioactive decay law Answer the following questions which requires the application of the some of the laws. 1. 100cm3 of a sample of ethane gas diffuses through a porous pot in 100 seconds. What is the molecular mass of gas Q if 100cm3 of the gas diffuses through the same porous pot in 121 seconds under the same condition? ( C=12.0, H=1.0). (3 marks) 2. A certain mass of a gas occupies 146dm3 at 291K and 98.31kPa. What will be its temperature if its volume is reduced to 133dm3 at 101.325kPa. ( 2 marks) 3. When 15cm3 of gaseous hydrocarbon, P, was burnt in 100cm3 of oxygen, the resulting gaseous mixture occupied 70cm3 at room temperature and pressure. When the gaseous mixture was passed through potassium solution, its volume decreased to 25cm3. a. What volume of oxygen was used during the reaction? (1 mark) b. Determine the molecular formula of the hydrocarbon. (2 marks) 4. The pressure of nitrogen gas contained in a 1dm3 cylinder at -1960C was 107 Pascals. Calculate the: a. volume onfittrhoeggeansgaats2(5M0Colaanr dga1s0v5oPlausmcaelsof(1ga1/s2ims 2a4rkdsm)3, b. mass of N=14.0). (11/2 marks) 5. A fixed mass of gas occupies 105cm3 at -140C and 650mmHg. At what 40

Learning chemistry made easier temperature will it have a volume of 15cm3 if pressure is adjusted to 690mmHg? (3marks) 6. 60cm3 of oxygen gas diffused through a porous portion in 50 seconds. How long would it take 60cm3 of sulphur (IV) oxide gas to diffuse through the same portion under same conditions. (S=32.0, O=16.0 7. s2toe0cd0oicnfmfdu³ss.eoHfthnoriwtoruologgnhegnthw(eIi)lsloaixtmitdaeekpe(lNuth2gOeu)snpadmaesrestvthhoerluosmuagmeheoafcpsouonlrdpoihtuiusornp(slIu.Vg()Nion=x2i1dm4e,i(OnSuO=t2e1)s6g1a, 5Ss = 32). (3mks) 8. 60cm3 of oxygen diffused through a porous plate in 20 seconds . How long will it take 120cm3 of carbon (iv) oxide gas to diffuse through the same plate under the same conditions? (C=12 , O=16) (2mks) 9. When 300cm³ of chlorine gas was completely reacted with element C. 1,875g of the produce was formed. Determine the relative atomic mass of element C. (Cl = 35.5, M.G.V = 24000cm³). (3 marks) 10. When 100cm3 of gaseous hydrocarbon o(fCsxtHeyambuarrnesfoinrm3e0d0. c(m3 m3 oarfkosx)ygen, 200cm3 of carbon (IV) oxide and 200cm3 11. A factory produces calcium oxide from calcium carbonate as shown in the equation below:- CaCO3(s) CaO (s) + CO2 (g) What volume of Carbon (IV) Oxide would be produced from 1000kg of calcium carbonate at s.t.p (Ca = 40, C = 12, O = 16, Molar gas volume at s.t.p = 22.4dm3) ( 3 marks) 12. Gas K diffuses through a porous material at a rate of 12cm3s-1 where as S diffuses through the same material at a rate of 7.5cm3s-1. Given that the molar mass of K is calculate the molar mass of S 13. Explain the distance between solids, liquids and gases in terms of kinetic theory of matter. (3marks) 14. The volume of 200cm3 of a gas is at 3atm and 250C is reduced by 20atm. Find the new volume at 250C. (2 marks) 15. The temperature of 40dm3 of a gas is 300C. The gas is cooled to -100C at constant pressure. Find the new volume in cm3. (2 marks) 16. A fixed mass of nitrogen gas occupies 100cm3 at -200C and 250mmHg. At what temperature will it have a volume of 300cm3 at 650mmHg. (2 marks) 17. 475cm3 of a gas have a pressure of 800mmHg. Calculate the pressure that changes the volume to 150cm3 at constant temperature. 41

Tactics of Chemistry III Describe Description questions are very common in any exam, and most frequent in KCSE. Description is a technique which you must have to be able to answer questions requiring a deep elaboration of a process. During a description, the following method is necessary and you need to keep it at the back of your minds to be able to answer the question correctly . Description involves • Procedure(s), P • Observation(s), O • Deductions(s), D Sample question 16 Describe a test for hydrogen gas. The Wrong Answers It burns with a pop sound. This statement generalizes that whatever burns with a pop sound is hydrogen gas which is not true. We can briefly follow the method highlighted to describe the test for hydrogen.. The correct Answer Procedure - Insert a burning splint in a gas jar containing the gas Observation - the splint burns/extinguished with a pop sound Deduction - Hydrogen gas present. Sample question 17 Describe how you can obtain a pure sample of sodium chloride crystals from a mixture of sodium chloride and sand. The following answer is commonly given by students, take your time to understand the mistakes committed. 42

Learning chemistry made easier The Wrong Answers • P- Dissolve the mixture in water • O- Sodium chloride dissolves while sand does not. • P- Filter the mixture • O-Obtain residue as sand and the filtrate-sodium chloride solution • P - Evaporate the filtrate to dryness. • O- Obtain solid sodium chloride. This answer may look so correct but it is very wrong. There is a contradiction of statements. • The word dissolve in procedure I concludes that even sand dissolves of which it doesn’t. The word dissolve must be replaced with Add. • In the question crystals are to be obtained. The salt is neither evaporated nor heated to dryness but it is heated to saturation, then left to crystallize. Other common questions which requires a description are the salt preparation questions. Sample question 18 Starting with lead (II) oxide, describe how you can prepare a solid sample of lead sulphate. The Wrong Answers • React lead (II) oxide with dilute sulphuric acid to obtain lead sulphate. • Filter and dry the residue between filter paper. Most student use this neutralisation reaction without knowing that the reaction starts, slows down and finally stops due a coating of the insoluble lead (II) sulphate. The correct Answer React lead (II) oxide with dilute nitric acid to form lead (II) nitrate. Add aqueous sodium sulphate in to the aqueous lead (II) nitrate. A white precipitate of lead (II) sulphate is formed. Filter and wash the residue with distilled water and dry the residue between dry filter papers. 43

Tactics of Chemistry Sample question 19 aIfre4.s5ugbojefcatemdmtoonsaiam, eNHte3mapnedra4t.u5rge of hydrogen chloride, HCl gases and pressure. Which gas would occupy a larger volume at room temperature and pressure. (Molar gas volume at rtp=24dm3, H=1.0, N=14.0, Cl=35.5) Answer The volume of the gases Molar mass of NH3 = 14 + (1 x 3) = 17g = 0.2647 x 24.0dm3 Molar mass of HCl = 1 + 35.5 = 36.5g N H3 = 6.3528dm3 Moles of the gases N4.H5 3 HCl HCl = 0.1233 x 24dm3 4.5 = 2.9592dm3 36.5 17 0.1233 NH3 occupies a larger volume Moles 0.2647 In the above question you cannot judge the volumes unless you work out the volume first. Sample question 20 Describe how the formation of nitrogen (II) oxide in the internal combustion engine leads to gaseous pollution. Answer In the exhaust tube, nitrogen combines with atmospheric oxygen due to high temperature of the exhaust pipe to form Nitrogen (II) oxide which is then oxidized by atmospheric oxygen to form nitrogen (IV) oxide. Nitrogen (IV) oxide is poisonous. Sample question 21 Describe the behaviour of sodium metal on water. Answer Sodium floats, melts and dats on the surface of water, producing a hissing sound and a stream of fumes which burns with a blue flame. It violently reacts with water and produces hydrogen gas which burns with a blue flame. It has a lower density than water thus it floats, a low melting point and the hydrogen gas produced propels it producing a hissing sound. 44

