Tactics of Passing Chemistry

Learning chemistry made easier 2. QUALITATIVE ANALYSIS Qualitative analysis also known as test for ions is the process of identifying chemical components in a given substance after carrying out a series of chemical tests. The practicality of qualitative analysis is highly based on your capability to make the right observation and conclusion. Qualitative analysis usually involves identification of the following in a compound • Cations • Anions • Functional group in an organic compound The observations made are based on • Gases emitted during chemical reactions or on heating. • Precipitates formed • Molecules of water of crystallization in a substance. • Effervescence formed • Colour changes during heating. A series of tests are performed before a confirmatory one is arrived at. This confirmatory test singles out the particular ion or functional group present from a list of possible ones in the previous tests. Before you deal with qualitative analysis, you need to have a theoretical knowledge on; • What happens when salts are heated • The behaviour of cations with few drops and excess NaOH solution and aqueous ammonia. • The common confirmatory tests for anions and cations • Uses of specific compounds in a practical eg bromine water, acidified potassium manganate (VII) etc Let us go through the tactics which can also be performed in your school laboratory. 139

Tactics of Chemistry i. Analyzing flames Saturation. Flames can be used to test the saturation of compounds. Unsaturated compounds burn in luminous flame while saturated compounds burn with non-luminous flame. Luminous flame Non -luminous flame Flames for cations Flame tests are used to identify a limited number of metals because not all metal ions give flame colours. The easiest ones are group I elements. Other metals have other easier methods to test them. Procedure. Clean a platinum or nichrome wire by dipping it in to concentrated hydrochloric acid, and then holding it in to a hot Bunsen flame. Repeat this until the wire does not produce any colour in the flame. Moisten the wire again by dipping it in the acid, then dip it in to a small amount of solid you are testing so that some sticks to the wire. Place the wire back in the flame again and observe the colours. Lithium Potassium Calcium Magnesium Strontium Sodium red violet Orange Lilac Purple Gold yellow 140

Learning chemistry made easier Barium Rubidium Caesium Copper Lead Iron green Red/violet blue/violet Blue green Grey white Orange Other common flames for gases Zinc Hydrogen Colourless or Blue sometimes bluish green Action of heat on salts When you are heating salts. The following should be noted: • Production and the colour of any gas evolved. • Production of water of crystallization as a colourless liquid at the cooler parts of the test tube or boiling tube. • The colour of the solid remaining when hot and cold. 141

Tactics of Chemistry TEST FOR CATIONS AND ANIONS The following chemicals can be provided for this section. CATIONS 10. Sodium hydrogen carbonate 1. 0.5g ammonium salt 11. Iron (II) sulphate solution 2. 0.5g copper salt 12. Barium chloride solution 3. 0.5g zinc salt 4. 0.5g calcium salt 13. Barium nitrate solution 5. 0.5g lead salt. (Soluble) 14. Copper turning 6. 0.5g iron (II) salt 15. Lead nitrate solution 7. 0.5g iron (III) salt 8. 0.5g aluminium salt. 16. Magnesium sulphate 17. Silver nitrate 18. Bromine water 19. Universal indicator ANIONS 20. Litmus papers 1. 0.5g sulphate salt 21. Unsaturated hydrocarbon (any) 2. 0.5g sulphite salt 22. Sodium carbonate 3. 0.5g chloride salt 4. 0.5g bromide salt 23. Solid Z: 0.5g zinc chloride. 5. 0.5g iodide salt SSSZooonlllSiiidddO4BPXi;:n:01t1.hg5gegmrFmaiexitSxtioOutur4oe.rf5e1oHf:o12fOPb.CCuOO3 6. 0.5g nitrate salt 24. & 7. 0.5g carbonate salt 25. & 8. 0.5g hydrogen carbonate 26. 27. Z2nMSaOm4 imn othneiurmatisoolouft1io:1n OTHER REAGENTS 1. 2M NaOH solution 28. Hydrogen peroxide. 2. Aluminium foil 3. 2M hydrochloric acid OTHER APPARATUS 1. Glass rod/stirring rod 4. 1M nitric acid 2. Test tubes & test tube rack 5. Concentrated H2SO4 3. Spatula 6. Potassium Iodide solution 4. Source of heat 7. Acidified K2Cr2O7 solution 5. Droppers 6. Filter paper (Whatman No. 1 size 8. Acidified KK2MCnrOO44 solution. 9. Acidified 11.0cm) NOTES Some of these solutions can be prepared as follow 1. Preparation of 2M NaOH solution. Dissolve 80.0g of sodium hydroxide in 500cm3 distilled water and add the water to make up one litre of solution. 2. Bromine water is prepared by adding 1cm3 of liquid bromine in 100cm3 of distilled water and shaking well in a fume cupboard. Label this as bromine water. 142

Learning chemistry made easier 3. 2.0M hydrochloric acid is prepared by adding 172.0cm3(1.28g/cm3) of concentrated hydrochloric acid to about 500cm3, of distilled water an diluting to one litre of solution 4. 1.0M nitric (V) acid is prepared by adding 45.0cm3 (1.42g/cm3) of concentrated hydrochloric acid to about 500cm3, of distilled water an diluting to one litre of solution. 5. Acidified potassium manganate (VII) is made by adding 3.16g of solid potassium manganate (VII) to 400cm3 of 2M sulphuric acid and diluting to one litre of solution using distilled water. 6. Acidified potassium dichromate (VI) is made by dissolving 25g of solid potassium dichromate (VI) in about 600cm3 of 2M H2SO4 and diluting to one litre of solution. 7. Acidified potassium chromate (VI) is made by dissolving 12g of solid potassium chromate (VI) in about 600cm3 of 2M H2SO4 and diluting to one litre of solution. 8. Barium nitrate solution is prepared by dissolving 26.0g of solid barium nitrate in about 800cm3 of distilled water and diluting to one litre of solution. Label this as aqueous barium nitrate. 9. Barium chloride solution is prepared by dissolving 20.0g of solid barium chloride in about 800cm3 of distilled water and diluting to one litre of solution. Label this as aqueous barium chloride. 10. Lead (II) nitrate is prepared by dissolving 33.0g of solid lead (II) nitrate in about 800cm3 of distilled water and diluting to one litre of solution. Label this as aqueous lead (II) nitrate. 11. Magnesium sulphate is prepared by dissolving 12.0g of solid magnesium sulphate in about 800cm3 of distilled water and diluting to one litre of solution. Label this as aqueous magnesium sulphate. 12. Silver nitrate is prepared by dissolving 17.0g of solid silver nitrate in about 800cm3 of distilled water and diluting to one litre of solution. Label this as aqueous silver nitrate. 13. Iron (II) sulphate solution is prepared by dissolving 16.0g of solid iron (II) sulphate in about 800cm3 of distilled water and diluting to one litre of solution. Label this as aqueous silver nitrate. 14. Potassium iodide is prepared by dissolving 5g of solid potassium iodide in about 800cm3 of distilled water and diluting to one litre of solution. Label this as potassium iodide 15. Hydrogen peroxide is prepared by adding 200cm3 of fresh 20 volume hydrogen peroxide to about 600cm3 of distilled water and diluting to one liter of solution. (This solution should be prepared one day before the day of practical and stored in stoppered container). 143

