Cambridge A Level Chemistry 9701 — 2021 Oct/Nov Paper 3 · Variant 4

1 The thiosulfate ion, S2O32–, is unstable in the presence of acid. The following reaction occurs. S2O32–(aq) + 2H+(aq) → S(s) + SO2(aq) + H2O(l) The rate of this reaction can be measured by timing how long it takes for the solid sulfur that is formed to make the mixture too cloudy to see through. You will investigate how the concentration of the thiosulfate ions affects the rate of this reaction. Throughout these experiments care must be taken to avoid inhaling any SO2 that is produced. It is very important that as soon as each experiment is complete, the contents of the beaker are emptied into the quenching bath and the beaker is rinsed thoroughly. FB 1 is 0.100 mol dm–3 sodium thiosulfate, Na2S2O3. FB 2 is 2.00 mol dm–3 hydrochloric acid, HCl. distilled water (a) Method Experiment 1 ● Label one burette FB 1 and fill it with FB 1. ● Run 45.00 cm3 of FB 1 from the burette into the 100 cm3 beaker. ● Use the 25 cm3 measuring cylinder to measure 10.0 cm3 of FB 2. ● Add FB 2 to FB 1 and start timing immediately. ● Stir the mixture once and place the beaker on the printed insert. ● View the print on the insert from above the mixture. ● Stop timing when the print on the insert is no longer visible. ● Record this reaction time to the nearest second. ● Empty the contents of the beaker into the quenching bath. ● Rinse and dry the beaker so it is ready for use in Experiment 2. Experiment 2 ● Fill the second burette with distilled water. ● Refill the burette labelled FB 1 with FB 1. ● Run 20.00 cm3 of FB 1 into the 100 cm3 beaker. ● Run 25.00 cm3 of distilled water into the same beaker. ● Use the 25 cm3 measuring cylinder to measure 10.0 cm3 of FB 2. ● Add FB 2 to the beaker and start timing immediately. ● Stir the mixture once and place the beaker on the printed insert. ● View the print on the insert from above the mixture. ● Stop timing when the print on the insert is no longer visible. ● Record this reaction time to the nearest second. ● Empty the contents of the beaker into the quenching bath. ● Rinse and dry the beaker so it is ready for use in the next experiment. Experiments 3–5 ● Carry out three further experiments to investigate how the reaction time changes with different volumes of FB 1. The combined volume of FB 1 and distilled water must always be 45.00 cm3. Do not use a volume of FB 1 that is less than 20.00 cm3. Record all your results in a table. You should include the volume of FB 1, the volume of distilled water, the reaction time and the reaction rate for each of your five experiments. Calculate the rate of reaction using the following formula. 1000 rate = reaction time I II III IV V VI VII VIII [8] (b) On the grid opposite, plot the rate on the y-axis against the volume of FB 1 on the x-axis. Identify any anomalous points. Draw a line of best fit. (c) In these experiments, the volume of FB 1 is related to the concentration of the thiosulfate ions. Use your graph to suggest the relationship between the rate of reaction and the concentration of the thiosulfate ions. .................................................................................................................................................... .............................................................................................................................................. [1] (d) The quenching bath contains an aqueous mixture of sodium carbonate and universal indicator. (i) How does the quenching bath prevent the further production of SO2 from the reaction? ............................................................................................................................................. ....................................................................................................................................... [1] (ii) Suggest why the mixture contains universal indicator. ............................................................................................................................................. ....................................................................................................................................... [1] (e) (i) In each experiment the acid is in large excess. Show, by calculation, that the acid is in large excess in Experiment 1. [2] (ii) Suggest a reason why the acid used should be in large excess. ............................................................................................................................................. ....................................................................................................................................... [1] [Total: 18]