Learning chemistry made easier Sample question 22 Describe what happens when sulphur (IV) oxide is bubbled through acidified potassium manganate (VII) The answer 5SO2(g) + 2MnO4-(aq) + 2H2O(l) 5SO42-(aq) + 2Mn2+(aq) + 4H+(aq) The purple colour of potassium manganate (VII) is decolourised. wSsMuuhallpnipchghhaauantrerae(tiIeocVno)(sloVo(uxISIir)Odlee4si2o-ds)n.iasssnod(lMvietnsiOsi4ni-)tsweoalxfteirdreidsfoeurscmesdiuntlgophsmuitlaepnhgiioatennseiosen(Ss(OI(I3S)2O-)io32nt-)os. The Tactic Proper description has a procedure, an observation and a deduction. Tactical Practise D 1. Starting with zinc sulphate solution, describe how a sample of zinc oxide can be obtained. (3 marks) 2. One of the ores of copper hinasthteheorfeorims ruelma,oCvueFdedS2u.ring extraction of copper a. Describe how iron metal (1 mark) b. State two environmental problems associated with extraction of copper metal (2 marks) 3. Describe an experimental procedure that can be used to extract oil from a nut seed. (2 marks) 4. Describe how a sample of sodium chloride can be prepared in the laboratory by direct synthesis. (2 marks) 5. By Using aqueous sodium chloride, describe how a student can distinguish calcium ions from lead ions. ( 2 marks) 6. A mixture contains solid iodine, sand and potassium chloride. Describe how each of the substances can be obtained from a mixture. ( 3 marks) 7. Hydrated cobalt (II) chloride exists as pink crystals and anhydrous cobalt (II) chloride is a blue powder. Describe a laboratory experiment that can be used to show that the action of heat on hydrated cobalt (II) chloride is a reversible 45

Tactics of Chemistry reaction. (3 marks) 8. A sample of fertilizer is suspected to be calcium ammonium nitrate. Describe chemical tests for each of the following ions in the sample. a. calcium ions; (2 marks) b. ammonium ions. (1 mark) 9. Describe how a solid sample of double salt, ammonium iron (II) sulphate can be prepared using the following reagents: Aqueous ammonia, sulphuric (VI) acid and iron metal. (3 marks) 10. a) You are provided with the following reagents; dilute nitric acid, dilute sulphuric acid, and lead (II) oxide. Describe how you would prepare a sample of lead (II) sulphate. (3 marks) b. Given a mixture of lead (II) chloride, iodine, ammonium chloride and sodium chloride crystals. Describe how you would separate all the four solids using methyl benzene, a source of heat and water. (Hint; Step one involves addition of methyl benzene) (3 marks) 11. Magnesium hydrogen carbonate is responsible for the temporary hardness of water. This type of hardness can be removed by addition of ammonia solution Describe how temporarily hard water is formed. (2 marks) 12. Describe how you would prepare a dry sample of crystals of potassium sulphate starting with 100cm3 of 1M sulphuric (VI) acid. (3 marks) 13. Water from a town in Kenya is suspected to contain chloride ions but not sulphate ions. Describe how the presence of chloride ions in the water can be shown. (2 marks) 14. When hydrogen gas burns in air it forms a colourless liquid. Describe a chemical test that can be used to identify the colourless liquid. ( 2 marks) 15. Describe a simple test that can be used to distinguish between carbon (II) oxide and carbon (IV) oxide. 46

Learning chemistry made easier IV Uses of substance When giving uses of a substance, give it`s direct use the first priority. These uses must be stated with clarity to avoid contradictions and ambiguity. Sample questions 23 Give one use of helium The Wrong Answers Helium is used in filling air balloons. The use given is not direct, which air balloons are you referring to? The correct Answer Helium is used in filling meteorological or weather balloons. Sample question 24 Give uses of Ethene gas. The Wrong Answers Ethene is used in the manufacture of plastics Which plastics are you referring to? Ensure you specify polyethene. The correct Answer Ethene is used in manufacture of polyethene/polythene plastics. When stating the uses, write them in a complete form. Sample question 25 Mention 3 uses of concentrated sulphuric (VI) acid. Answer. It is used as a: 1. Drying agent for gases due to its stronger affinity for water. 2. Oxidizing agents when hot for some metals and non metals. 3. To release more volatile acids from their salts. 47

Tactics of Chemistry The4T.o pASsecaredt ehydrating agent by removing elements of water in a compound. (Mark any 3 correct) In other questions you are required to give the use according to a particular process. Sample question 26 Study the nuclear equation below and answer the questions that follow. U235 + 10n 15421Ba + mnKr + 310n +Energy 92 a. Determine the values of n and m. Show your working. n is (235 +1)={141 + n +(3x1)}, n = 92 m is (92 +0)={52 + m +(3x0)}, m = 40 b. State two useful applications of this type of reaction. • The heat produced is harnessed to produce electricity • energy source is CO2 free so no harm to the environment Avoid stating more uses than required. It is advisable to state the exact and the correct points as required. The Tactic Check the penalties likely to happen for giving more points than required on specificity. Uses can also be in form of advantage or a disadvantage. In such questions give their direct advantages or disadvantages. Look at the question below. Sample question 27 In terms of functional group give an advantage of soapless detergent over soapy detergent in removing grease from fabrics when used with water (3 marks) Answer Soapy detergent has carboxylate group while soapless detergent has sulphonate group. They have polar end which dissolve in polar water 48

Learning chemistry made easier and a non polar end that dissolve in non polar grease thus removing it in water. The sulphonate formed with grease dissolves in water while the carboxylate formed with grease does not dissolve in water forming scum. A use can be given and an examiner wants an explanation of why it is used. Sample question 28 State two reasons why aluminium is preferred to copper in making overhead electric cables. - Aluminium is lighter than copper - Aluminium is a better conductor, it has 3 delocalised electrons while copper has 2. - Aluminium forms an oxide coating which prevents corrosion. Tactical Practise E 1. State one use of polystyrene ( 1 mark) 2. Complete the nuclear reaction below Q a.226 P222 88 86 + b. State two uses of radioisotopes in health (2 marks) 3. In the contact process, during the production of sulphur (VI) oxide, a catalyst is used. Give two reasons why vanadium (V) oxide is preferred to platinum. (2 marks) 4. Cotton is a natural polymer. State one advantage and one disadvantage of this polymer (2 marks) 5. State two uses of oxygen. ( 1mark) 6. State two uses of plovinylchloride. (1 mark) 7. Explain why graphite can be used as lubricant while diamond cannot. (2 marks). 8. Name one alloy of copper and state its use. ( 1 mark) 9. Other than production of sulphuric (VI) acid, state one commercial use of sulphur. ( 1 mark) 10. (a) what is the purpose of the glass beads in the manufacture of hydrochloric acid. (1 mark) 49

