Cambridge A Level Chemistry 9701 — 2023 May/June Paper 3 · Variant 1

1 The thiosulfate ion, S2O32–, decomposes when an acid is added. S2O32–(aq) + 2H+(aq) S(s) + SO2(aq) + H2O(l) The rate of this reaction can be investigated by measuring how long it takes the solution to produce enough sulfur so that it cannot be seen through. You will investigate how the concentration of the thiosulfate ion affects the rate of the reaction. Note: A small amount of sulfur dioxide gas may be formed in the experiment. It is very important that you avoid inhaling any fumes. As soon as each experiment is complete, empty the reaction mixture into the quenching bath and rinse the beaker thoroughly. FA 1 is 0.100 mol dm–3 sodium thiosulfate, Na2S2O3. FA 2 is 2.00 mol dm–3 hydrochloric acid, HCl. (a) Method Experiment 1 • Fill a burette with FA 1. • Run 40.00 cm3 of FA 1 into the 100 cm3 beaker. • Use the 25 cm3 measuring cylinder to measure 10.0 cm3 of FA 2. • Add the FA 2 to the FA 1 in the beaker and start timing immediately. • Stir the mixture once and place the beaker on the printed insert. • View the printing on the insert from above, through the solution. • Stop timing when the print on the insert becomes obscured. • Record this reaction time to the nearest second in the space for results on page 4. • Empty the contents of the beaker into the quenching bath. • Rinse and dry the beaker and glass rod so they are ready to use in Experiment 2. Experiment 2 • Refill the burette with FA 1. • Fill the other burette with distilled water. • Run 20.00 cm3 of FA 1 into the 100 cm3 beaker. • Run 20.00 cm3 of distilled water into the same beaker. • Use the 25 cm3 measuring cylinder to measure 10.0 cm3 of FA 2. • Add the FA 2 to the solution in the beaker and start timing immediately. • Stir the mixture once and place the beaker on the printed insert. • View the printing on the insert from above, through the solution. • Stop timing when the print on the insert becomes obscured. • Record this reaction time to the nearest second. • Empty the contents of the beaker into the quenching bath. • Rinse and dry the beaker and glass rod so they are ready to use in the next experiment. Experiments 3–5 • Carry out three further experiments to investigate how using different volumes of FA 1 affects the reaction time. Note that the combined volumes of FA 1 and distilled water must always be 40.00 cm3. Do not use a volume of FA 1 that is less than 15.00 cm3. Record all your results in a table. You should include the volume of FA 1, the volume of distilled water, the reaction time and the reaction rate for each of your five experiments. The rate of reaction can be calculated using the following formula. 1000 rate = reaction time Results I II III IV V VI VII VIII [8] (c) In these experiments, the volume of FA 1 is a measure of the concentration of thiosulfate ions. A student suggested that the graph shows that the rate of reaction is directly proportional to the concentration of thiosulfate ions. Explain, using your graph, whether you agree with this student. ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [1] (d) Use your graph to calculate the time you would expect to record if you had used 12.50 cm3 of FA 1 and followed the same method. Show clearly on the graph how you calculated this time. time = .............................. s [2] (e) Another student broke the beaker and decided to use a Petri dish instead. beaker Petri dish Fig. 1.1 What effect, if any, would this have on the times measured in the experiment in (a)? Explain your answer. ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [2] [Total: 17]