Tactics of Chemistry ii. Test for Cations Ammonium ions (NH4+) Dissolve the ammonium salt in about 10cm3 of water. Add about 2cm3 of 2M sodium hydroxide and a few pieces aluminium foil. Warm the mixture. Dip a glass rod in hydrochloric acid and bring it to the mouth of the heated mixture. Observation Inference Bubbles of a colourless gas NH4+ present produced, which produces white dense fumes with hydrogen chloride. Zinc ions (Zn2+) Dissolve the zinc salt to about 10cm3 of distilled water. To about 2cm3 of the zinc salt solution in a test tube add 2M aqueous ammonia drop wise until in excess. Observation Inference White precipitate formed, which dissolves in excess to form a colourless solution. Zn2+ present Calcium ions (Ca2+) Dissolve the calcium salt to about 10cm3 of distilled water. To about 2cm3 of the calcium salt solution in a test tube add 2M aqueous sodium hydroxide drop wise until in excess. To another about 2cm3 in a test tube add 2M ammonia solution drop wise until in excess. 144

Learning chemistry made easier Observation Inference Forms white precipitate with Ca2+ present sodium hydroxide insoluble in excess. No white precipitate formed with aqueous ammonia. Aluminium ions (Al3+) Dissolve the aluminium salt in about 10cm3 of water. Divide mixture in to three portions. To the first portion test the resulting solution with litmus paper. Observation Inference The blue litmus paper changes H3O+, Al3+ present red and the red litmus paper does not change. Note: For Fe3+ ions to be present the solution must be yellow/brown and green solution for Fe2+ ions. To the second portion, add few drops of ammonia solution until in excess. Observation Inference White precipitate Al3+ present forms which is insoluble in excess. To the third portion, add sodium hydrogen carbonate or sodium carbonate. 145

Tactics of Chemistry Observation Inference Effervescence formed Al3+ present. and a colourless gas is evolved. Copper (II) ions (Cu2+) Dissolve the copper salt in about 10cm3 of distilled water. To about 2cm3 of the copper salt solution in a test tube add 2M aqueous ammonia drop wise until in excess. Observation Inference Pale blue precipitate formed, which dissolves in excess to form a deep blue solution Cu2+ present Lead ions (Pb2+) Dissolve the lead salt in about 10cm3 of distilled water. Potassium iodide or potassium chromate (VI) is added to the solution containing lead ions. Observation Inference In both cases, the yellow Pb2+ present precipitate is formed. 146

Learning chemistry made easier Iron (II)ions (Fe2+) Dissolve the iron (II) salt in about 10cm3 of water. To about 2cm3 of iron (II) salt solution in a test tube add aqueous sodium hydroxide drop wise until in excess. Observation Inference Green precipitate formed which do not dissolve in excess//Green precipitate persists Fe2+ present Iron (III) ions (Fe3+) Dissolve the iron (III) salt in about 10cm3 of water. To about 2cm3 of iron (III) salt solution in a test tube add aqueous sodium hydroxide drop wise until in excess. Observation Inference Brown precipitate formed which do not dissolve in excess// Brown precipitate persists Fe2+ present 147

Tactics of Chemistry iii. Tests for Anions. SSuullpphhaatteeio(SnOs 4a2r-)e tested from a solution. i. Add the sulphate solid in about 10cm3 of water and shake. ii. To about 2cm3 of the resulting solution add few drops of aqueous barium chloride or barium nitrate and preserve. Observation Inference White precipitate formed CO32-, SO42- , SO32- present iii. To the mixture above, add nitric acid or hydrochloric acid drop wise until in excess. Observation Inference White precipitate does not SO42- present dissolve in nitric acid or hydrochloric acid. iS. ulphitAedsd(SthOe32s-u) lphite solid in about 10cm3 of water and shake. ii. To about 2cm3 of the resulting solution add few drops of aqueous barium chloride or barium nitrate and preserve. Observation Inference White precipitate formed CO32-, SO42- , SO32- present iii. To the mixture above, add nitric acid or hydrochloric acid 148

Learning chemistry made easier drop wise until in excess. Observation Inference White precipitate dissolves CO32-, SO32- present in dilute nitric acid or dilute hydrochloric acid to form a colourless solution. iv. To the 2nd portion add 2-3 drops of acidified potassium dichromate (VI). Observation Inference Acidified potassium dichromate SO32- present (VI) changes from orange to green. Carbonates(CO32-) i. Add the carbonate solid in about 10cm3 of water and shake. ii. To about 2cm3 of the resulting solution add few drops of aqueous magnesium sulphate. Observation Inference White precipitate formed. CO32- present Carbonate can also be tested as. i. To about 2cm3 of the carbonate solution add few drops of silver nitrate solution followed by few drops of dilute nitric acid. 149

Tactics of Chemistry Observation Inference White precipitate formed which CO32- present dissolves to form a colourless solution NNiittrraatteesca(nNbOe3-)confirmed using the brown ring method. For this test aqueous nitrate is used. i. Add the nitrate solid to 10cm3 of water. Add few drops of freshly prepared iron (II) sulphate solution. ii. Slowly add concentrated sulphuric (VI) acid poured down the slides of the test tube containing the mixture. Inference Observation A brown ring forms NO3- present at the junction of the two layers An alternative method which is safer is to: i. Put the solid suspected to be a nitrate in a dry test tube. ii. Add a few drops of concentrated sulphuric acid. iii. Warm the mixture and add a few pieces of copper turnings. Observation Inference Brown fumes NO3- confirmed are produced. 150

Learning chemistry made easier Hydrogen carbonate(HCO32-) i. Add the hydrogen carbonate solid in about 10cm3 of water and shake. ii. To about 2cm3 of the resulting solution add few drops of aqueous magnesium sulphate. Heat the resulting solution until it boils Observation Inference No white precipitate formed when magnesium sulphate is added. On boiling, bubbles of a colourless gas HCO3- Confirmed are produced and white precipitate is formed. Chlorides (Cl-) Add the chloride salt in about 10cm3 of water and shake. To about 2cm3 of the resulting solution add 3 drops of dilute nitric (V) acid and shake. Add two drops of and silver nitrate solution followed by ammonia solution. Observation Inference White precipitate formed, Cl- present Soluble in ammonia solution. Chloride ions can also be tested in solid form. By adding a spatulaful of a chloride salt in 1cm3 concentrated sulphuric acid in a test tube. Heat the mixture and test for any gas formed. Dip a class rod in a ammonia solution and place it in the test tube. Observation Inference A gas which fumes in damp air is Cl- present produced and turns blue litmus paper red on heating and forms thick fumes with the ammonia gas from the glass rod. 151