Tactics of Chemistry (b) Give two uses of hydrochloric acid. (1 mark) 11. State a disadvantage of each of the cleansing agents L1 and L2 below. L1 R – CH2 – CH3 L2 R – COONa+ OSO3Na+ L1 ___________________(1mark) L2 _____________________(1mark) 12. State one use of sulphuric (VI) acid. (1 mark) 13. State two uses of ethanol other than as an alcoholic drink. (2 marks) 14. What role does nitrogen play in air. (1 mark) 15. Give one industrial use of carbon (IV) oxide gas other than as fire extinguisher and as a refrigerant. ( 1 mark) 16. Why is anhydrous calcium chloride used to dry hydrogen chloride gas? (1 mark) 17. Give two industrial use of methane. ( 2 marks) 18. Other than making pipes, state one use of lead. (1 mark) 19. Aluminium is a good conductor of electricity. State two uses of aluminium based on that property. ( 2 marks) 20. What is the use of molten cryolite extraction (smelting) of aluminium. (1 mark) 50

Learning chemistry made easier V. Identify, Name and Give Identification requires a part in question to be recognized and the answer given as either a name or a chemical formulae if possible. Identification is very common in flow charts questions. Ensure to write the oxidation numbers where applicable, Sample question 29 Study the diagram below and answer the questions that follow. Ice cold water Dry hydrogen gas Liquid P Identify Liquid P. The correct Answer Liquid P is Water or H2O Any of the two answers is correct, provided it is identified correctly. Sample question 30 Name the reducing agent in the reaction below. Mg(s) + Fe2+(aq) Mg2+(aq) + Fe(s) The correct Answer and not The Wrong Answer Magnesium/Magnesium metal Mg When required to name, don`t write the formula, give the name of that compound, element or the ion or any other answer you may have been required to name. 51

Tactics of Chemistry Sample question 31 Name the product formed when charcoal burns in limited supply of air. The Wrong Answer The correct Answer CO gas Carbon (II) oxide gas Tactical Practise F 1. When dilute hydrochloric acid was reacted with solid B, colourless gas which extinguished a burning splint was produced. When a aqueous solution of solid B was tested with a blue litmus paper, the paper turned red/pink a. Identify the anion present in solid B (1 mark) b. Write an ionic equation for the reaction between solid B and dilute hydrochloric acid. (1 mark) 2. Use the part of the periodic table given below to answer the questions that follow. (Letters are not the actual symbols of the elements. QM NP R a. Identify the element that forms giant covalent structures. (1 mark) b. Identify one element that does not form compounds. (1 mark) c. Write the formula for the nitride of M. ( 1mark) 3. a. A student electrolysed dilute sodium chloride solution using inert carbon electrode. Name the products at: i. anode ii. cathode b. If the experiment was repeated using concentrated sodium chloride instead of dilute sodium chloride, write the half equation at the anode. ( 1 mark) 4. a. Name a suitable solvent for extracting an indicator from flowers; (1 mark) b. Give a reason why the solvent named in (a) above is used. (1 mark) 5. The set up below can be used to prepare oxygen gas. Study it and answer the questions that follow. 52

Learning chemistry made easier X Oxygen Gas jar Water Spoedriouxmide Bee hive shelf a. identify X (1 mark) b. What property of oxygen makes it possible for it to be collected as shown in the above set up. ( 1 mark) 6. The table below gives the solubilities of substance T and U at 100C and 400C. Substance Solubility g/100g water 100C 400C T 40 65 U 15 17 When an aqueous mixture containing 55g of T and 12g of U at 800C was cooled to 100C a. Identify the crystals formed. (1 marks) b. Determine the mass of the crystals formed. ( 1 marks) c. Name the method used to obtain the crystals. ( 1marks) 7. The chromatogram below was obtained from a contaminated Solvent front PQR S T a. Identify the contaminant in mixture P.( 1 mark) b. Which is the most soluble contaminant in P. ( 1 mark) 8. On heating a pale green solid K, carbon (IV) oxide gas and black solid M were formed. On reacting K with dilute hydrochloric acid, carbon (IV) oxide gas and green solution S were formed. When excess aqueous ammonia was added to solution S, a deep blue solution was formed. 53

Tactics of Chemistry a. identify the cation in solid K. (1 mark) b. Identify the two onions in solution S. (2 mark) 9. a. Name two ores from which copper is extracted from. (1 mark) b. During extraction of copper metal, the ore is subjected to froth flotation. Give a reason why this is necessary. ( 1 mark) 10. Hydrogen sulphide is a highly toxic and flammable gas. It is normally pre- pared in a fume chamber. a. Name two reagents that can be used to prepare hydrogen sulphide in the laboratory. (1 mark) b. One of the uses of hydrogen sulphide is to produce sulphur as shown in the following equation. 2H2S(g) + SO2(g) 3S(s) + 2H2O(l) Identify the reducing agent in this reaction and give a reason for your answer. (1 mark) 11. Consider the diagram below. a Name the regions labeled a, b, c. (3 marks) b a ……………………………….... b …………………………………. c c ................................................. 12. The structure below represents two cleansing agents, L1 and L2. L1 R – CH2 – CH3 OSO3Na+ L2 R – COONa+ Identify each of the two cleansing agents, L1 and L2. L1 _________________________(½ mark) L2 ______________________________(½ mark) 13. Magnesium was burnt in air forming a white residue T. When put in a boil- ing tube with water effervescence was noticed and a colourless gas D with a characteristic pungent smell was evolved. The gas turned a wet red litmus paper blue. Identify; Residue T and Gas D. (2 marks) 54

Learning chemistry made easier VI. Flow charts Flow charts will never miss in your chemistry exam. Many students leave such questions because either they appear confusing or students are unable to discover the point to start. Flow charts require a student to show a clear understanding of a concept or a relationship between a number of concepts. Flow charts demands the student to use the physical and chemical properties of elements and compounds as well as how these properties are linked with others. In short flow charts are words or processes in form of a diagram. For you to be able to answer questions correctly, you must identify the origin of the flow chart. Consider the question below Sample question 32 The figure below shows the stages in the manufacture of sodium carbonate in a Solvay process. Study the diagram below and use it to answer the questions that follow. Brine REACTION CHAMBER 1 Filtration Residue V CO2 Filtrate U HEATING REACTION REACTION LIMESTONE CHAMBER 2 H2O CHAMBER 3 REACTION Sodium CHAMBER 4 Carbonate Calcium chloride 55

Tactics of Chemistry To be able to answer this question correctly, discover the origin of the chart as indicated by the arrows or your concept grasping. Using a pencil, work by filling all you know of every step on the chart to understand every process before you even start answering the questions. Once you are through now answer the questions below. a. i. Name three starting materials in the manufacture of sodium carbonate. - Ammonia gas - Limestone. - Brine or Concentrated sodium chloride. - Coke (Any three materials) ii. Which substances are recycled in this process? - Carbon (IV) oxide. -Ammonia gas. iii. Identify the chambers in which the recycled substances are regenerated. - Chamber 3 - Chamber 4 iv. Name the substances U and V. U – Ammonia chloride V – Sodium hydrogen carbonate. b. Give an equation for the reaction which occurs: i. In the reaction chamber 1 NH3(g) + H2O(l) + CO2(g) + NaCl(aq) NH4Cl(aq) + NaHCO3(s) Or NH4HCO3(aq) NH3(g) + H2O(l) + CO2(g) NH4HCO3(aq) + NaCl(aq) NH4Cl(aq) + NaHCO3(s) 56

