Cambridge A Level Chemistry 9701 — 2024 May/June Paper 3 · Variant 1

1 In this experiment you will determine the enthalpy change, ∆Hr , for the reaction shown. NaOH(aq) + CO2(g) NaHCO3(aq) enthalpy change = ΔHr You will react each of sodium hydroxide and sodium hydrogencarbonate with excess dilute sulfuric acid. You will determine the enthalpy change for each reaction, then use Hess’s law to calculate ΔHr . (a) Reaction of sodium hydroxide with sulfuric acid 2NaOH(aq) + H2SO4(aq) Na2SO4(aq) + 2H2O(l) enthalpy change = ΔH1 FA 1 is 2.00 mol dm–3 sodium hydroxide, NaOH. FA 2 is 2.00 mol dm–3 sulfuric acid, H2SO4. Method ● Support a cup in the 250 cm3 beaker. ● Use the 50 cm3 measuring cylinder to transfer 30.0 cm3 of FA 1 into the cup. ● Place the thermometer in FA 1 and tilt the cup, if necessary, so that the bulb of the thermometer is fully covered. Record the temperature of FA 1. ● Use the 25 cm3 measuring cylinder to add 20.0 cm3 of FA 2 to the FA 1 in the cup. ● Stir the mixture. ● Measure and record the maximum temperature reached. ● Calculate and record the change in temperature. Results I II [2] (b) Calculations (i) Calculate the energy change, in J, in your experiment. energy change = .............................. J [1] (ii) Calculate the amount, in mol, of sulfuric acid that reacted with FA 1 in your experiment. amount of H2SO4 = .............................. mol [1] (iii) Calculate the enthalpy change of reaction, ΔH1, in kJ mol–1 of sulfuric acid, for the neutralisation of NaOH(aq) with H2SO4(aq). Show your working. ΔH1 = ......... ................... kJ mol–1 of H2SO4 [1] sign value (c) Reaction of sodium hydrogencarbonate with sulfuric acid 2NaHCO3(s) + H2SO4(aq) Na2SO4(aq) + 2H2O(l) + 2CO2(g) enthalpy change = ΔH2 FA 2 is 2.00 mol dm–3 sulfuric acid, H2SO4. FA 3 is sodium hydrogencarbonate, NaHCO3. Method ● Support the other cup in the 250 cm3 beaker. ● Use the 25 cm3 measuring cylinder to transfer 25.0 cm3 of FA 2 into the cup. ● Place the thermometer in FA 2 and tilt the cup, if necessary, so that the bulb of the thermometer is fully covered. Record the temperature of FA 2. ● Weigh the container with FA 3. Record the mass. ● Adding small quantities at a time, tip all the FA 3 from the container into the FA 2 in the cup. ● Stir the mixture. ● Measure and record the minimum temperature reached. ● Calculate and record the change in temperature. ● Weigh the container with any residual FA 3. Record the mass. ● Calculate and record the mass of FA 3 added. Results I II III [3] (d) Calculations Use your data to calculate the enthalpy change, ΔH2, in kJ mol–1 of sulfuric acid, for the reaction of NaHCO3(s) with H2SO4(aq). Show your working. ΔH2 = ......... ................... kJ mol–1 of H2SO4 [3] sign value (e) The enthalpy change when one mole of sodium hydrogencarbonate dissolves in water is ΔH3. NaHCO3(s) + aq NaHCO3(aq) enthalpy change = ΔH3 Using the symbols ΔH1, ΔH2 and ΔH3 in your answer, use Hess’s law to deduce an expression for ΔHr . NaOH(aq) + CO2(g) NaHCO3(aq) enthalpy change = ΔHr ∆Hr = ...................................................................... kJ mol–1 [1] (f) A student suggested that the experiment in (c) would be more accurate if 25.0 cm3 of 3.00 mol dm–3 sulfuric acid was used instead of 25.0 cm3 of 2.00 mol dm–3 sulfuric acid. State whether the student’s suggestion is correct. Explain your answer. ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [1] [Total: 13]