Tactics of Chemistry Bromide (Br-) Add the bromide solid in about 10cm3 of water and shake. To about 2cm3 of the solution in a test tube add three drops of dilute nitric (V) acid followed by two drops of aqueous silver nitrate. To the mixture add aqueous ammonia solution drop wise until in excess. Observation Inference Pale yellow precipitate formed, Br- present slightly soluble in ammonia solution. Iodide (I-) Add the iodide solid in about 10cm3 of water and shake. To about 2cm3 of the solution in a test tube add three drops of dilute nitric (V) acid followed by two drops of aqueous silver nitrate. To the mixture add aqueous ammonia solution drop wise until in excess. Observation Inference Yellow precipitate formed, I- present which is soluble in ammonia solution. iv. Test for Saturation. In a chemistry practical, some chemicals should always give you a hint of what exactly you might be expected to test. For instance when you see bromine water, there is likely you are required to test saturation. All these are discussed below. Saturation can be tested using the following. Bromine water. To 2cm3 of the solution add 2 drops of bromine water and warm. 152

Observation Learning chemistry made easier Inference The reddish brown colour of CC bromine water is decolourised. CC Present Potassium manganate (VII), KMnO4 To 2cm3 of the solution add 2-3 drops of acidified potassium manganate (VII) Observation Inference The purple colour of potassium CC manganate (VII) water is CC decolourised. Present Acidified potassium chromate (VI), (K2CrO4) Add the solution in the acidified potassium chromate (VI) Inference Observation The Yellow colour of potassium CC chromate (VI) changes to CC orange Present 153

Tactics of Chemistry Acidified potassium dichromate (VI) K2Cr2O4 To the solution, add few drops of acidified K2Cr2O7 (aq) Observation Inference The orange colour of potassium C OH chromate (VI) changes to green Present When testing the saturation using litmus papers and universal indicator, the following measures are considered. If you are provided with an organic salt and you are required to test to identify it, the following testing agents should give you a hint that you are testing an organic compounds. • Acidified potassium manganate (VII) • Bromine water • Universal indicator • Acidified potassium dichromate (VI) • Burning of a salt on a spatula Consider the following tests. If the solid you are testing is an organic solid, the following observations and inferences are acceptable. i. Put the solid in a boiling tube, add water and shake. Observation Inference The solid dissolves to form a Soluble solid colourless liquid Polar solid/substance ii. Test the resulting solution with a litmus paper. 154

Learning chemistry made easier Observation Inference The blue litmus paper turns - COOH, H3O+ red, red litmus paper does not change. iii. Test the solution with a universal indicator. Observation Inference pH of 5 Weakly acidic In case you are heating an organic compound and Observation Inference It burns with a yellow High hydrogen to carbon ratio Or sooty smoky flame. Un saturated compound or C C, CC N/B; Universal indicator tests the strength of a substance e.g strongly acidic, weakly acidic, strongly alkaline or weakly alkaline. Sample question 76 (hint: solid ZnCl2) carry You are provided with a solid Z, inferences. Z is out the following tests and indicate the The Tactic You should be in a position to describe how Zn2+ and Cl- are identified. i. Add all the solid Z in about 10cm3 of water and stir. Observation Inference The white solid dissolves to a Cu2+, Fe2+, Fe3+ Absent colourless liquid 155

Tactics of Chemistry In this case the list of cations which dissolves in water to form a colourless solution are quite many and you may fail to mention the ion present. It is therefore recommended to state a negative inference as stated and not as: Observation Inference The white solid dissolves to form K+, Na+ Mg2+, Pb2+,Ca2+ NH4+ a colourless solution present Note Cu2+, Fe2+ and Fe3+ are coloured even in solution while ions which dissolve to form a colourless solution are quite many. ii. Divide the mixture into two 2cm3 portions. To the first portion, add drops of 2M sodium hydroxide solution drop wise until in excess. Observation Inference dWishsiotelvperseicnipeixtcaetessfotormfoerdmwahich Pb2+, Al3+ Zn2+ ions present colourless solution iii. To the second portion add aqueous ammonia solution drop wise until in excess. Observation Inference Wdishsiotelvperseicnipeixtcaetessfotormfoerdmwahich Zn2+ ions present colourless solution The knowledge of the chemical and physical properties of ions are very crucial in this topic. v. DOs and DONTs of qualitative analysis The DOs of a qualitative analysis. • Make brief and precise record of observations. • Only record observations which you can draw a deduction/ inference on. • Be alert and focused so as to study the progress of stated the tests • Make deductions at every stated observation in relations to 156

Learning chemistry made easier theoretical background information. • Test for gas immediately it is evolved. The DON`Ts of a qualitative analysis. • Do not record elements, compounds or gases on the inference column. Record ions only which exhibit the properties observed and recorded as an observations. • The standard recommended way on the inference column is writing chemical symbols for ions and correct charge rather than words, for instance Zn2+ rather than Zinc ions, CO32- rather than carbonate ion... • No inference is marked correct unless the observation is correct. • Avoid contradictory ion or incorrect alternative ion. They do penalize the right one. For instance if Mg2+ is included in the above test for solid Z, Observation Inference Wdishsiotelvperseicnipeixtcaetessfotormfoerdmwahich Pb2+, Al3+, Zn2+, Mg2+ ions colourless solution present Mg2+ ions contradicts with the other three ions which exhibits the observed property, while Mg2+ do not. Mg2+ cancels the correct ions. The Tactic Contradictory inference earns ZERO marks, avoid them vi Statements rejected in observations. When you are doing your analysis avoid the following statements in the observations. Wrong Statements Correct Statements •• CWlehaitressoolulutitoionn • Colourless solution • Solution turns milky/ • White precipitate formed cloudy 157

Tactics of Chemistry ••••• TWeasttetrucboensdweenasteisng • Colourless vapour/gas MWoaitsetrufroercmosndenses • Cpcthoaoenrlotdcsuoeoronlfleseteshrsseploiatnqurutbtsihedeo. ffcotorhomeleetredsatt Moist gas present tube. •••• No change • The right observation here No observable change depends on the expected Nothing happens observation. Look at the No reaction next test below: • To 2cm3 of the provided filtrate in a test tube, add 3 drops of barium nitrate solution. In this test due to barium nitrate ewxepeacrtewtehsittienpgrtehceippirtaestieonncetoobf eSO42-, SoObs3e2-r, vCeOd3.2- ions and therefore we Observation Inference No white precipitate formed SO42-, SO32-, CO32-absent Avoid observations such as No observable change No change No reaction Nothing happens To 2cm3 of solution provided add two drops of bromine water Inference Observation Bromine water is decolourised C = C or C = C but not C = C , C = C, = C = C = Show the complete bonding in organic compounds. 158