Mark scheme: 1(a) I Single table to show results for five experiments (volume of FA 1, volume of water, time and rate) 8 II Appropriate headings and units for recorded data given Volume units for headings: / cm3 or (cm3) or in cm3 Time units for headings: / s or (s) or in seconds Rate units for headings: / s–1 or (s−1) or per second OR units of cm3, s or s–1 against each entry III All times recorded to nearest second AND all volumes given to 2 dp with the final digit being 0 or 5 cm3 IV Three additional volumes chosen with intervals not less than 5 cm3 AND all volumes of FA 1 not less than 15.00 cm3 V In all additional experiments water is added to make a total of 40.00 cm3 VI All rates correctly calculated using 1000 / time All recorded to minimum of two significant figures or integer value AND from at least three experiments Accuracy marks Calculate ratio: 3 3 Time for 20.00 cm of Time for 40.00 cm of FA 1 FA 1 to two decimal places VII Award for ratio 1.70–2.40 VIII Award for ratio 1.90–2.20 1(b) I Rate on y-axis and volume of FA 1 on x-axis with unambiguous labels or units 4 II Linear scales (starting at 0,0) chosen so that the graph occupies at least half the available length for both axes III Five recorded points plotted correctly IV Draws a line of best fit straight line or smooth curve AND anomalous point(s), if any, should be ringed Question Answer Marks 1(c) Yes / directly proportional and as straight line through / close to origin OR no / not directly proportional and line not through origin / curve 1 1(d) M1 Line drawn at 12.50 cm3 and line drawn across to rate axis OR marks on rate and volume axes within half a small square M2 Time correctly calculated from candidate’s rate 2 1(e) M1 Solution is less deep / shallower M2 Time would be longer because of the need to have more S / solid to obscure insert owtte 2

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2 Many salts contain water of crystallisation which can be removed by heating to form the anhydrous salt. You will determine the enthalpy change of dehydration for hydrated magnesium sulfate. MgSO4•7H2O(s) MgSO4(s) + H2O(l) You will determine this enthalpy change by measuring the changes in temperature when samples of hydrated magnesium sulfate and anhydrous magnesium sulfate are dissolved separately in excess water. FA 3 is hydrated magnesium sulfate, MgSO4•7H2O. FA 4 is anhydrous magnesium sulfate, MgSO4. (a) Method Experiment 1 • Weigh the container with FA 3. Record the mass in the space below. • Support the cup in the 250 cm3 beaker. • Rinse the 25 cm3 measuring cylinder with distilled water. 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. • Tip all of the FA 3 into the water in the cup. Stir the mixture thoroughly. • Record the highest or lowest temperature of the mixture. • Calculate the change in temperature. Record this change. • Weigh the container with any remaining FA 3. Record the mass. • Calculate the mass of FA 3 used. Record this mass. Experiment 2 • Repeat the method using FA 4 in place of FA 3. Use the second cup. Results I II III IV V [5] (b) Calculations (i) Calculate the heat energy transferred, in J, in each experiment. Assume that 4.18 J of heat energy changes the temperature of 1.0 cm3 of solution by 1.0 °C. Experiment 1 with FA 3 Experiment 2 with FA 4 heat energy = .............................. J heat energy = .............................. J [1] (ii) Calculate the enthalpy change, ∆H, in kJ mol–1, when 1.0 mol of solid dissolves in water in each experiment. Experiment 1 with FA 3 Experiment 2 with FA 4 ∆H1 = ...... ..................... kJ mol–1 ∆H2 = ...... ..................... kJ mol–1 sign value sign value [3] (iii) Use your answers to (b)(ii) to calculate the enthalpy change when 1.0 mol of hydrated magnesium sulfate is dehydrated to form 1.0 mol of anhydrous magnesium sulfate. MgSO4•7H2O(s) MgSO4(s) + 7H2O(l) Show clearly, by a Hess’s energy cycle or any other suitable means, how you calculated your answer. If you were unable to complete the calculation in (b)(ii) then assume that the enthalpy change in Experiment 1, ∆H1, is +8.7 kJ mol–1 and that the enthalpy change in Experiment 2, ∆H2, is –15.5 kJ mol–1. These may not be the correct values. enthalpy change of dehydration of MgSO4•7H2O = ...... ..................... kJ mol–1 [2] sign value [Total: 11] 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. Rinse and reuse test-tubes and boiling tubes where possible. No additional tests should be attempted.