Mark scheme: 1(a) I Three pieces of data shown  Two thermometer readings recorded AND temperature rise correctly calculated, all listed in the space provided, with units.  ‘Correct’ headings are not required for this mark, but it must be clear what the three figures refer to.  Ignore precision of readings (but see II of 1(c)(i)).  Do not award if either thermometer reading is below 10 °C.  Temperature units: / °C OR in °C OR (°C) OR °C by every reading. Accuracy mark Calculate the difference  between temperature rises for supervisor and candidate. Supervisor value ⩾ 10 °C, award mark II if ⩽ 1.5 °C. Supervisor value ⩾ 5 °C and < 10 °C, award mark II if ⩽ 1.0 °C. Supervisor value < 5 °C, award mark II if ⩽ 0.5 °C. 1(b)(i) correctly calculates energy released energy = 50  4.18  temp rise (J) AND answer given to 2–4 significant figures 1 1(b)(ii) amount H2SO4 used = (0.030  2  0.5) = 0.03(000) (mol) 1 1(b)(iii) correct use of data to calculate H1 H1 = energy/1000  (b)(ii) AND answer must have negative sign AND be expressed to 2–4 significant figures 1 Question Answer Marks 1(c) I Six correct headings shown, with units, in list / table  (mass of) container + FA 3 / solid / NaHCO3  (mass of) container (empty or with residual FA 3)  (mass of) FA 3 / solid / NaHCO3 OR mass used  initial temperature / thermometer reading  lowest / minimum / final temperature / thermometer reading  temperature change/decrease Mass units: / g OR in g OR (g) OR g by every reading Temperature units: / °C OR in °C OR (°C) OR °C by every reading II All readings shown to appropriate precision  all four balance / thermometer readings shown  all four thermometer readings [including those in 1(a)] to .0 or .5  both masses to 2 decimal places OR both masses to 3 decimal places III Subtractions correct  mass of solid correct AND between 5.00–7.00 g  temperature fall correct AND between 8.0–12.0 °C 3 1(d) Correct use of data expressions for M1 and M2 need not be evaluated to score M1 energy = 25  4.18  temp decrease M2 amount of H2SO4 used = 0.5  mass NaHCO3 used/84 (mol) M3 enthalpy change = energy / amount of H2SO4 used  1000 AND answer has positive sign AND expressed to two or more significant figures 3 1(e) Hr = 0.5 H1 – 0.5 H2 + H3 1 1(f) (student is not correct) because acid is used in excess (in candidate’s experiment (c)) (so temperature fall is unchanged) 1

