Cambridge A Level Chemistry 9701 — 2024 May/June Paper 3 · Variant 5

1 Iodide ions in aqueous solution are oxidised to iodine by a variety of oxidising agents. One of these is the peroxodisulfate ion, S2O82−, which reacts as shown. 2I−(aq) + S2O82−(aq) I2(aq) + 2SO42−(aq) Sodium thiosulfate is added to the reaction mixture to react with iodine as it is produced. When all of the thiosulfate has reacted, further iodine produced reacts with starch indicator to give a dark colour. You will carry out two experiments to investigate how the rate of this reaction is affected by changing the concentration of the peroxodisulfate ion. FA 1 is 0.0200 mol dm−3 potassium peroxodisulfate, K2S2O8. FA 2 is 0.00500 mol dm−3 sodium thiosulfate, Na2S2O3. FA 3 is 1.00 mol dm−3 potassium iodide, KI. FA 4 is starch indicator. (a) Method Experiment 1 ● Label one of the 100 cm3 beakers A and the other 100 cm3 beaker B. ● Fill one burette with FA 1. Label this burette FA 1. ● Run 20.00 cm3 of FA 1 from the burette into beaker A. ● Fill the second burette with FA 2. Label this burette FA 2. ● Run 10.00 cm3 of FA 2 from the burette into beaker B. ● Use the measuring cylinder to add 20.0 cm3 of FA 3 to beaker B. ● Add 10 drops of FA 4 to beaker B. ● Add the contents of beaker A to beaker B and start timing immediately. ● Stir the mixture once and place the beaker on the white tile. ● Stop timing as soon as the solution turns a dark colour. ● Record this time to the nearest second in the space for results. ● Wash out both beakers and dry them using paper towel. Experiment 2 ● Run 10.00 cm3 of FA 1 from the burette into beaker A. ● Run 10.00 cm3 of FA 2 from the burette into beaker B. ● Use the measuring cylinder to add 20.0 cm3 of FA 3 into beaker B. ● Use the same measuring cylinder to add 10.0 cm3 of distilled water to beaker B. ● Add 10 drops of FA 4 to beaker B. ● Add the contents of beaker A to beaker B and start timing immediately. ● Stir the mixture once and place the beaker on the white tile. ● Stop timing as soon as the solution turns a dark colour. ● Record this time to the nearest second. Record all your data in a table. You should include the volume of FA 1, the volume of distilled water, the reaction time and the rate of reaction for both experiments. Use the following formula to calculate the rate of reaction. 1000 rate = reaction time Results I II III IV V VI [6] (b) (i) Explain why the concentration of potassium peroxodisulfate used in each experiment is proportional to the volume of FA 1 used. ........................................................................................................................................... ..................................................................................................................................... [1] (ii) A student thinks that the rate of reaction is proportional to the concentration of FA 1. Complete Table 1.1 to suggest volumes of reactants that could be used in a further experiment to confirm whether the student is correct. Do not carry out this experiment. Table 1.1 volume / cm3 volume FA 1 FA 2 FA 3 distilled water FA 4 10 drops [2] (c) A student correctly carried out the method in (a) but had been given a more concentrated solution of sodium thiosulfate. State how you would expect the student’s times to differ from yours. Explain your answer. ................................................................................................................................................... ............................................................................................................................................. [1] (d) The potassium iodide is in a large excess in Experiments 1 and 2. Suggest why a large excess of iodide ions is needed in these experiments. ................................................................................................................................................... ............................................................................................................................................. [1] [Total: 11]