Learning chemistry made easier ii. When solid V is heated. 2NaHCO3(s) Na2CO3(s) + CO2(g) + H2O(l) iii. In the reaction chamber 3. Ca(OH)2(s) + 2NH4Cl(aq) CaCl2(aq) + 2NH3(g) + 2H2O(l) c. State one commercial use for Sodium carbonate. - Manufacture of glass. - Softening of hard water. - Manufacture of papers. - Manufacture of soap. - Refining of metals. Now use the same procedure to answer the question 33 which is on chemistry of metals. Flow charts of Metals As you study all the other metals it is important to deal with them in a flow chart form. The process of extraction is easier to understand in a flow chart format instead of statements. Sample question 33 The flow chart below is for the extraction of iron metal. Study it and answer the questions that follow. K Gas L Blast hot air Gas L + Slag CO Hot air V T Gas L Roasting W furnace 57

Tactics of Chemistry Questions. ores of iron. MTh oisstqoufets htieonquceo svteiorsncsh ienmthisitsr ayroefamaertealisn. 1. Identify two Fe2O3 - Haematite, flow chart form. - Magnetite, Fe3O4 We recommend you to study the six 2. Identify Gas LO.xi-de or CO mdiaetgarlasm atmyaotuirc alellvyeilnana dflpoowssci hbalyrtd.o them - Carbon (II) 3. Identify solid V, T and W , V– Coke T– Calcium Carbonate W– Calcium Oxide. 4. Write equation for the reaction taking place to produce solid K and W. W. CaCO3 (s) CaO(s) + CO2 (g) K. Fe2O3 (s) + 3CO(g) 2Fe(s) + 3CO2(g) 5. Give the use of limestone in the extraction of iron. To produce calcium oxide which reacts with silica to form calcium silicate (slag). 6. What name is given to solid K Pig iron Flow charts on salts Preparation of salts or the reactions involving salts are mainly represented in flow a chart form. It is important to understand the processes involving salts preparation for effective answering of the questions. These questions may be mixed with test of ions. It is therefore crucial to understand the effects of ammonia and sodium/potassium hydroxide on metal cations as well as how anions are tested. Salts or compounds colour can also assist be used to identify it. Sample question 34 Study the flow chart below and answer the questions that follows. 58

Learning chemistry made easier Solid Colourless X Solution Heat Solution K Brown gas A dilute Few drops of Excess Colour+less gas HNO3(aq) NH3(aq) NH3(aq) White Solid White precipitate 1. Name the brown gas B - Nitrogen (IV) oxide 2. Identify Solution K - Zinc -nOitxryagteenororZnO(2NO3)2 Colourless gas C 3. Write the reaction between the white solid and dilute HNO3. ZnO(s) + 2HNO3(aq) Zn(NO3)2(aq) + H2O(l) 4. Write the formula of the ions in the colourless solution The ions are [Zn(NH3)4]2+ Note NOT tetra ammine zinc (II) ion Flow Charts on Organic Chemistry. Organic chemistry is well tested using flow charts. Common questions combine organic chemistry from hydrocarbons to alkanols and alkanoic acids. Understand the following, 1. The preparation of; • Hydrocarbons; methane, ethane, ethene, ethyne, • Alcohols; ethanol • Alkanoic acid; ethanoic acid. 2. Properties of hydrocarbons, alkanols and alkanoic acids. 3. Chain reaction from one organic compound to another. For instance from ethanol ethene ethane, ethene ethanol ethanoic acid esters etc Sample question 35 Study the scheme below and answer the questions that follow. 59

Tactics of Chemistry Substance P n Solid C H Cl CC Water HH Gas G Step I Gas H CH3CH3 H2 CH2CH2 CH2CHCl Ni Complete combustion StepII Reagent W and catalyst B CH3CH2OH Step III Compound E Heat H+ MnO-4(aq) Substance A 1. Name substances • A - Ethanoic acid • B - Phosphoric acid • C - Calcium carbide • E - Ethylethanoate • H - Hydrogen chloride gas • P - Polyvinyl chloride/ polychloroethene • W - Water 2. Write down the equation that evolves gas G CaC2(s) + H2O(l) C2H2(g) + Ca(OH)2(aq) 3. The molecular mass of P is 67,500. Calculate the value of n 62.5n = 67,500 n = 67,500 62.5 = 1,080 molecules 60

Learning chemistry made easier 4. Explain the difference in the flames obtained in the combustion of CH3CH3 and gas G. • Gas G burns with a yellow sooty/smoky flame. It is unsaturated while CH3CH3 burns with a blue flame it is saturated. 5. Give two uses of P • Making pipes/clothes/handbags/suitcases/car seats/hose pipes/tiles/carpets Majority of flow chart questions come from the following topics. • Industrial chemistry • Organic chemistry • Chemistry of metals • Solvay process • Chemistry of gases Tactical Practise G 1. Study the flow chart below and answer the questions that follow Acidified Potasium Yellow Heat dichromate (VI) Green Substance Y O2 Z solution Water Solution M pH=5 Identify Z na M M............................................... Z......................................., 2. Study the flow diagram below and answer the questions that follow: Salt M Cold dilute HCl MnO2 Gas L NaOH NaCl(aq) a. Name gas L ………………………………………………………(1 mark) b. Write a balanced equation for the reaction between hydrochloric acid and manganese (IV) oxide. (1 mark) 61

Tactics of Chemistry c. Explain what happens to coloured petals when dropped into a solution of M. (2 marks) 3. Study the flow chart below and answer the questions that follow. Propanoic acid Step I Solid sodium NaOH(aq) propanoate Step II Ethane + sodium a. Name the process in SM tep I. (1 mark) carbonate b. Identify the reagent in step II. (1 mark) c. Give one use of ethane. (1 mark) 4. Study the flow chart below and answer the question that follow. C2H2 X + water HCl Y Process Z Polyvinylchloride Identify i. X ii. Y 5. Study the scheme below and answer the questions that follow. FeSO4(aq) Step I Cl2(g) Yellow Step II Brown solid Step III Solid Solution F Heat + Add NaOH (aq) and filter Water a. Write the formula of the cation present in the yellow solution F. (1 mark) b. What property of chlorine is shown in Step II? (1 mark) c. Write an equation for the reaction in step III. (1 mark) 62