Learning chemistry made easier • Note carefully it is not sufficient to state observations which no valid deductions can be made such as Invalid Observation Valid observation White precipitate formed, White precipitate formed soluble in soluble in excess excess to form a colourless solution. Once again you are reminded that. • Use of the negative deductions must be avoided as much as possible only where the list of possible ions is too large. For instance. A salt is dissolved in water to form a colourless solution. There are a number of ions, where when their compounds are dissolved in water to form a colourless solution, but the list of ions which do not exhibit such property are few, hence the negative deduction as earlier discussed. Observation Inference The solid dissolves to form a Cu2+, Fe2+, Fe3+ absent colourless solution This is so because the ions which forms colourless solutions such as Nfaail+,toK+m, NenHt4i+o, nMtgh2e+, Ca2+ are quite many and in the list student may ion under test. In this case negative inference is accepted as earlier mentioned. viii Othe areas of great importance during qualitative analysis a. Avoid contradictions, make deductions after every test. Where there are difficulties to make deductions repeat the test until concrete deductions can be made from the observations. b. Give the right and consistent observations only. c. Don’t forget to indicate i. Colours of the gas evolved as an observation ii. Colours of the precipitate as an observation iii. Final colour of the solution as an observation iv. Deductions which are related to the observation given. 159

Tactics of Chemistry For your testing and assessment of the grasping level, do the following practical in your school laboratory. Sample question 77 Request your teacher to prepare about 0.5g of Solid B. Solid B is FeSO4 .5H20. You are provided with solid B. carry out the tests below. Write your observations and inferences in the spaces provided. i. Place about half of the solid B in a clean dry test tube. Heat the solid gently and strongly. Test any gases produced with both blue and red litmus paper. Observation Inference Colourless liquid formed on the cooler parts of the test tube Solid contains water of crystalisation// hy- Blue litmus paper turns red drated salt and red litmus remains red. ii. Put the remaining solid B in a test tube. Add about 5cm3 of distilled water and shake the mixture well . Retain the mixture for use in test iii. Observation Inference Solid dissolves to form a Fe2+ present green solution. iii. Divide the mixture into four potions. To about 2cm3 of the mixture in a test tube, add aqueous ammonia solution drop wise until in excess. Observation Inference Green precipitate formed Fe2+ Present Insoluble in excess 160

Learning chemistry made easier iv. To about 1cm3 of the mixture in a test tube add about 1cm3 of hydrogen peroxide followed by aqueous ammonia drop wise until in excess. Observation Inference or Brown solution formed Fe2+ oxidised to Fe3+ or Brown precipitate insoluble in excess Fe2+ Fe3+ + e- Fe3+ present v. To about 1cm3 of the mixture in a test tube, add three drop of barium nitrate solution Observation Inference White precipitate formed SO42-, SO32-, CO32- present. vi. To a mixture obtained in (v) above. Add about 4cm3 of 2M hydrochloric acid. Observation Inference White precipitate does not SO42- confirmed. dissolve// white precipitate persists Let us go through another qualitative analysis question, which will help you understand it fully. Sample question 78 Inform your teacher to provide you with Solid P, which is a mixture of PbCO3 and ZnSO4. a. Put the mixture in a test tube and about 10cm3 of water to the mixture and shake, Filter the mixture and preserve the filtrate and the residue for the next tests. 161

Tactics of Chemistry Observation Inference Solid is slightly soluble to form Cu2+, Fe2+, Fe3+ absent a colourless solution. White residue Colourless filtrate b. Divide the filtrate into three portions i. To the first portion add aqueous NaOH solution drop wise until in excess Observation Inference White precipitate formed Pb2+, Zn2+, Al3+ present which dissolves in excess to form a colourless solution ii. To the second portion add aqueous NH3 solution drop wise until in excess Observation Inference White precipitate formed Zn2+ present which dissolves in excess to form a colourless solution iii. To the third portion add three drops of aqueous barium nitrate and preserve for the test (iv). Observation Inference White precipitate formed CO32-, SO42-, SO32- present iv. To the portion in (iii) above add few drops of dilute hydrochloric acid. 162

Learning chemistry made easier Observation Inference White precipitate does not SO42- present dissolve//white precipitate persists. This confirms that the soluble part of the mixture is ZnSO4. c. Add all the residue in to a test tube, add about 5cm3 of dilute nitric acid and shake. Observation Inference Effervescence and bubbles of a CO32-, SO32- present colourless gas produced i. To the first portion add a three drops of acidified potassium dichromate (VI) solution. Observation Inference Orange acidified potassium oSrO32- absent dichromate (VI) does not turn CO32- present green. ii. To the second portion add aqueous NaOH solution drop wise until in excess. Observation Inference White precipitate formed Pb2+, Zn2+, Al3+ present which dissolves on excess to a colourless liquid. 163

Tactics of Chemistry iii. To the third portion add aqueous NH3 solution drop wise until in excess. Observation Inference White precipitate formed which Pb2+, Al3+ present does not dissolve in excess iv. To the fourth portion add three drops of aqueous potassium iodide. Observation Inference Yellow precipitate formed. Pb2+ present The residue part of the mixture is confirmed to be PPbbCCOO33.sTahlti.sTihs ethe mixture is the combination of two salts ZnSO4 and purpose of the qualitative analysis. Sample question 79 Inform your teacher to provide you with Solid X, which is a mixture of copper (II) oxide and zinc sulphate in the ratio 1:1 You are provided with solid X which is a mixture of two solids. Carry out the following tests to identify the cations and anions present in the mixture. a. Put all the mixture in Add about 10cm3 of water, stir and then filter. Keep both the residue and the filtrate for further reactions. Observation Inference Black residue When a portion is blocked this way Colourless solution filtrate no inference is required. b. Place the residue in a boiling tube and add dilute nitric acid and warm. Divide the solution into two portions 164

Learning chemistry made easier Observation Inference Blue solution formed Cu2+ present No effervescence HCO3-, CO32-, SO32-absent c. To the 1st portion add NaOH (aq) till in excess Observation Inference Blue precipitate formed soluble Cu2+ present in excess d. To the 2nd portion add aqueous ammonia till in excess Observation Inference Blue precipitate soluble in excess to form a deep blue solu- Cu2+ present tion Divide the filtrate in to 5 portions. i. To the first portion add sodium hydroxide solution drop wise until in excess Observation Inference White precipitate formed Pb2+,Zn2+, Al3+ present soluble in excess ii. To the second portion add aqueous ammonia till in excess Observation Inference White precipitate formed Zn2+ present soluble in excess iii. To the third portion add dilute HCl 165

Tactics of Chemistry Observation Inference No white precipitate formed Ag+, Pb2+, SO32- absent, No effervescence formed CO32-, HCO3- absent iv. To the fourth portion add lead (II) nitrate solution Observation Inference White precipitate formed Cl -, SO42- present v. To the fifth portion add barium chloride solution Observation Inference White precipitate formed SO42- present Tactical Practice O Question 1 1. You are provided with solid E. Carry out the following tests and write your observations and inferences in the table below. a. Divide solid E into two portions. Place the first portion in a clean dry test-tube and heat gently and then strongly. Observation Inference (1 mark) (1 mark) b. Place the second portion in a boiling tube and add about 10cm3 of distilled water. RETAIN the mixture. Observation Inference (1 mark) (1 mark) 166