Mark scheme: 1(a) I A single table with 4 headings for results with indication of the four items of data to be recorded (volume of FA 1, volume of water, time and rate) AND two experiments carried out II Correct headings and units for four required items of data:  volume of FA 1 (used) AND / cm3 OR (cm3) OR in cm3  volume of (distilled) water AND / cm3 OR (cm3) OR in cm3  (reaction) time AND / s OR (s) OR in seconds  rate AND /s–1 OR (s–1) OR in s–1 III Precision of data recorded:  Volume of FA1 recorded to 2 decimal places with the final digit being 0 or 5  Volume of Water recorded to 1 decimal place with the final digit being 0 or 5  all times to the nearest second IV Rates correctly calculated using 1000 / time. Accuracy marks Correct times for each of experiments 1 and 2 to the nearest second. Calculate time for experiment 2 time for experiment1 correct to 2 decimal places. V Award if ratio, t2 / t1 = 1.80 – 2.20 VI Award if ratio, t2/ t1 = 1.90 – 2.10 1(b)(i) total volume was the same for both experiments OR the volume of water + FA1 is 20 cm3 in both experiments 1 1(b)(ii) M1 FA 1 + distilled water = 20.00 cm3 AND volume of FA 1 is at least 2.5 cm3 from other experiments M2 FA 2 = 10.0 cm3 AND FA 3 = 20.0 cm3 2 1(c) time would increase AND (extra) thiosulfate reacts with more iodine/more iodine has to be formed to / before it can react with starch 1 Question Answer Marks 1(d) so concentration of KI / I− remains almost constant OR only the concentration of S2O82−/K2S2O8 affects the rate 1

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2 You will carry out an experiment to determine the enthalpy change, ΔH, when one mole of ammonium chloride dissolves in water. FA 5 is ammonium chloride, NH4Cl. (a) Method ● Weigh the container with FA 5. Record the mass in the space for results. ● Support the cup in the 250 cm3 beaker. ● Use the measuring cylinder to transfer 25.0 cm3 of distilled water into the cup. ● Place the thermometer in the water and tilt the cup, if necessary, so that the bulb of the thermometer is fully covered. Record the temperature of the water at time t = 0. ● Start the stop-clock and leave it running for the whole experiment. ● Measure and record the temperature of the water in the cup every half minute for 2 minutes. ● At t = 2½ minutes, tip all the FA 5 into the cup. Stir the contents of the cup. ● Measure and record the temperature of the contents of the cup at t = 3 minutes and then every half minute up to and including t = 8 minutes. ● Weigh the container with any residual FA 5. Record the mass. ● Calculate and record the mass of FA 5 added. Results I II III IV V [5] (b) Plot a graph of temperature (y-axis) against time (x-axis) on the grid. You should choose a scale that allows you to plot 2 °C below the minimum temperature reached. Label any points you consider to be anomalous. Draw two straight lines of best fit. One line is for the temperature before adding FA 5 and the other line is for the warming of the solution once the minimum temperature has been reached. Extrapolate both these lines to t = 2½ minutes. (ii) Calculate the energy change, in J, in the reaction. energy change = .............................. J [1] (iii) Calculate the amount, in mol, of ammonium chloride used. amount of NH4Cl = .............................. mol [1] (iv) Calculate the enthalpy change, ΔH, in kJ mol−1, when one mole of ammonium chloride dissolves in water. ΔH = ......... ................... kJ mol−1 [1] sign value (d) Use your results in (a) to calculate the maximum percentage error for the temperature change from 0 to 4 minutes. Assume that the maximum uncertainty in a single thermometer reading is ±0.5 °C. Show your working. maximum percentage error = .............................. % [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 Records 16 thermometer readings with appropriate times and correctly displayed units. II Records two balance readings and correctly calculated mass of FA 5 added with correctly displayed unit. III Balance readings consistent to either 2 or 3 decimal places AND thermometer readings to .0 °C or .5 °C. At least one must be .0 °C and one .5 °C. Accuracy marks Correct relevant thermometer readings to nearest .5 °C before subtraction. Calculate the difference  between supervisor and candidate values. Tsup / ºC <5.0 5.0–9.5 10.0–14.5 15.0–19.5 IV 0.5 1.0 2.0 2.5 V 0 0.5 1.5 2.0 2(b) I Temperature on y-axis and time on x-axis with unambiguous labels and units II Linear scales based on 1, 2 or 5 AND scale chosen so that plotted points occupy more than half the available space along each axis including 2 °C below minimum temperature recorded. III All points recorded plotted correctly to within half a small square AND a minimum of 9 temperatures recorded. IV Draws two lines of best fit AND both lines are extrapolated to 21/2 minutes AND temperature at 21/2 minutes is less than or equal to lowest temperature recorded. 4 Question Answer Marks 2(c)(i) Correct value of T from graph to 1 decimal place 1 2(c)(ii) Correctly calculates energy = 25  4.18  T (J) AND answer to 2–4 significant figures. 1 2(c)(iii) Correctly calculates amount of ammonium chloride n(NH4Cl) = mass from (a)/53.5 (mol) AND answer to 2–4 significant figures. 1 2(c)(iv) Correctly uses H = (c)(ii)/(c)(iii)  1000 (kJ mol–1) AND correct sign (+) AND answer correct to 2–4 significant figures 1 2(d) Correct expression {(0.5  2)/(T0 – T4)}  100 1