Mark scheme: 1(a)(i) I Constructs a single 4 × 5 table for results for 5 experiments AND at least one experiment attempted 1 II Correct headings and units for data given. • volume of FB1 and in cm3 or / cm3 or (cm3) • volume of water and in cm3 or / cm3 or (cm3) • time and in seconds or / s or (s) • rate and /s–1 or (s–1) 1 III All times recorded to the nearest second AND all volumes given to nearest 0.05 1 IV 3 additional volumes chosen with intervals not less than 5.00 cm3 AND all additional volumes of FB 1 ⩾ 25.00 cm3 AND In all 3 additional experiments water is added to make a total of 45.00 cm3 1 V All rates correctly calculated using 1000 / time AND all recorded to the same number of dp or same number of sf. 1 VI All five times increase with decreasing volume of FB 1 1 VII & VIII Compare ratio of time for 20.00 cm3 of FB 1 / time for 45.00 cm3 of FB 1 to 2 dp. Award VII for ratio 2.10 – 2.60 Award VIII for ratio 2.20 − 2.50 2 Question Answer Marks 1(b) I Rate on y axis and volume of FB 1 on the x axis With correct labels and units. Do not penalise missing /incorrect unit if penalised in 1(a) II (ecf) 1 II Linear scales chosen so that the graph occupies more than half the available length for both axes (7 × 5 big squares) 1 III All points recorded plotted correctly to within half a small square and in the correct square or on the line if it should be on the line and not on the line if it shouldn’t. 1 IV Draws a line of best fit. This can be a straight line or smooth curve. 1 1(c) Rate is proportional to concentration of thiosulfate 1 1(d)(i) (Carbonate/ quenching bath) removes H+ /neutralises acidic solution (so reaction stops). 1 1(d)(ii) To know when all the carbonate has been neutralised / no more carbonate (solution) remains / more carbonate needs to be added OR mixture becomes acidic /pH drops below 7 /pH is no longer ⩾ 7 1 1(e)(i) M1: moles of H+ = 2(.00) × 10–2 AND moles of S2O32– = 4.5(0) × 10–3 M2: 2(.00) x 10–2 > 2 × 4.5(0) × 10–3 / 9(.0) × 10–3 Other valid methods exist (e.g., comparison of volume of acid required to react and volume used). 2 1(e)(ii) So acid concentration remains (almost) constant throughout the experiment OR only the conc of thiosulfate affects the rate. 1

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2 In this experiment you will determine the concentration of a solution of copper(II) sulfate. You will react an excess of zinc with copper(II) sulfate as shown. Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s) FB 3 is zinc powder. FB 4 is aqueous copper(II) sulfate, CuSO4. (a) Method ● Support the cup in the 250 cm3 beaker. ● Using the 50 cm3 measuring cylinder, transfer 40 cm3 of FB 4 into the cup. ● Measure and record the temperature of the solution in the cup. ● Add all of FB 3 to the cup. ● Use the thermometer to stir the mixture gently. ● Measure and record the maximum temperature reached. ● Calculate and record the change in temperature. [3] (b) Calculations (i) Use your results from (a) to calculate the heat energy produced in the reaction. (Assume that 4.2 J are required to change the temperature of 1.0 cm3 of solution by 1.0 °C.) heat energy produced = .............................. J [1] (ii) You can assume that under the conditions of your experiment the molar enthalpy change for the reaction is –218.7 kJ mol–1. Use this value to calculate the concentration, in mol dm–3, of FB 4. concentration of FB 4 = .............................. mol dm–3 [2] (c) (i) Calculate the maximum percentage error in the temperature rise that you recorded in (a). Assume that the maximum error in a single thermometer reading is ± 0.5 °C. maximum percentage error = .............................. % [1] (ii) The maximum percentage error becomes smaller if the temperature rise is increased. A student suggests that using a greater volume of FB 4 would increase the temperature rise as long as the zinc remains in excess. Explain whether the student is correct. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] [Total: 9] Qualitative analysis Where reagents are selected for use in a test, the name or correct formula of the element or compound must be given. At each stage of any test you are to record details of the following: ● colour changes seen ● the formation of any precipitate and its solubility in an excess of the reagent added ● the formation of any gas and its identification by a suitable test. You should indicate clearly at what stage in a test a change occurs. If any solution is warmed, a boiling tube must be used. Rinse and reuse test-tubes and boiling tubes where possible. No additional tests for ions present should be attempted.