Learning chemistry made easier c. Divide the mixture obtained in (b) above into four portions. i. To the first portion add aqueous sodium hydroxide drop wise until in excess. Observation Inference (1 mark) (1 mark) ii. To the second portion, add about 1 cm3 of sodium chloride solution. Observation Inference (1 mark) (1 mark) iii. To the third portion add aqueous ammonia solution drop wise until in excess. Observation Inference (1 mark) (1 mark) d. i. To the fourth portion add about 1cm3 of Pb(NO3)2 and RETAIN the mixture for test (d) (ii) below. Observation Inference (1 mark) (1 mark) ii. To the mixture obtained in (d) (i) above, add about 2cm3 of 2M nitric (V) acid and warm Observation Inference (1 mark) (1 mark) Question 2 2. You are provided with solid R. Carry out the following tests and record your observations and inferences in the spaces provided. a. i. To half of solid R put in a dry test tube, add 5cm3 of absolute ethanol and shake the mixture thoroughly. Divide the mixture into two equal parts. 167

Tactics of Chemistry Inference Observation (1 mark) (1 mark) ii. To the first portion, dip universal indicator paper and determine its PH. Observation Inference (1 mark) (1 mark) iii. To the second portion, add half of sodium carbonate solid provided. Observation Inference (1 mark) (1 mark) b. Place the remaining solid R into a boiling tube and add 10cm3 of distilled water and shake thoroughly. i. To the first 2cm3, add 4-5 drops of acidified potassium manganate (VII) solution U, then warm. Observation Inference (1 mark) (1 mark) ii. To the second portion, add 2cm3 of acidified potassium dichromate (VI) solution. Observation Inference (1 mark) (1 mark) iii. To the third portion, add 2cm3 of solid sodium carbonate. Observation Inference (1 mark) (1 mark) Confidential for question 1 & 2 In addition to the general apparatus in a chemistry laboratory each student is required to have the3.f oTellsotwtuinbeg.holder 1. Solid E in a stoppered container 4. Boiling tube -1 2. Six clean dry test tube on a test tube rack. 5. Spatula 168

Learning chemistry made easier 6. Solid R 5. 2M Nitric (V) acid solution with a 7. About 5cm3 of absolute ethanol dropper 8. PH chart 6. Universal indicator paper. 9. Solid sodium carbonate (about 0.1g) 7. 2M acidified potassium dichromate (VI) 10. Distilled water. solution Each student should have access to: NOTE 1. 2M sodium hydroxide solution with a 1. Solid R is about 1g of malleic acid. dropper 2. Solid E is about 1g of hydrated Zinc sulphate. 2. 2M Sodium Chloride solution with a dropper 3. Solution U is prepared by dissolving 0.1g 3. 2M Ammonia solution with a dropper 4. 1M Lead (II) nitrate solution with a of KMnO4 in 400cm3 of 2M H2SO4 and then top up to 1 litre dropper Question 3 3. a. i. Put a very small amount of solid P at the tip of a clean metallic spatula and perform the flame test. Observation Inference (1 mark) (1 mark) ii. Put all the remaining solid P in a clean boiling tube and add 5 cm3 of distilled water and shake to dissolve the solid P. Label the solution formed P2. Observation Inference (1 mark) (1 mark) iii. Put 1 cm3 P2 into a clean test-tube. Add one drop of Barium chloride solution followed by few drops of 2M HNO3. Observation Inference (1 mark) (1 mark) iv. Put 1 cm3 of P2 in a clean test tube and add one drop of Bromine water. 169

Tactics of Chemistry Inference Observation (1 mark) (1 mark) v. Put 1 cm3 of fresh P2 into a clean test-tube and add one or two drops acidified potassium manganate VII. Observation Inference (1 mark) (1 mark) vi. Put 1 cm3 of P2 in a clean test-tube and add one or two drops of acidified potassium chromate (VI) and shake. Observation Inference (1 mark) (1 mark) 4. a. i. Put one spatula of solid R at the tip of a clean metallic spatula and place the solid at the tip of a non-luminous Bunsen burner using the spatula. Remove the solid from the Bunsen burner flame once the reaction starts. Observation Inference (1 mark) (1 mark) ii. Put all the remaining R into a clean boiling tube and add 5 cm3 of distilled water. Shake gently the contents of the boiling water and allow to settle. Label the solution formed R2. Observation Inference (1 mark) (1 mark) b. i. Put 1 cm3 of R2 into a clean test-tube and add sodium hydrogen carbonate solution. Observation Inference (1 mark) (1 mark) ii. Put 1 cm3 of fresh R2 into a clean test-tube and one or two drops of universal indicator and use the pH-chart and record the pH of the solution. 170

Learning chemistry made easier Observation Inference (1 mark) (1 mark) iii. Put 1 cm3 of fresh R2 into a clean test-tube and add one drop of acidified potassium manganate (VII) and warm gently. Allow to cool. Observation Inference (1 mark) (1 mark) iv. Put 1 cm3 of R2 into a clean test tube, add one drop of bromine water and warm gently. Allow to cool. Observation Inference (1 mark) (1 mark) Confidential for question 3 & 4 In addition to the general apparatus in a chemistry laboratory each student is required to have the following. 1. Solid P (0.5g of pure sodium sulphite. PREPARATION Label P) 1. Potassium manganate VII solution. 2. 0.25M Barium Chloride Weigh 0.18g of potassium manganate 3. Clean metallic spatula VII crystals, dissolve in 500 cm3. 4. Bunsen burner, producing non-lumi- 2. Potassium Chromate (VI) nous flame Weigh about 30g of potassium 5. Six test-tubes chromate VI and dissolve in 500 6. Two boiling tube, one for question 3 cm3 of 1.0M H2SO4 and shake to dissolve. Add more water to 1000 and the other for question 4 cm3 and shake. 7. Six test-tubes clean and dry test-tube 3. Bromine Water holder 4. 1 cm3 of bromine in 1000 cm3 of water. 8. 2M HNO3 Use fume chamber 9. Potassium manganate VII solution 10. Potassium Chromate VII solution 5. Sodium hydrogen carbonate solution. 11. Sodium hydrogen carbonate solution Weigh 10g of sodium hydrogen 12. Universal indicator full range with carbonate powder and dissolve 1000 cm3. matching pH chart 13. Bromine water 14. Solid R. 0.5g of malleic acid C 171

Tactics of Chemistry PHYSICAL CHEMISTRY i. Thermal chemistry This deals with energy changes as heat is either evolved or absorbed during a chemical reaction. This is noticed by temperature change. 1. Temperature - is the quantity that tells how hot or cold an object is with respect to some standards. 2. Heat is the energy transferred from one body to another due to temperature difference between them, when in contact. For you to be able to handle these questions effectively, you need to understand the mole concept. Accuracy is checked in both readings and calculation. To earn more marks ensure that 1. Your table (s) is completely filled. 2. Your temperature readings show a trend, for instance • Continuous rise in temperature up to a maximum value then a fall. • Constant value followed by a drop or rise etc 3. Use consistent decimal places. When doing thermal chemistry questions, you need to note where the change takes place. Check for changes such as precipitation, crystallization, highest temperature attained, lowest temperature attained etc When calculating the energy change, use of signs is recommended to distinguish exothermic and endothermic reactions. Common questions in this topic has the following format. Sample question 80 Mg2+(aq) ∆ Hhyd = -1920 kJ mole-1 Given that: Mg2+(g) + (aq) Cl-(g) + (aq) Cl-(aq) ∆Hhyd = - 364 kJ mole-1 MgCl2(s) Mg2+(aq)+ 2Cl-(aq) ∆ Hsoln= -155 kJ mole-1 Use the above information given to answer the questions below. 172