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3 (a) FA 6 is a salt containing a Group 1 ion and an anion that consists of a transition metal and a non-metal element. (i) Transfer FA 6 into a hard-glass test-tube. Heat the tube gently at first and then strongly. Record all your observations and identify the gas produced. The residue is FA 7. You will use FA 7 in (a)(ii). observations ...................................................................................................................... ........................................................................................................................................... gas produced .................................................................................................................... [3] Allow FA 7 to cool before starting (a)(ii). While FA 7 is cooling you may wish to continue with (b)(i). (ii) Put a 2 cm depth of acidified aqueous potassium manganate(VII) in a test-tube. Add the same depth of aqueous sodium hydroxide. Then add FA 7 and stir using the glass rod for about 30 seconds. Filter the mixture and collect the filtrate. Record your observations. ........................................................................................................................................... Put a 1 cm depth of the filtrate in a test-tube. Add sulfuric acid until in excess. Record your observations. ........................................................................................................................................... [2] (b) (i) FA 8 and FA 9 are both aqueous solutions of salts. FA 8 contains one cation and one anion. FA 9 contains two cations and one anion. One of the cations and both anions are listed in the Qualitative analysis notes. Carry out the following tests and record your observations in Table 3.1. For each test use a 1 cm depth of FA 8 or FA 9 in a test-tube. Table 3.1 observations test FA 8 FA 9 Test 1 Add sulfuric acid. Test 2 Add aqueous sodium hydroxide, then transfer the mixture into a boiling tube and warm. Test 3 Add a few drops of aqueous barium chloride or aqueous barium nitrate, then add nitric acid. Test 4 Add FA 8 with shaking until in excess. [7] (ii) Deduce the identity of the three ions listed in the Qualitative analysis notes that are present in FA 8 and FA 9. Suggest the identity of one other cation. Give the formula of each ion. If you cannot identify an ion write ‘unknown’. FA 8 contains .............................. and .............................. . FA 9 contains .............................. and .............................. and .............................. . [2] (iii) Write an ionic equation for one reaction that occurred in Test 2 in Table 3.1. Include state symbols. ..................................................................................................................................... [1] [Total: 15]

Mark scheme: 3(a)(i) M1 Any two of:  FA 6 / solid is (dark) purple crystals  solid jumps around  solid becomes darker / black residue forms M2 gas relights glowing splint M3 oxygen 3 3(a)(ii) M1 (filtrate is dark) green (solution) M2 (solution) turns pink/ purple 2 3(b)(i) FA 8 FA 9 Test 1: + acid(aq): effervescence (owtte) * tests gas/ CO2 with limewater* gas/ CO2 gives white ppt (with limewater) * no change / solution stays blue * Test 2: + NaOH(aq) then excess no change * on warming: no change / no gas (turning red litmus blue)* (pale) blue ppt * insoluble in excess * on warming: (ppt) turns black * Test 3: + BaCl2/Ba(NO3)2 then HNO3 white ppt * effervescence (with acid) * forms (colourless) solution / ppt dissolves * white ppt * insoluble (in acid) / no change * Test 4: FA 8 + FA 9 effervescence * (pale) blue ppt * 2* = 1 mark (round down) (16 * available MAX 7 marks) 7 Question Answer Marks 3(b)(ii) FA 8 unknown and CO32− FA 9 Cu2+ and H+ and SO42− All correct = 2 marks, 3 correct = 1 mark 2 3(b)(iii) Any one of: Cu2+(aq) + 2OH−(aq) → Cu(OH)2(s) Cu(OH)2(s) → CuO(s) + H2O(l) 1

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