Mark scheme: 2(a) M1: Unambiguous headings and units: / °C or (°C). • two thermometer readings • change / increase in temperature / DT AND subtraction correct. M2: Both thermometer readings recorded to either 0.0 or 0.5 °C M3: Award 1 mark for 15.0 ⩽ ΔT ⩽ 35.0 2(b)(i) Correctly calculates ΔT × 4.2 × 40 AND answer expressed to 2 – 4 sf 1 2(b)(ii) M1: Shows use of moles of CuSO4 = ans (b)(i) / (218.7 × 1000) OR uses (b)(i) / (40/1000) M2: Correctly uses conc = (b)(i) / (218.7 × 1000) × 0.04 AND answer expressed to 2 – 4 sf (unless already penalised in (b)(i)). 2 2(c)(i) Shows use of ((2 × 0.5) / temp rise) × 100 AND answer is correct 1 2(c)(ii) M1: The student is incorrect AND The temperature rise would remain the same M2: The heat given out per unit volume is the same OR Although more heat is given out the volume is greater 2

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3 (a) FB 5 is an aqueous solution containing three cations and a single anion. Two of the cations are listed in the Qualitative Analysis Notes. The anion is either the sulfate ion, SO42–, or the sulfite ion, SO32–. (i) Carry out tests to identify the three cations. Record your tests and observations. I II III IV V VI VII The formulae for the cations present in FB 5 are .............. , .............. and .............. . [7] (ii) Carry out tests to identify whether FB 5 contains the sulfate ion, SO42–, or the sulfite ion, SO32–. Record your observations. The anion present in FB 5 is .............................. . [2] (b) (i) Carry out the following tests on FB 6 and FB 7 and record your observations. test observations Test 1 Add a small spatula measure of FB 6 to a hard‑glass test‑tube. Heat the sample gently at first and then more strongly. Test 2 Add a small spatula measure of FB 7 to a hard‑glass test‑tube. Heat the sample strongly. [3] (ii) State the type of reaction observed with FB 6 in (b)(i). ....................................................................................................................................... [1] [Total: 13]

Mark scheme: 3(a)(i) I Selects (aqueous) NaOH and / or (aqueous) NH3. 1 II Selects named carbonate or Mg strips or named pH indicator 1 III With either NaOH or NH3: green ppt AND turning brown /insoluble in excess 1 IV Warm with NaOH AND gas /NH3 that turns (damp red) litmus blue 1 V With carbonate: effervescence / fizzing / bubbling OR gas / CO2 gives white ppt with limewater / limewater turns milky / chalky / cloudy white With Mg: effervescence / fizzing / bubbling OR (gas / H2) pops with a lighted splint With named indicator: change to correct colour to indicate acidic solution 1 VI mark for any two ions correctly identified. VI and VII for all three ions correct: NH4+, H+ and Fe2+ Minimum evidence: • green ppt or ppt turns brown or insoluble in excess with NaOH / NH3 for Fe2+ • fizz with Mg or CO32– or A / or correct final colour of indicator • litmus turns blue 2 Question Answer Marks 3(a)(ii) M1: Selects BaCl 2 or Ba(NO3)2 followed by HCl or HNO3 (or names) (H2SO4 CONS both marks) OR Add either specified acid with BaCl 2 or Ba(NO3)2 OR Warm solution with any named mineral acid and test gas with litmus paper / (H+) / KMnO4 paper M2: White ppt insoluble in (unspecified) acid OR White ppt with acidified BaCl 2 or Ba(NO3)2 (or names) OR Gas does not turn blue litmus red or does not decolourise (H+) / KMnO4 paper AND Sulfate identified. 2 3(b)(i) Observations for FB 6 Melts / turns liquid * Turns yellow * Fizzing / effervescence * Gas / O2 relights glowing splint * Observations for FB 7 Sublimation / white fumes / white smoke forms / white solid forms near top of tube * Gas / NH3 turns (red) litmus blue * Gas turns (Blue) litmus red (must be in correct order) * 3 3(b)(ii) Thermal decomposition OR Redox 1

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