Cambridge A Level Chemistry 9701 — 2024 Feb/March Paper 3 · Variant 3

1 The ionic equation for the reaction between sodium thiosulfate and hydrochloric acid is: S2O32–(aq) + 2H+(aq) S(s) + SO2(aq) + H2O(l) The solid sulfur formed causes the reaction mixture to become cloudy and opaque. You will carry out experiments to investigate the relationship between the concentration of sodium thiosulfate and the rate of reaction. Small amounts of SO2 gas are released during this reaction. Take care to avoid inhaling this gas. It is 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. FA 1 is 0.10 mol dm–3 sodium thiosulfate, Na2S2O3. FA 2 is 2.00 mol dm–3 hydrochloric acid, HCl . distilled water (a) Method Prepare a table for your results in the Results section on page 4. For each experiment the table should include: ● volume of FA 1 used ● volume of distilled water used ● reaction time ● relative rate. Relative rate can be calculated using the expression: 1000 relative rate = reaction time Experiment 1 ● Label a burette FA 1. Fill the burette with FA 1. ● Transfer 25.00 cm3 of FA 1 into a 100 cm3 beaker. ● Place the beaker on the printed insert. ● 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 immediately start the stop-clock. Stir the mixture once. ● Look vertically down through the solution in the beaker at the print on the insert. ● Stop the stop-clock as soon as the print on the insert is no longer visible. ● Record the reaction time to the nearest second. ● Empty the contents of the beaker into the quenching bath. ● Rinse the beaker with water. Dry the beaker so that it is ready to be used in Experiment 2. Experiment 2 ● Transfer 12.50 cm3 of FA 1 into the 100 cm3 beaker. ● Label a second burette ‘water’. Fill this burette with distilled water. ● Transfer 12.50 cm3 of distilled water into the 100 cm3 beaker. ● Place the beaker on the printed insert. ● 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 immediately start the stop-clock. Stir the mixture once. ● Look vertically down through the solution in the beaker at the print on the insert. ● Stop the stop-clock as soon as the print on the insert is no longer visible. ● Record the reaction time to the nearest second. ● Empty the contents of the beaker into the quenching bath. ● Rinse the beaker with water. Dry the beaker so that it is ready to be used in the next experiment. Experiments 3–5 Carry out three further experiments to investigate how reaction times change with different volumes of FA 1. Do not use a volume of FA 1 less than 12.50 cm3. Results I II III IV V VI VII VIII [8] (b) Plot a graph, on the grid, of relative rate (y-axis) against volume of FA 1 (x-axis). The graph should not include the origin. Identify any anomalous point. Draw a line of best fit. I II III IV [4] (c) Use your graph to predict the reaction time if an experiment is carried out using 23.50 cm3 of FA 1 and distilled water. Show clearly on the grid how you determined the relative rate. reaction time = .............................. s [2] (d) The final instruction for each experiment is to rinse and dry the beaker. State the effect on the reaction time of not drying the beaker before carrying out each of Experiments 2–5. Explain your answer. ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [1] (e) A student repeats Experiment 1 but uses a 250 cm3 beaker in place of the 100 cm3 beaker. All other conditions remain the same. State whether each statement below is correct. Explain your answers. (i) The student records a longer time for this experiment because the 250 cm3 beaker is used. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] (ii) A longer time is recorded because the rate of production of sulfur is slower. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [1] [Total: 17]