More questions on Enthalpy change, ΔH

2 Sodium sulfite is oxidised when it reacts with excess iodine. Na2SO3(aq) + I2(aq) + H2O(l) Na2SO4(aq) + 2HI(aq) The remaining iodine is then titrated using aqueous sodium thiosulfate. I2(aq) + 2Na2S2O3(aq) Na2S4O6(aq) + 2NaI(aq) You will determine the integer value of x in the formula of hydrated sodium sulfite, Na2SO3•xH2O, by titration. FA 4 is aqueous sodium thiosulfate containing 14.24 g of Na2S2O3 in 1.00 dm3. FA 5 is aqueous iodine, prepared as shown. ● 5.00 g of hydrated sodium sulfite is added to 600 cm3 of 0.100 mol dm–3 aqueous iodine. ● The mixture is allowed to stand to ensure that all the sodium sulfite has been oxidised. ● The mixture containing the remaining iodine is made up to 1.00 dm3 with distilled water. FA 6 is starch indicator. (a) Method ● Fill the burette with FA 4. ● Pipette 25.0 cm3 of FA 5 into a conical flask. ● Add FA 4 from the burette into the conical flask until the colour of the solution changes to yellow. ● Add 10 drops of FA 6 to the conical flask. Continue titrating until the blue-black colour just disappears. ● Perform a rough titration and record your burette readings in the space below. The rough titre is .............................. cm3. ● Carry out as many accurate titrations as you think necessary to obtain consistent results. ● Make sure any recorded results show the precision of your practical work. ● Record, in a suitable form in the space below, all your burette readings and the volume of FA 4 added in each accurate titration. I II III IV V VI VII [7] (b) From your accurate titration results, calculate a suitable mean value to use in your calculations. Show clearly how you obtain the mean value. 25.0 cm3 of FA 5 required .............................. cm3 of FA 4. [1] (c) Calculations (i) Give your answers to (c)(ii), (c)(iii), (c)(iv) and (c)(v) to an appropriate number of significant figures. [1] (ii) Calculate the amount, in mol, of sodium thiosulfate present in the volume of FA 4 in (b). amount of Na2S2O3 = .............................. mol [1] (iii) Calculate the amount, in mol, of iodine in 1.00 dm3 of FA 5. amount of I2 in 1.00 dm3 = .............................. mol [1] (iv) Use the information given and your answer to (c)(iii) to calculate the amount, in mol, of iodine that reacted with sodium sulfite when solution FA 5 was prepared. amount of I2 reacted with sodium sulfite = .............................. mol [1] (v) Use your answer to (c)(iv) to calculate the relative formula mass, Mr , of hydrated sodium sulfite. Mr of Na2SO3•xH2O = .............................. [1] (vi) Calculate the value of x. Show your working. x = .............................. [1] [Total: 14] Qualitative analysis For each test you should record all your observations in the spaces provided. Examples of observations include: ● colour changes seen ● the formation of any precipitate and its solubility (where appropriate) in an excess of the reagent added ● the formation of any gas and its identification (where appropriate) by a suitable test. You should record clearly at what stage in a test an observation is made. Where no change is observed, you should write ‘no change’. Where reagents are selected for use in a test, the name or correct formula of the element or compound must be given. If any solution is warmed, a boiling tube must be used. If a solid is heated, a hard-glass test-tube must be used. Rinse and reuse test-tubes and boiling tubes where possible. No additional tests should be attempted.

Mark scheme: 2(a) I The following data are recorded:  two burette readings AND titre for the rough titration  initial and final burette readings for two (or more) accurate titrations. II Titre values recorded for accurate titrations AND correct headings and units in the accurate titration table.  initial / start AND (burette) reading / volume  final / end AND (burette) reading / volume  titre OR volume used / added / OR FA 4 used / added  unit: / cm3 OR (cm3) OR in cm3 (for each heading) OR cm3 unit given for each volume recorded. III All accurate burette readings recorded to 0.05 cm3. IV The final accurate titre recorded must be within 0.10 cm3 of any other accurate titre. 7 Accuracy marks Check and correct titre subtractions. Select the ‘best’ titres using the hierarchy (ignoring the rough titre value): 2 identical titres, 2 titres within 0.05 cm3, 2 titres within 0.10 cm3, etc. Calculate candidate’s mean titre to 2 decimal places. Calculate the difference  between the candidate’s titre and the supervisor’s titre. V Award if  ⩽ 0.60 cm3 VI Award if  ⩽.0.40 cm3 VII Award if  ⩽ 0.20 cm3 2(b) Candidate calculates mean titre correctly to 2 decimal places  Candidate must take the average of two (or more) titres that are within a total spread of not more than 0.20 cm3.  Working/ explanation must be shown OR ticks must be put next to the two (or more) accurate readings selected.  The mean should be quoted to 2 decimal places and be rounded to the nearest 0.01 cm3. 1 2(c)(i) All answers given for (c)(ii), (iii), (iv) and (v) given to 3 or 4 significant figures. 1 2(c)(ii) Correctly calculates amount of sodium thiosulfate used = 14.24/158.2  mean titre/1000 (mol) 1 Question Answer Marks 2(c)(iii) Correct use of 2(c)(ii) amount of iodine (in 1 dm3) = (c)(ii)/2  1000/25(mol) 1 2(c)(iv) Correct use of 2(c)(iii) amount of iodine reacting with sodium sulfite = 0.0600 – (c)(iii) 1 2(c)(v) Correct use of 2(c)(iv) Mr of sodium sulfite = 5.0/(c)(iv) 1 2(c)(vi) Correct use of 2(c)(v) x = [(c)(v) – 126.1]/18 answer must be correctly rounded to an integer 1