Learning chemistry made easier 6. Study the flow chart below and answer the questions that follow Gas M Ammonia Copper (III) Oxide Copper Step VI Heat Water Step I Air Platinum-Rhodium Gas J Step II Air Nitrogen (IV) Water, air Nitric (V) Ammonia Ammonium Oxide acid Step IV nitrate i. Identify gas J, M and liquid. (2 marks) Step V ii. Using oxidation numbers, show that ammonia is Products the reducing agent in step VI. (1 mark) iii. Write the equation that occurs in step V. (1 mark) iv. Give the one use of ammonium nitrate. (1 mark) 7. Study the flow chart below and answer the questions that follow. Glucose Ethanoic acid S EthanFoelrmentatioCnHon2cSeOnt4rated Gas P Process Sodium R Hydroxide solution Sodium ethanoate Dry Gas T Ethane Sodium hydroxide i. State the conditions necessary for fermentation of glucose to take place. (1 mark) ii. State one reagent that can be used to carry out process S. (1mark) iii. Identify gases P.................. T.................. iv. How is sodium hydroxide kept dry during the reaction? (1 mark) 63

Tactics of Chemistry v. Give one commercial use of process R. ( 1 mark) 8. a. The scheme below shows some of the reactions of solution D. Study it and answer the questions that follow. Residue Ore N NaOH Q Step II Solution X Molten Electrolysis Molten Step III Al2O3 Aluminium Step IV Mixture a. Name one Ore of N. (1mark) b. Explain why the ore is first dissolved in excess sodium hydroxide solution. (1mark) c. Name the major compound present in the residue. (1mark) d. Give the formula of the aluminium compound present in solution X. (1mark) e. Explain how to obtain aluminium hydroxide from solution X. (1mark) f. i. Write an equation of the reaction that takes place in (e) above (1mark) ii. What is the role of cryolite (Na3AlF6) in the extraction of Aluminium: (1mark) iii. Explain why Na+ and F– ions are not discharged during electrolysis in step (IV). (1 mark) iv. Aluminium is a good conductor of electricity. State two uses of aluminium based on that property. (2 marks) 9. Do the flow chart for preparation of the following compounds. i. Ammonia ii. Nitric (V) acid iii. Sulphuric (VI) acid iv. Hydrochloric acid 10. Do the flow chart for extraction of the following metals. i. Sodium ii. Lead iii. Copper iv. Zinc v. Iron 64

Learning chemistry made easier The Top Secret VII. Compare. This is a question whose answer requires comparative terms or words to create a contrast between two items. The student must start with what the two have in common. In comparison questions, comparative words are used. They include • Higher • Smaller • Larger etc. The Tactic If no comparative term used, your answer attracts zero mark. Sample question 36 Sodium and Potassium have atomic numbers 11 and 19, Compare their atomic radius. The correct Answer The Wrong Answer Both K and Na are group I The atomic radius of potassium elements. Atomic radius for is large than that of sodium or potassium is larger than that of atomic radius increases down the sodium. Potassium has 4 energy group. levels compared to sodium that has 3. Note the underlined comparative term Sample question 37 Two elements K and L have atomic number 13 and 17 respectively. a. Write their electronic configuration. K - 2. 8. 3 or 2, 8, 3 avoid writing ratio as 2 :8: 3 L - 2. 8. 7 or 2, 8, 7 Avoid writing ratio as 2: 8: 7 b. Compare the atomic radii of K and L. Explain. Both elements K & L belong to the same period 3 of the periodic table. The atomic radius of element L is smaller than that of K. L has more protons at the nuclear which causes more effective force of attraction to the outermost electron , making the atom shrink. 65

Tactics of Chemistry Sample question 38. State properties of duralumin that make it more suitable than pure aluminum in aeroplanes construction. The correct Answer The Wrong Answers • It is more durable • It is durable • It is stronger • It is strong • It is harder • It is hard Giving answer without comparative terms does not distinguish which material is better to be used, between aluminium and duralumin. Remember they are both durable, strong and hard but duralumin is more durable, stronger and harder than aluminium. This creates a boundary between the two and this contrast is the examiners trick. It is also the student`s tactics to understand materials. The Tactic Comparative terms creates a contrast. This is the examiner`s trick. In other questions requiring parallel comparison either statements or tables may be used. Sample question 39 Compare chemical reaction and the nuclear reactions. (3 marks) Chemical reaction Nuclear reaction 1. Protons and neutrons react Electrons react outside the inside the nucleus. nucleus. 2. It is affected by physical It is not affected by physical factors such as temperature and factors such as temperature and pressure. pressure. 3. Releases relatively lower Releases enormous amount of energy between 10 -103kJ energy equal to 108kJ 4. The Same number of each kind Elements transmute into other of atom appear in the reactant elements. and products. 66

Learning chemistry made easier Statement may also be used. 1. Chemical reaction takes place at the outermost energy levels while nuclear reaction takes place at the nuclear of an atom. 2. Chemical reaction is affected by physical factors such as temperature and pressure while nuclear reaction is not. 3. Chemical reaction produces relatively lower energy between 10-103kJ while nuclear reaction produces enormous amount of energy equal to 108kJ. 4. The same number of each kind of atoms appear in the reactant and products while in nuclear reactions elements transmute into other elements. Though either of the method is accepted, a table is easier to use, it saves time and also gives examiner an easier time to mark Tactical Practise H 1. The table below shows the relative molecular masses and boiling point of pentane and ethanoic acid. Relative molecular Boiling point (0C) mass Pentane 72 36 Ethanoic acid 60 118 Explain the large difference in boiling between ethanoic acid and pentane. (2 marks) 2. The table below contains atoms that form common radicals. Complete the table to show radicals formed from various atoms. Element N S H NH4+ O 3. In terms of structure and bonding, explain the following observations: a. the melting point of aluminium is higher than sthualpthouf rs.o(d1i1u/m2 m:(a1r1k/s2m) arks) b. melting point of chlorine is lower than that of 4. The plot below were obtained when the atomic radii of some element in groups 67

Tactics of Chemistry I and II were plotted against atomic numbers. K Atomic radius (mm) Na Ca Li Mg Be Explain Atomic number a. the trend shown by Li, Na and K. ( 1 mark) b. Why the atomic radii of elements Be, Mg and Ca are lower than those of Li, Na and K. ( 2 Marks) 5. Study the data in the table below and answer the questions that follow. The letters do not represent actual symbols of the element. Element Protons Melting point Boiling point Ionic radius (0C) ()C) (nm) I 11 98 890 0.095 J 12 650 1110 0.065 K 13 660 2470 0.050 L 14 1410 2360 0.041 M 15 590 and 280 0.032 44.2 0.0212 N 16 113 and 119 445 0.184 P 17 -101 -35 0.181 Q 18 -109 -186 - i. Explain the chemical structure of the chloride of elements J and L. (1 mark) ii. Why do the elements represented by the letters M and N have two values of the melting point? ( 1 marks) iii. Explain the following observations in terms of structure and bonding. I. There is an increase in boiling point from I to K.(1 mark) II. Element L has a high boiling point (2 marks) III. The is a decrease in boiling point from N to Q. (1 mark) 68