Learning chemistry made easier a. Draw the energy cycle diagram for the reaction ∆Hlatt MgCl2(s) Mg2+(g) + 2Cl-(g) ∆Hsoln ∆Hhyd Mg2+(aq)+ 2Cl-(aq) b. Calculate the Lattice energy for Magnesium Chloride. ∆Hsoln = ∆Hlatt + ∆Hhyd ∆Hlatt = ∆H -soln(MgCl2) ∆Hhyd(Mg + 2Cl) ∆Hlatt = -155 - {-1920 + (2 x -364) = -155 - {-1920 - 728} = - 155 - {-2648} = +2,493kJmol-1 c. Draw the energy level diagram for the reactions Energy (kJmol-1) Mg2+(g) + 2Cl-(g) ∆Hlatt = +2,493 MgCl2(s) ∆Hhyd = -2,648 ∆Hsoln = -155 Mg2+(aq)+ 2Cl-(aq) Reaction path 173

Tactics of Chemistry Sample question 81 Some average bond energies are given below. Bond Energy in kJmol-1 C – C 348 C – H 414 Cl – Cl 243 C – Cl 432 H – Cl 340 Calculate the energy change for the reaction below. C2H6 (g) + Cl2 (g) CH3CH2Cl(g) + HCl(g) The best way is to write the equation in an open structure for clear bond view. HH HH H C C H + Cl Cl H C C H + H Cl HH H Cl NOTE: the bond energy provided above are positive, meaning they are bond breaking processes. When the same bonds are formed same energy is given out. We can calculate the enthalpy change Enthalpy = Bond breaking + Bond formation Change energy energy C - H = 6 x 414 = +2484 C - H = 5 x -414 =-2070 C - C =1 x 348 =+ 348 C - C = 1 x -348 =-348 Cl - Cl =1 x 243 = +243 Cl - Cl = 1 x -243 =-243 +3075 -2661 Enthalpy change = +3075 + (-2661) = +414kJmol-1 The Tactic The correct way to get the overall enthalpy is to get the sum of the total bond breaking energy and total bond formation energy using their signs. 174

Learning chemistry made easier Tactical Practise P 1. a. Define molar heat of combustion. (1 mark) b. Use the following data to determine the enthalpy of combustion of ethanol. Volume of water in calorimeter = 100cm3 Initial temperature of water = 22.50C Final temperature of water = 50.50C Change in temperature of water= x0C Mass of water = 100g Mass of lamp before burning= 40.46g Mass of lamp after burning= 40.06g i. Work out the value of X i.e. change in temperature of water. (2 marks) ii. Calculate the mass ethanol burnt. (2 marks) iii. Determine the heat evolved given that specific heat capacity of water =4.2 Jg-1K-1 (4 marks) iv. Calculate the molar heat of combustion of ethanol. (2 marks) 2. a. In an experiment to determine the molar heat of reaction when magnesium displaces copper, excess magnesium powder was added to 25.0cm3 of 2.0M copper (II) chloride solution at 230C. The temperature rose to 380C. i. Apart from increase in temperature, state and explain the observations which were made during the reaction(2marks) ii. Calculate the heat change during the reaction. (2 marks) (Specific heat capacity of the solution =4.2 J g-1 K-1, density of the solution =1g/cm3). iii. Write the ionic equation for the reaction. (1mark) iv. Determine the molar heat of displacement of copper by magnesium. (2marks) b. Use the thermochemical equations below to answer the questions that follow. C2H6 (g) + 7/2O2 (g) 2CO2 (g) +3H2O (g) H1 = -1560 kJmol-1 C (s) +O2 (g) CO2 (g) H2 = -394 kJmol-1 H2 (g) + ½O2 (g) H2O (g) H3 = -286 kJmol-1 i. Name two types loefvheledatiacghraanmgefosrrethpererseeanctteiodnbryepHr3e.s(e2nmteadrkbsy) equation ii. Draw an energy (2 marks) iii. Calculate the standard enthalpy of formation of ethane. (2marks) 175

Tactics of Chemistry 4. Study the information in the table below then answer the questions that follows. Bond Bond energy (kJmolˉ¹ 435 H–H 243 Cl – Cl 431 H - Cl a. Calculate the enthalpy change for the reaction. (2marks) H 2(g) + Cl 2(g ) → 2HCl (g ) b. On the axis given below draw an energy level diagram for the reaction above. (1mark) 5. Matter exists in three states which can be related as shown in the diagram below. Solid P→ Gas RQ Liquid a. Name processes: P: _____________________________________________________ (1mark) R: _____________________________________________________ (1mark) b. Explain whether process Q is exothermic or endothermic. (1mark) 6. 6.0g of Potassium nitrate solid was added to 120cm3 of water in a plastic beaker. The mixture was stirred gently and the following results were obtained. Initial temperature = 21.50C Final temperature = 17.00C a. Calculate the enthalpy change for the reaction. (3 marks) (Density =1g/cm3, C= 4.2jg-1K-1) b. Calculate the molar enthalpy change for the dissolution of potassium nitrate. (K=39, N= 14, O =16) (2 marks) 176

Learning chemistry made easier 7. The table shows the results obtained when 20.2g of potassium nitrate was added in 50cm3 of water. Time in (min) 0.0 0.3 1.0 2.0 2.3 3.0 3.3 4.0 Temperature (oC ) 25.0 25.0 25.0 17.0 17.0 20.0 20.0 20.0 (i) Draw the graph of temperature against time (4 marks) (ii) Using the graph, determine the temperature change. (1 mark) (iii) Calculate the heat change. (2 marks) (iv) Find the molar heat of solution of potassium nitrate. ( 2 marks) 8. Calculate the latent heat of vaporization of water. (3 marks) H2O(l) H2O(g) Given the following thermo chemical equations:- H2(g) + ½O2(g) H2O(g) ΔH= -242KJ/Mol H2(g) + ½O2(g) H2O(l) ΔH= -286KJ/Mol 9. The equation below represents changes in the physical state of ions metal: Fe(s) Fe(l) ΔH= +15.4kjmol-1 Fe(l) Fe(g) ΔH=+354kjmol-1 a. Calculate the amount of heat energy required to change 10kg of solid iron to gaseous iron. Fe = 56 (3 marks) b. Iodine can react with chlorine as shown below:- I2(g) + Cl2(g) 2ICl(s) ΔH= -68kJ Determine the molar enthalpy change for this reaction. (1 mark) c. Draw an energy level diagram for the reaction in (b) above. (3 marks) 177