Mark scheme: 2(a) I Unambiguous headings and units covering all entries  (mass of) container + solid FA 3 / FA 4  (mass of) container (+ residue)  initial thermometer reading / temperature  lowest / highest reading / temperature  mass solid (used / added) / FA 3 / FA 4  change in temperature / T Units: / g, (g), in g and / °C, (°C), in °C II All four balance readings to the same number of decimal places (2 or 3) AND thermometer readings to .0 °C or.5 °C AND written in the table / results space III Correctly calculates for both experiments  mass solid  change in temperature Accuracy marks Correct all temperatures to nearest .5 °C. Calculate T for supervisor and for candidate. T = initial temperature – lowest / highest temperature in table Calculate the difference, between supervisor and candidate values IV Award if ⩽ 1.0 °C for experiment 1 V Award if ⩽ 1.0 °C for experiment 2 2(b)(i) Correctly calculates Q = mcT for both experiments = 25  4.18  T AND answers to 2–4 significant figures 1 Question Answer Marks 2(b)(ii) M1 Correctly calculates Mr for each: 246.4 AND 120.4 M2 Shows correct working (may be in more than 1 step) 246.4 mass 1000   (b)(i) expt 1 FA 3 AND 120.4 mass 1000   (b)(i) expt 2 FA 4 M3 Shows correct sign for both values of H AND answers from attempted calculation to 2–4 significant figures 3 2(b)(iii) M1 Shows attempt at use of Hess’ law / energy level diagram / reverse equation (H =H1 – H2) M2 Correct answer to 2–4 significant figures with appropriate sign 2

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3 (a) (i) A student finds a container of a compound which is thought to be FA 3. The student labels the container FA 5. Carry out tests to determine whether your sample of FA 5 contains magnesium ions, sulfate ions and water of crystallisation. Record your tests and observations in a suitable form in the space below. If any solid is heated, a hard-glass test-tube must be used. [4] (ii) Complete Table 3.1 to indicate the contents of FA 5. Put a tick (3) in one box in each row. Table 3.1 yes no FA 5 contains magnesium ions FA 5 contains sulfate ions FA 5 contains water of crystallisation [1] (b) FA 6 contains one cation and one anion, both of which are listed in the Qualitative analysis notes. (i) In a hard-glass test-tube heat a spatula measure of FA 6. Heat gently at first and then heat strongly. Record all your observations. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (ii) Carry out the following tests and record your observations. Table 3.2 test observations Test 1 To a small spatula measure of FA 6 in a test-tube, add 5 cm depth of dilute hydrochloric acid. Keep the solution for Test 2. Test 2 Put 1 cm depth of the solution from Test 1 in a test-tube. Add aqueous ammonia dropwise at first and then until it is in excess. [2] (iii) Use your observations from (b)(i) and (ii) to identify the ions present in FA 6. cation .............................. anion .............................. [1] (iv) Write the ionic equation for the reaction that occurs in Test 2 before excess ammonia is added. Include state symbols. If you were unable to identify the cation in the solution then use the symbol M2+. ..................................................................................................................................... [2] [Total: 12]

Mark scheme: 3(a)(i) M1 Test for water Heat solid sample of FA 5 AND either (gentle heating) solution / liquid formed or solid / FA 5 dissolves / melts or observes condensation / steam produced M2 Add water / make a solution M3 Test for magnesium Add (aqueous) sodium hydroxide AND white ppt soluble in excess OR add (aqueous) ammonia AND white ppt soluble in excess M4 Test for sulfate Add (aqueous) barium chloride / nitrate AND white ppt insoluble in hydrochloric / nitric acid OR KMnO4 is not decolourised owtte 4 3(a)(ii) yes no FA 5 contains magnesium ions FA 5 contains sulfate ions FA 5 contains water of crystallisation 1 3(b)(i) green (solid) * (goes) black (solid) * condensation * gas / carbon dioxide tested with limewater * gives white ppt * Two marks for 4 or 5 points One mark for 2 or 3 points 2 Question Answer Marks 3(b)(ii) test observations Test 1 Hydrochloric acid fizz / bubbles / effervescence * use limewater * blue solution * (formed) Test 2 Ammonia (pale) blue ppt * (dissolves in excess) to dark / deep blue solution (as final observation) * Two marks for 4 or 5 points One mark for 2 or 3 points 2 3(b)(iii) cation: copper(II) / Cu2+ AND anion: carbonate / CO32− 1 3(b)(iv) Cu2+(aq) + 2OH–(aq) → Cu(OH)2(s) OR M2+(aq) + 2OH–(aq) → M(OH)2(s) Use of OH– scores one mark Complete correct equation scores two marks 2

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