Learning chemistry made easier iv. Write the formula and electron arrangement of each of the two ions of M whose ionic radii are shown in the table above. (2marks) b. The table below shows the atomic and ionic radii of some elements in a group of the periodic table. Element Atomic radii (nm) Ionic radii (nm) A 0.112 0.031 B 0.160 0.065 C 0.197 0.099 D 0.215 0.113 i. Explain why the ionic radius is less than the atomic radius for each of the elements. (1 mark) ii. Explain the variation in atomic radius for elements A and D. (1 ½ Marks) iii. Explain the increase in reactivity down the group. (1 ½ Marks) 6. State two similarities between rusting and combustion. (a) ……………………………………………………………..….. (1 mark) (b) ………………………………………………………………….. (1 mark) 7. The nitrates of the following metals were heated strongly and observation made accordingly. The nitrate of metal P produced the metallic oxide, Nitrogen (IV) oxide and oxygen gas; and that of metal Q produced the metallic nitrite and oxygen gas. The nitrate of R produced metal R, nitrogen (IV) oxide and oxygen gas. Arrange the metals in order of reactivity beginning with the most reactive. (2 marks) 8. The set up below was used to show that metal P is more reactive than Q. V Metal Q Metal P Fresohf courtansegection of an orange a. Show the direction of flow of electrons on the diagram. (1mark) b. Explain your answer in (a) above. (1mark) 69

Tactics of Chemistry VIII. Definitions This is where you are required to give a meaning of a term or a statement. Your definition must be succinct. ie should be to the point. For instance The Tactic In definitions avoid re using the term or statement to be defined. Sample question 40. Define a hydrocarbon The Wrong Answers The correct Answer This is a compound consisting This is a compound consisting of hydrogen and carbon. of carbon and hydrogen only. Hydrocarbons have only two elements. If you don`t put the word only, you don`t exclude hydrocarbons from other compounds with other elements apart from carbon & hydrogen. Good examples are: Glucose-C6H12O6, Alkanols -CnH2n + 1OH, Alkanoic acids - CnH2n-1COOH, Amino acids NH2CH2COOH... They have carbon, Hydrogen and Oxygen. Sample question 41 Define molar enthalpy of neutralization. The Wrong Answers This is the heat energy evolved when an acid is neutralised by a base. Giving such an answer shows that the student never understood the concept fully. A question arises what is the measure for enthalpy change. 70

Learning chemistry made easier In all neutralisation reaction the end products is water and a salt. Note the measure of enthalpy of such reactions in based on the formation of one mole of water. The correct Answer This is the heat energy change that occur when an acid reacts with the base to produce one mole of water. Or This is the heat energy change that occur when one mole of an acid is neutralised by a base. Study and understand the other definitions of different enthalpy to avoid confusion. Definition can also be used to distinguish terms which are closely related to each other. Sample question 42 Distinguish between a deliquescent and a hygroscopic substance. In such a question it is important to use connectors of contrast when defining the two terms. Answer. A deliquescent substance is a substance which absorbs the water from the surrounding and dissolve in it to form a solution, while a hygroscopic substance is a substance which absorbs the water from the surrounding but do no dissolve into it. In other questions an examiner may give you a definition then for you to give a term for that statement. Sample question 43 What is the name given to each of the following. 1. Ability of a metal to be made in to a wire Answer. Malleable 71

Tactics of Chemistry 2. Minimum energy required for a chemical reaction to start Answer. Activation energy 3. Type of force that holds atoms of neon together. Answer. Van der waals force Tactical Practise I 1. Diamond and graphite are allotropes of carbon. What is meant by an allotrope. ( 1 mark) 2. What is meant by the terms: (2 marks) (i) element. (ii) atomic number? 3. Distinguish between a strong acid and a concentrated acid. (1 mark) 4. What is meant by term radical? (1 mark) 5. Define half life of radioisotopes. (1mark) 6. What is a saturated solution. (1mark) 7. Define the term “Solubility of salt” (1mark) 8. Differentiate between the bleaching action of chloride and sulphur (IV) oxide gas. (2 marks) 9. Define the term acid and base. (1mark) 10. Distinguish between the following terms • Enthalpy and enthalpy change. (2 marks) • Heat of solution and molar heat of solution. (2 marks) • Heat of displacement and molar heat of displacement. (2 marks) • Heat of neutralisation or molar heat of neutralisation. (2 marks) • Lattice energy and hydration energy. (2 marks) 11. Differentiate between electrolyte and non-electrolyte. (2 marks) 12. What is electrolysis. (2 marks) 13. Define the following terms: • Crystallization (1mark) • Salting out as used in soap making (1mark) 14. What is meant by isomerism (1mark) 15. Distinguish between the isotopes and isomers. (2 marks) 16. Name the type of polymerization undergone by ethene. 17. What is meant by the term, esterification? (1mark) 18. What is a nuclide? (1mark) 19. Distinguish between a covalent bond and a co-ordinate bond. (2 marks) 20. Define the term melting point of a substance (1mark) 21. What name is given to different crystalline forms of the same element? (1mark) 22. Give Bronsted and Lowry definition of an acid. ( 1 mark) 72

Learning chemistry made easier IX. Specificity & Tricky terms. Some questions requires you to be very specific. Such specificity is required to indicate the exact limits, measurements, controls or the exact representation of an item which would otherwise require an explanation. Examples of questions which require specificity includes; Sample Question 44 Give the optimum conditions for Haber process. The Wrong Answers • High temperature • High pressure This is unacceptable it is not specific. How much is high temperature & pressure? In fact in science don`t use high or low, give the temperature then the person can judge whether the temperature is high or low The correct Answer • Optimum temperature of 4500C - 5000C • Optimum pressure of 200 atmospheres Sample question 45 Study the reaction beRloewageanntdAanswer the question that follow. C2H5OH(l ) C2H4(g) + H2O(l) Name reagent A. The correct Answer Concentrated sulphuric (VI) acid The Wrong Answer Sulphuric (VI) acid You need to understand that sulphuric (VI) acid is only hygroscopic when in concentrated form. It can only be able to remove molecules of water or water elements from a compound when in concentrated form. It is recommended to write a full word; concentrated and not conc. 73

Tactics of Chemistry When dealing with structure and bonding for instance it is always important to specify between a cross and a dot the electron they each represent. Sample question 46 Using cross (x) and dots (•) show the bonding in the formation of ammonium ion. HH + Appropriate answer. N Let H x represent nitrogen electron H • represent hydrogen electrons. This shows your understanding of the topic and avoids any question about what represents what? This way an examiner can be able to track the electron sharing or transfer. Sample question 47 An element P and K has atomic number 11 and and 17. Using cross (x) and dots(.), Show the bonding between the two elements. Let + - x represent P electron • represent K electrons. P K Other mistakes students commit is the mixing up of the particles responsible of passage of currents in substances. Note that substances which conduct electricity in solid or liquid form without being decomposed are called conductors. The particles responsible for this conduction are free/delocalised electrons. 74

Learning chemistry made easier Example of such materials are copper, graphite, mercury etc Substances which conduct electricity in liquid form or in aqueous state and are decomposed by electricity are known as electrolytes. The particles responsible for conduction are free/mobile ions. Example of such substances are molten lead bromide, solution of sodium chloride, copper sulphate solution, dilute sulphuric (VI) acid... Sample question 48 Mention particles that conduct electric current in the following substances. The correct answer a. Molten Calcium Chloride Mobile/free ions b. Graphite The correct answer Mobile/free/delocalised electron. Free ions in aqueous solution is due to dissociation of ions in water. The Tactic Note that mobile/free ions are found in electrolytes while mobile/free/delocalised electrons are found in conductors. Other specificity you need to ensure is the common mistake done by generalization of addition polymerization. Sample question 49 Propene undergoes self addition reaction to form a polymer. Draw the structure of the polymer. In such a question students confuse the polypropene and Polyethene HH HH CH3 H CH3 H C=C-C-H C=C-C-H C - C- C- C H HH H HH H H H Hn Propene Propene Polypropene 75