Tactics of Chemistry ii. Solubility and solubility rules Knowledge of solubility is very important because a lot of analysis in chemistry is based on it. Here is the solubility rule which you can always consult when determining whether an ionic compound is soluble in water or not. Solubility rules for some ionic compounds SOLUBLE 1 All salts of sodium (Na+), Potasium (K+) and ammonium (NH4+) are SOLUBLE 2 (AClllOs3a-)latsndopf enricthraotreaste((NCOlO3-4)-,) aarceetSaOteLU(BCHLE3C. O2-), chlorate 3 All chlorides (Cl-), bromide (Br-) and iodide (I-) are SOLUBLE except those of lead (Pb2+) , silver (Ag+) or mercury (Hg22+). 4 All salts of floride (F-) are SOLUBLE except those of magnesium (Mg2+), Calcium (Ca2+), barium (Ba2+) or lead (Pb2+) All salts (oCfas2u+)l,pshilavteer((SAOg4+2)-), are SOLUBLE except those of 5 calcium mercury (Hg2+), barium (Ba2+) and lead (Pb2+) INSOLUBLE 6 All hydroxides (OH-) and oxides (O2-) are INSOLUBLE except those of potasium (K+), sodium (Na+), and barium (Ba2+) 7 All sulphides (S2-)are INSOLUBLE except those of potasium (K+), sodium (Na+), ammonium (NH4+)and barium (Ba2+) 8 AINllSOcLaUrBboLnEaetxecsep(tCtOho32s-)e oafnpdotapsshiousmph(aKt+e)s, so(dPiOum43-)(Naa+r)e, or ammonium (NH4+) 178

Learning chemistry made easier Solubility questions are very common in any chemistry exam. The technique behind this is your knowledge on • Measurements of temperature, volumes, concentrations etc • Graphs drawing and their analysis. Knowledge on the above area as dealt with earlier is very crucial. Questions on solubility appear as follows. Sample question 82 Solubility of potassium nitrate and copper (II) sulphate were determined at different temperatures. The following data was obtained. Temperature (0C) 0 10 20 30 40 50 Solubility of KNO3 6.0 16.0 36.0 62.0 92.0 126.0 salt/100g of water CuSO4 8.0 12.0 18.0 26.0 34.0 46.0 130 120 110 100 90 80 Solubility /100g of water 70 60 solubility curve 50 40 KNO 3 solubility curve 30 20 CuSO 4 10 0 10 20 30 40 50 Temperature (0C) 179

Tactics of Chemistry a. On the graph paper provided; i. Plot solubility curves for both salts, where solubility (vertical axis) is plotted against temperature. (3 marks) From the graph check the following marking areas. ◊ Labelling of axis ◊ Appropriate scale ◊ Plotting ◊ Curve/line ii. Determine from the graph the solubility of each salt at 35°C. I. KNO3 = 76g/100g of water (1mark) II. CuSO4 = 30g/100g of water (1mark) Note the dotted lines used to identify how you got your answer. iii. At what temperature was the solubility of both salts equal? (l mark) - at 30C Again note the dotted lines used to identify how you got your answer. iv. 120g of potassium nitrate solution at 50°C was cooled to 10°C. What mass of the crystals will be deposited? (4 marks) The saturation has not been reached and therefore we get the difference. The solubility of KNO3 at 500C = 120.0g /100g of water The solubility of KNO3 at 100C = 16.0g/100g of water The salt deposited is = 104.0g 180

Learning chemistry made easier Tactical Practice Q 1. (I) Faith, a form four student, carried out an experiment to determine the solubilities of salts P and Q and obtained the following results. Use them to answer the questions that follow. Salt P: Temperature (0C) 0 10 40 60 80 100 120 Solubility of water, g/100g of H2O 36 33 30 28 26 23 18 Salt Q Temperature (0C) 0 10 20 30 40 50 60 Solubility of water, g/100g of H2O 10 21 36 50 63 81 97 a. On the same axes, plot the graphs for the solubility curves of the two salts. (5 marks) b. From your graph determine the temperature at which equal amount of P and Q dissolve in 100 g of water. (1 mark) c. Explain how to prepare a saturated solution containing 70 g of Q in 100g of distilled water. (1 mark) d. 12.5 g of Q and 30 g of P were dissolved in 50 g of water at 500C. Describe how a pure sample of Q can be obtained. (2 marks) e. Give one practical application of the process applied in (d) above and name the process. (2 marks) 6. Below is a table showing the solubilities of salts Q and R at different temperatures. Temperature (0C) 0 10 20 30 40 50 Solubility of water, W 3.0 5.0 7.4 10.0 14.0 19.0 g/100g of water R 15.0 17.0 20.7 25.7 28.5 33.0 If both salts W and R are present in 100cm3 of saturated solution at 500C, what will be the total mass of crystals formed if the solution was cooled to 200C? (3marks) 7. The following results were obtained during an experiment to determine the solubility of potassium chlorate (V) in water at 300C. Mass of evaporating dish =15.86g Mass of evaporating dish + saturated solution at 300C = 26.8g Mass of evaporation dish +solid potassium chlorate (V) after evaporation to dryness=16.86g Calculate the mass of the saturated solution containing 60.0g of water at 300C. (3 marks) 181

Tactics of Chemistry 8. a. What is meant by the term solubility of salts? (1 mark) Calculate the solubility of salt given that 15g of the salt can saturate 25cm3 of water. (2 marks) i. The table below gives the solubility of salt X in grams per 100g of water at different temperatures Temperature 10 20 30 40 50 60 70 80 90 100 (0C ) Solubility 5.0 7.5 14.0 18.5 24.0 30.0 38.0 46.0 46.0 50.1 (g/100g) water i. Plot a solubility curve for salt X (solubility in g /100g water Y- axis) (temp 0C. (X –axis) (4 marks) ii. What is meant by the points plotted in (i) above? (2 marks) iii. From your graph determine the solubility of salt X at the following temperatures I. 440C………………………………………... (1 mark) II. 620C ………………………………………. (1 mark) iii. What mass of crystals of the salt will be formed if the solution was cooled from 620C to 440C. (1 mark) iv. Name two areas where knowledge of solubility curves is applied. (2 marks) 5. a. The table below shows the solubility of potassium chlorate (V) at different temperatures Temperature (0C ) 10 20 30 40 50 60 70 Solubility (g/100g) water 27 30 36 55 80 110 140 i. Plot a graph of solubilities of potassium chlorate (V) against temperature. (4 marks) ii. Using your graph: I. Determine the solubility of potassium chlorate (V) at 470C. (1 mark) II. Determine the concentration in moles per litre of potassium chlorate (V) at 470C (K= 39, Cl = 35.5, O= 16) density of solution = 1g/cm3. ( 3 marks) III. Determine the mass of potassium chlorate (V) that would crystallize if the solution is cooled from 620C to 450C. (1 mark) 182