More questions on Reacting masses and volumes (of solutions and gases)

3 (a) FA 7, FA 8 and FA 9 are dilute ethanoic acid, dilute hydrochloric acid and aqueous silver nitrate but not necessarily in that order. The solutions of acids have equal concentrations. (i) You are supplied with strips of magnesium ribbon. You must not use any other reagents in this part of the question. Carry out tests to identify each of the three solutions, FA 7, FA 8 and FA 9. Obtain as much evidence as you can for your identifications. Use a 1 cm depth of solution in a test-tube for each test you carry out. Record all your observations. FA 7 is ....................................... . FA 8 is ....................................... . FA 9 is ....................................... . [5] (ii) Give the ionic equation for the reaction of magnesium with FA 8. Include state symbols. ..................................................................................................................................... [1] (b) FA 10 and FA 11 are both aqueous solutions of salts, each of which contains one cation and one anion listed in the Qualitative analysis notes. (i) Carry out the following tests and record your observations in Table 3.1. For Tests 1 and 2, use a 1 cm depth of FA 10 or FA 11 in a test-tube. For Test 3, use a 1 cm depth of FA 10 or FA 11 in a boiling tube. Table 3.1 observations test FA 10 FA 11 Test 1 Add aqueous ammonia. Test 2 Add a few drops of aqueous barium chloride or aqueous barium nitrate, then add dilute hydrochloric acid. Test 3 Add aqueous sodium hydroxide, then warm the mixture carefully, then add one piece of aluminium foil. [5] (ii) Use your observations in (b)(i) to complete Table 3.2 by identifying the formulae of the ions present in FA 10 and FA 11. If you cannot identify an ion write ‘unknown’. Table 3.2 cation anion FA 10 FA 11 [2] [Total: 13]

Mark scheme: 3(a)(i) M1 adds Mg to FA7: records dark grey / black AND solid / precipitate / deposit / coating / layer M2 adds Mg to FA8 and FA9: records fizzing / effervescence in both reactions M3 clear indication that the reaction with magnesium of FA9 is faster / more vigorous than the reaction with magnesium of FA8 (ORA) M4 Any one of the following:  (gas) pops with lighted splint in either the Mg / FA8 or Mg / FA9 reactions  Mg dissolves / disappears AND colourless solution formed / heat produced in either the Mg / FA8 or Mg / FA9 reactions  white precipitate forms when FA7 and FA9 are mixed. M5 all three identities correct: FA7 = AgNO3, FA8 = CH3COOH and FA9 = HCl 5 3(a)(ii) Mg(s) + 2H+(aq) → Mg2+(aq) + H2(g) 1 Question Answer Marks 3(b)(i) FA10 FA11 Test 1 NH3  white ppt AND insoluble in excess  white precipitate  soluble in excess Test 2 Ba2+  white precipitate AND  no reaction / no change / no ppt AND HCl (white ppt) insoluble / no change no reaction / no change no ppt Test 3 NaOH  white ppt AND insoluble in excess  white precipitate  soluble in excess Heat Ignore anything written in either of these boxes, unless any positive gas test is recorded (including “litmus goes red”), in which case it contradicts the gas test bullet in the box below the gas test – mark by column. + Al  fizzing / effervescence  (gas) pops with a lighted splint OR gas does not turn (red) litmus blue  fizzing / effervescence  gas turns (red) litmus blue. Two points needed for each mark. 5 3(b)(ii) cation anion FA 10  Mg2+  SO42– FA 11  Zn2+  unknown Two points needed for each mark. 2

More questions on Some reactions of the halide ions