Tactics of Chemistry The mistake students do is failure to understand these structure formation but a lot of time students represent the polypropene and other polymers as polyethene after opening up the double bond. Avoid such mistakes they will cost you marks. Don`t generalize all polymers to polyethene/polythene. HH + HH HHHH C=C C=C C - C- C- C HH HH H H H Hn Ethene Ethene Polyethene Other area you have to be specific is on discipline of giving the exact number of points as requested. Don’t give more that expected. Sample question 50 Give two uses of zinc metal - 2 marks. • Galvanizes iron to prevent it from rusting. • Mixed with copper to make an alloy; brass. • In making aircraft. The Tactic For the first two points you score 2 marks. The 3rd point is wrong. This reduces the 2 marks to 1. You better give what you are requested. Sometimes the examiner may draw a line after marking the required points and ignores the rest of the points. If you give the first two points wrong while the following points are correct. It is might be ignored. Giving more points than the ones required is a waste of time and energy which may end up failing you. Avoid it. Where you are supposed to give an exact value, don`t use the word about. For instance temperature of about 200C is different from temperature of 200C. Sample question 51 Give two uses of oxygen gas (2 marks) 76

Learning chemistry made easier Answer I 1. used in filling meteorological balloons 2. used in manufacture of hydrochloric acid 3. used in hospitals for patients with respiratory problems 4. used by mountain climbers and deep sea divers. The examiner has two options depending with the marking panel proposal. 1. Mark the first two and ignore the rest whether right or wrong. In this case the student gets zero; the first two points are wrong. 2. Mark all the points but get the difference between the correct and the wrong points. In the above case, the student gets zero. If the student had given point 3 and 4 as the first two uses, both are correct but only one marks is scored. The two points can be termed as a single use because they are both respiratory uses. Note that uses such as used in hospitals, germination, mountain climbers and deep sea divers is one use, ie respiration. Suppose another student had done as below Answer II 1. used by mountain climbers and deep sea divers. 2. used in welding, it combines with acetylene to produce a very hot flame (oxy-acetylene). 3. used in manufacture of hydrochloric acid. 4. used in filling metrological balloons. In this case the student scores -2 Correct points - 2 marks -Any wrong points - deduct one mark This student gets Zero. How I wish the student did not add the other two points. The Tactic The examiner does this to remain neutral. Remember there are so many students sitting for that exam and measures must be put in place to avoid favoritism. Follow instructions carefully. 77

Tactics of Chemistry Tactical Practise J 1. Explain how conduction of electricity takes place in the following a. Iron metal. (1 mark) b. Molten lead (II) iodide. (1 mark) 2. Given that the atomic number of Y is 13 and that of Z is 9; a. write the electronic arrangement of Y; (1 mark) b. Draw the dot(.) And cross (x) diagram for the compound formed by Y and Z. (1 mark) 3. A student investigated the effects of an electric current by passing it through some substances. The student used inert electrodes, and connected a bulb to the circuit. The table below shows the substances used and their states. Experiment Substance State 1 Potassium Carbonate Solid 2 Copper (II) sulphate Solution 3 Sugar Solution 4 Lead (II) iodide Molten a. In which experiments did the bulb not light? (1 mark) b. Explain your answer in (a) above. ( 2 marks) 4. Some animal and vegetable oils are used to make margarine and soap. Give the reagents and conditions necessary for converting the oils into: a. margarine (2 marks) b. soap (1 marks) 5. a. Using electrons in the outermost energy level, draw the dot( ) and ( ) diagrams for the molecules H2O and C2H4. (H=1, C=6, O=8) (2 marks) b. The formula of baectowmepenlexziinocnainsdZnam(NmHo3)n4i2a+. Name the type of bond that is likely to exist in the complex ion. ( 1 mark) 6. Using dots and crosses to represent electrons, draw the structures of the following: a. HPhyodsrpohxoonroiuums ciholno,r(iHde3O, (+P) Cl3) b. 7. Explain how electrical conductivity can be used to distinguish between magnesium oxide and silicon (IV) oxide 8. Name the type of bonding and structure found in: - a. Ice b. Magnesium chloride 9. Show bonding in Carbon (II) Oxide by use of ( ) or ( ) to represent bonds. 78

Learning chemistry made easier 3. DIAGRAMS The Tactic All the diagrams must be drawn using a pencil. Diagrams test students creativity and originality. When dealing with diagrams, the following are the marking areas. • Labelling and correct identification of substance & apparatus. • Methods of gas collection for gas preparation questions, • Workability - this is the ability of the set up to achieve its objective. Sample question 52 The set up below is used to prepare dry hydrogen chloride gas in the laboratory. Complete the diagram. When completing this diagram ensure • Labelling is done properly • Proper drying agent is used • Correct gas collection method • The prepared gas is finally obtained. Consider a student known as kutosoma kutojua from Kaignorant high school who completes his diagram as shown below 79

Tactics of Chemistry The student`s diagram has the following mistakes. • Labelling - No labeling is done at all. • Workability - The delivery tube and the cork have opening. This will make gas escape. The gas jar is sealed consequently no gas displacement can take place. See the correct diagram Concentrated sulphuric (VI) acid. Card board Dry HCl(g) Sodium H2SO4(l) chloride Other checking points or areas in a diagram are • The level of the apparatus. It is believed that the apparatus are on one level on a table. An ordinary lab has flat benches. • You may put the retort stand holding the round bottomed flask to be on the safer side. Some flasks may not stand on their own. In electrochemistry, ensure your connections of anodes and cathodes have no gaps. The cell arrangement should be observed very well. Note the longer one is a rearrangement of the positive sign (+) as and then joined and the shorter one is negative, thus 80

Learning chemistry made easier Other common mistakes committed by students on creativity and originality on diagrams are: Errors/ mistakes in gas collection Wrong diagram Correct diagram The student chose to use a trough with Trough is done well with the delivery a hole at the side. This does not exist. tube well placed. See also no water. Water has been included too Balloon Wrong diagram Correct diagram Can the gas be collected by a balloon? Balloon is not a laboratory apparatus. No it cannot. Use appropriate laboratory apparatus . Wrong diagram Correct diagram The syringe used has a bigger barrel with a plunger piston which is not The syringe used has a plunger fitting. Gas will just escape. piston fitting the barrel. The gas will not escape. 81

Tactics of Chemistry Placing the corks Wrong diagram Correct diagram How were these corks placed and how Fitting of the corks is always from will they be removed? outside but not inside the container. Other area which you must be very keen, is when the dry gas has to be collected. The gas is passed through a drying agent which does not react with the gas, and then NOT passed through water but to a dry gas jar for collection. Check the diagram below What is the importance of drying the gas, then pass it over water to be wet again? Water Drying agent Method of gas collection depends on the density or the molar mass of the gas compared to air (Oxygen). If the molar mass of a gas is more than that of oxygen, O2 (32), it is 82