Learning chemistry made easier iii. Reaction rates Though this has been widely covered in graph section of this book, it is important to note the factors which affect the rate of reaction and reactions at equilibrium. If required to plot a graph, time should be on x-axis since it is dependent of factors such as temperature, concentration, surface area, catalyst etc. Tactical Practice R 1. The factors which affect the rate of reaction between lead carbonate and di- lute nitric acid (V) acid were investigated by carrying out three experiments: Experiment number Lead carbonate Concentration of nitric (V) acid 1 Lumps 4M 2 Powdered 4M 3 Lumps 2M a. Other than concentration, name the factor that was investigated in the experiments. (1 mark) b. For each experiment, the same volume of acid (excess) and mass of lead carbonate were used and the volume of gas liberated measured with time. i. Draw a set up that can be used to investigate the rate of reaction for one of the experiments. (3 marks) ii. On the grid provided, sketch the curves obtained when the volume of gas produced was plotted against time for each of the three experiments and label each as 1,2 or 3. (4 marks) Volume of gas (cm3) Time (seconds) iii. Write an equation for the reaction that took place. (1 mark) c. If the experiment were carried out using dilute hydrochloric acid in place of dilute nitric (V) acid, the reaction would start, slow down and eventually stop. Explain these observations. (1 mark) d. A solution of bromine gas in water is an example of a chemical reaction in a state of balance. 183

Tactics of Chemistry The reaction involved is represented by the equation below. Br2(g) + H2O(l) 2H+(aq) + Br-(aq) + OBr-(aq) Yellow/orange Colourless State and explain the observation made when hydrochloric acid is added to the mixture at equilibrium. 2. a. The figure below shows how the rate of the following reaction varies with time. A(g) + B(g) 2C(g) + D(g) Rate of reaction Curve II y Curve I x Time (minutes) i. Which of the two curves represents the rate of the reverse reaction? Give a reason. (2 marks) ii. What is the significance of points X and Y on the figure?. (2 marks) b. State and explain the effects of an increase in pressure on the rates of the following reactions. i. H2(g) + Cl(g) 2HCl(g) ii. CH3OH(l) + CH3COOH(l) CH3COOCH3(l) + H2O(l) c. In an experiment to study the rate or reaction between barium carbonate and dilute hydrochloric acid, 1.97g of barium carbonate were reacted with excess 2M hydrochloric acid. The equation for the reaction is BaCO3(s) + 2HCl(aq) BaCl2(aq) + CO2(g) + H2O(l) i. The data in the table was obtained. Time in seconds 0 30 60 90 120 150 180 210 240 Volume of gas (cm3) 0 80 135 175 210 230 240 240 240 ii. From the graph, determine the rate of reaction at: I. 12 seconds ( 1 mark) II. 120 seconds (1 mark) III. Give a reason for the difference between the two values. (1mark) 184

Learning chemistry made easier ACCURACY OF CHEMISTRY INSTRUMENT i. Temperature Temperature is measured using a thermometer. The commonly used thermometer for KCSE has a range between –100C to 1100C. They are either mercury or alcohol. A section of such a thermometer is as shown below. Coloured alcohol thermometer Mercury thermometer These thermometers can measure temperature to the nearest 0.50C. From the section of a thermometer below: The thermometer reading shows that the exact readings which can be read are 20.0, 21.0, 22.0, 23.0 etc. This is because the reading between 20 and 30 is divided in to 10 units. The accuracy of these thermometers is therefore to the nearest 10C unit. It is accepted that if the reading is between say 20.0 and 21.0 it can be 20.5 this is why readings like 20.1 or 20.2 are invalid but readings like 20.0 or 20.5 are valid. If the accuracy is 0.50C, readings such as 20.1, 22.4 or 30.6 are invalid. Instead the values should be 20.0, 20.5, 22.0, 30.0 etc The reading during an experiment should be recorded with the first digit after the decimal point as either 0 or 5. Your temperature must also be recorded with same decimal places. Sample question 83 Measure 50cm3 of water and pour it in to a plastic beaker wrapped with a tissue paper. Determine the temperature of water and record it in the table below. Add 4g of NaOH in the water, stir using a thermometer and record the temperature at the intervals of 30 second in the table below. 185

Tactics of Chemistry Time (s) 0 30 60 90 120 150 180 210 (4marks 23 27.0 31.0 34 Temperature (0C) 38.0 38.0 The table is to be marked as follow: Complete table - 1 mark Half complete table - -1/02mmaarrkks Other wise complete Consistency of decimal places - 1 mark Accuracy mark ...... - 1 mark. (Compared to school value) Trend ..... 1 mark The way the table has been filled can only earn the following marks. CInocnosmisptelentceytoafbdlee-ci1m/2aml palrakc es - 0 mark Accuracy mark ...... - 0 mark. Proper filled table should be as Time (s) 0 30 60 90 120 150 180 210 Temperature (0C) 23.0 27.0 31.0 34.0 38.0 38.0 37.0 36.0 Complete table - 2 mark Complete table - 1 mark Consistency of decimal places - 1 mark Accuracy mark ...... - 1 mark Trend ..... 1 mark The Tactic Ensure your table is complete and the values well recorded with consistency of the decimal places. If two decimal places; keep the consistency. If one decimal place is used maintain that throughout. No values should be recorded without a decimal place. In the questions involving temperature, some temperature may not be determined directly by measuring but through a graph. 186

Temperature (cm3) Learning chemistry made easier For Instance, in the above question we may be required to determine the highest temperature attained. Depending with the intervals of time the maximum temperature must have been attained and does not coincide with the given time. Questions Plot a graph of temperature (y axis) against time. 40 35 30 25 20 0 30 60 9T0ime 1(s2)0 150 180 210 240 The highest temperature attained can now be read from the graph when the two lines are extrapolated. The highest temperature attained is 390C. Other questions that may follow is to calculate the enthalpy, which you can now attempt. Calculate the molar enthalpy of solution of sodium hydroxide. (Take density of water as 1g/cm3, specific heat capacity of the water as 4.2kJKg-1K-1, Na=23.0, O=16.0, H=1.0) 187

Tactics of Chemistry ii. Time NOTE: The accuracy of the time depends on the stopwatch provided. Time is a measure of duration of a change in a chemical reaction. The stop watch commonly used in the secondary school laboratory are shown below. Stop watch 1 Stop watch 2 Minutes Seconds Centiseconds Hours Minutes Seconds Centiseconds These watches have three control knobs and the function for each is are usually indicated. They include Reset - This takes the stopwatch reading back to zero. Start/Stop - Start timing or stop timing Mode - Changes the watch from the normal time to stop watch timing. Stop watch 1 read 0:0000, the readings are: zero minutes, zero When reset, it second and zero centisecond. When you start and stop immediately, if it reads 0:0045, these are 45centiseconds. If it reads 0:0515 , these are 5 seconds and 15 centiseconds or 5.15seconds. If it reads 3:4375, these are 3 minute, 43 seconds and 75 centiseconds. If required to give time in seconds you have to convert the minutes into seconds ie {(3 x 60) + 43} = 180 + 43 = 223.75seconds. (Though most question in an exam involves seconds only) 188