Learning chemistry made easier collected using upward displacement (downward delivery). Example hydrogen chloride gas; HCl, carbon (IV) oxide; CO2, Sulphur (IV) oxide;SO2, Nitrogen (IV) oxide; NO2, Nitrogen (I) oxide; N2O, Sulphur (IV) oxide;SO3, Hydrogen sulphide; H2S and chlorine;Cl2, Drying agent Upward displacement Gases which have lower density/molar mass than air (oxygen);32, are collected using downward displacement (upward delivery). They include hydrogen; H2, nitrogen; N2, ammonia;NH3, carbon (II) oxide; CO, nitrogen (II) oxide; NO, methane; CH4, ethane; C2H6, ethene; C2H4 and ethyne; C2H2. Downward displacement Drying agent 83

Tactics of Chemistry Another diagram that students make a lot of mistakes is the drawing or completing an electrochemical cell. Look at the diagram below as we also look at the common mistakes in electrochemistry. Sample question 53 Use the standard electrode potential below to answer the questions that follows. The letters do not represent the actual symbols of the elements. S2+ + 2e S(s) E0 = -0.76V (aq) Z2+ + 2e Z(s) E0 = + 0.34V (aq) Draw an electrochemical cell between S and Z and show the direction of flow of electrons. Flow of electrons v Z Z2+ S Salt bridgeKNO3(aq) (aq) S2+ (aq) Calculate the emf of the cell. Eoverall = Ered - Eoxid, = +0.34 -(-0.76), = 1.10v Electrochemistry questions are also very common in chemistry. Understanding the following areas will make the topic easier for you. • Oxidation numbers and how to track their change in a chemical equation. This will enable you to judge the change taking place in reaction or the change taking place in the ion species during a reaction. 84

Learning chemistry made easier Note. When an oxidation number increases, the electrons has been lost and this is called Oxidation. In an electrochemical cell oxidation takes place at anode. We can therefore summarize that at Anode; Loss of Electron; Oxidation takes place which can be abbreviated as ALEO or OIL ( Oxidation is Loss of electrons) When an oxidation number decreases, the electrons has been gained and this is called reduction. In an electrochemical cell reduction takes place at cathode. We can therefore summarize that Gain of Electron; Reduction at Cathode which can be abbreviated as GERC or RIG Reduction is Gain of electrons. Just for fun remember ALEO lion says GERC or simply OILRIG. Using the equations below we can classify the LEO and the GER i. K+ + e- K(s) E0 = - 2.92V (aq) ii. K(s) K+ + e- E0 = + 2.92V (aq) The oxidation number of potassium in (i) decreases from +1 to 0. Potassium has gained electrons hence reduction. Potassium in the equation (ii) on the other hand has lost electrons and the oxidation number has increase form 0 to +1 hence it has undergone oxidation. Notice the change of sign. Make a point to understand the laws for assigning oxidation numbers. • Understand what standard electrode potential (E0) is and its use. Electrode potential can either be negative or positive depending with the tendency to gain or lose electrons. Considering the two equations below i. Li+(aq) + e- Li(s) E0 = - 3.04V ii. F2(g) + 2e- 2F-(aq) E0 = +2.87V Lithium has a negative E0. This means that lithium has a higher tendency to lose electrons when connected to hydrogen half-cell. 85

Tactics of Chemistry Fluorine has a positive E0. This means that it has a higher tendency to gain electrons when measured with hydrogen. When drawing an electrochemical cell two cells are combined. One with a higher tendency to gain electrons and another one with a higher tendency to lose electrons. A half cell is an arrangement of a metal rod (or electrode) dipped into a solution of its own ions. It is usually written in short form as metal metal ion. The vertical line represents a phase boundary where the potential difference develop. To complete an electrochemical cell a notation is used of the two half cells joined together with a salt bridge as two vertical lines. The half-cell with a higher tendency to lose electrons (Oxidation) is always written on the left-hand side, while the half-cell with a higher tendency to gain electrons (Reduction) is always written on the right hand side. For instance a cell between lithium and copper, the half equations are Li+(aq) + e- Li(s) E0 = - 3.04V Cu2+(aq) + 2e- Cu(s) E0 = + 0.34V Lithium is more reactive than copper. Lithium has a higher tendency to lose electrons than copper. Therefore lithium will be oxidised. Its half cell is Li(s) Li+(aq) and that of copper is Cu2+(aq) Cu(s) the cell diagram should therefore be written as Li(s) Li+(aq) Cu2+(aq) Cu(s) E0 = +3.38V cell Oxidation Reduction Using the E0 for oxidation and reduction half cells, the formula, E0 = E0 - E0oxi, can be used to calculate the electromotive force of the cell. red Other common diagrams are electrolysis diagrams involving either binary electrolyte or aqueous electrolytes. Let us go through the question below. 86

Learning chemistry made easier Sample question 54 a. A student used the set up below to electrolyse copper (II) sulphate solution. Study it and answer the questions that follow. Gas V B A Platinum electrodes A Cells Before you answer this question, you need to understand the flow of current. The flow of current can be determined in two ways. • 1st- using the cells terminals • 2nd -using the products at the electrodes. We cannot use the first method because the cell terminals are not known. For the second method we must understand the preferential ions which are discharged. The solution has four ions; cations Cu2+ & H+ aarneddaisncihoanrOgeHd-.&InStOh4i2s-. The least reactive or most concentrated ions case copper ions and hydroxide ions are discharged. OH- migrate to electrode B where they loses electrons to form water and oxygen gas. Electrode B then has ability to attract anions hence become Anode. Copper ions are deposited to electrode A. Electrode A has ability to attract cations, it is therefore negatively charged hence become etrhelemecctaraionthdsoeidsnewt.hDeerueseotltouotbrioeenmreaopnvldaalccoeofdmCwbui2int+,hethtcooepfsoporelmur.tiTHoh2nSeOfaf4ad.deIfisn.tghHe+wpaonluadltdiSnOnuo4m2t- take place but anode will dissolve (‘eaten away’) and be deposited at the cathode. This is the tactics to understand the diagrams. With this understanding let us go through the questions now. 87

Tactics of Chemistry i. Identify the electrodes (1 mark) A……… Cathode (1 mark) B………Anode ii. Name gas V. Oxygen gas iii. Write the ionic equation of the reaction taking place at electrode B. (1 mark) 4OH-(aq) 2H2O + O2(g) + 4e- iv. What happens to the PH of the electrolyte during the electrolysis. Explain your answer. (1 mark) The pH decreases. Concentration of hydrogen ions; H+ increases lowering the PH of the solution. 2H+(aq) + SO42-(aq) H2SO4(aq) v. If the above set up had copper electrodes instead of platinum electrodes write the equation at the anode. Half equation at the anode. (1 mark) Cu(s) Cu2+(aq) + 2e- vi. State and explain two observations made when copper electrodes were used. (2 marks) • The anode is decreases in mass/dissolves. • Copper is deposited at the cathode • Cathode increases in mass as copper is deposited The copper ions discharged from the solution are deposited at the cathode and the discharged ions from the solution are compensated by the anode. b. i. State Faraday’s 1st law of electrolysis. (1 mark) It states that the mass of a substance produced during electrolysis is directly proportional to the quantity of electricity passed. 88