Mark scheme: Question Answer Marks 1(a) I A single table with 4 headings for results on page 4 with indication of the 4 items of data to be recorded (volume of FA 1, 1 volume of water, time and rate) AND the 2 specified experiments carried out II Correct headings and units for 4 required items of data: 1 • 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: 1 • all volumes recorded to 2 dp with the final digit being 0 or 5 cm3 • all times to the nearest second IV 3 additional volumes chosen with intervals not less than 2.00 cm3 1 AND no volume less than 12.50 cm3 AND water added in each additional experiment to make total volume of FA 1 and water equal to 25.00 cm3 V All rates correctly calculated using 1000 / time to minimum 2 sf 1 VI Reaction time decreases as volume of FA 1 increases for all five experiments 1 Examiner corrects times (if necessary) to the nearest second for Experiments 1 and 2 and calculates ratio t12.5 / t25 to 2 dp 2 Write the ratio, ringed, near the mark boxes Award VII for ratio between 1.70–2.40 Award VII and VIII for ratio between 1.90–2.20 1(b) I Axes unambiguously labelled 1 (relative) rate or s–1 on y-axis AND volume / FA 1 / (sodium) thiosulfate / Na2S2O3 / cm3 on x-axis AND some numbers for scales II Suitable scales chosen 1 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 III All points recorded in the table are accurately plotted. 1 All points recorded plotted correctly to within half a small square AND in the correct small square or on the line if it should be on the line AND minimum of 4 experiment points plotted IV Line of best fit drawn 1 1(c) M1 2 lines drawn on the graph – vertical from 23.50 on the x-axis to the line of best fit and horizontal from the line to the 2 y-axis. M2 Correctly uses time = 1000 / (relative) rate 1(d) Time is longer 1 AND extra water would lower the concentration / dilute (FA 1 or reactants) 1(e)(i) The student is correct (that time would be greater) 1 AND the depth (of solution / ppt / mixture) is less 1(e)(ii) The student is not correct / (The (relative) rate is smaller but) the rate is not slower 1 AND (the solution) concentrations have not changed / the frequency of successful collisions remains the same / more sulfur is required to obscure the insert owtte.

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2 In this experiment you will determine the enthalpy change, ΔH, for the reaction between aqueous copper(II) sulfate and magnesium. CuSO4(aq) + Mg(s) Cu(s) + MgSO4(aq) FA 3 is 1.0 mol dm–3 copper(II) sulfate, CuSO4. FA 4 is magnesium powder, Mg. (a) Method ● Support the cup in the 250 cm3 beaker. ● Use the 50 cm3 measuring cylinder to transfer 50.0 cm3 of FA 3 into the cup. ● Weigh the stoppered container of FA 4. Record the mass. ● Measure the temperature of FA 3 in the cup. Record the temperature. ● Add the FA 4 to the FA 3 in the cup and stir the mixture constantly. ● Measure and record the maximum temperature reached. ● Reweigh the stoppered container and any residual FA 4. Record the mass. ● Calculate and record the mass of FA 4 used. ● Calculate and record the maximum temperature change that occurs during the reaction. [3] (b) Calculations (i) Calculate the heat energy produced in the reaction. heat energy produced = .............................. J [1] (ii) Determine which reactant, FA 3 or FA 4, is in excess for the reaction. Show your working. [1] (iii) Calculate the enthalpy change, ΔH, in kJ mol–1, for the reaction. ΔH = ....... ......................... kJ mol–1 (sign) (value) [2] (c) A student suggests that the slow rate of the reaction using the method described in (a) means that heat energy is lost from the solution so the temperature change is inaccurate. Describe how you would change the method and processing of the results to improve the accuracy of the enthalpy change for this reaction. You should not change the quantities of FA 3 or FA 4 used. You may wish to illustrate your answer with a sketch graph. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... [3] [Total: 10] 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) M1 Unambiguous headings and correct units for: 1 • (Mass of) container + FA 4 and g / grams • (Mass of) container (+ residue) and g / grams • Initial / FA 3 (temperature) and °C • Maximum (temperature) and °C • (Mass of) magnesium / FA 4 (added) and g / grams • (Temperature) change and °C M2 Readings are appropriately recorded against headings 1 • Both temperature measurements recorded to the nearest 0.5 °C AND • Both balance readings recorded to a consistent number of decimal places (either 2 or 3) AND • Correct calculations of mass of FA 4 and temperature change M3 Examiner checks and corrects T if necessary 1 T within x °C of supervisor value Tsup / ºC <5.0 5.0–9.5 10.0–14.5 15.0–19.5 20.0–24.5 25.0–29.5 30.0–34.5 35.0–39.5 M3: 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 2(b)(i) Correctly calculates: 1 50  4.18  T AND answer to 2–4 sf 2(b)(ii) Correctly calculates: 1 amount of FA 3 = 0.05  1.0 = 0.050 mol AND amount of FA 4 = candidate’s mass / 24.3 AND FA 3 / CuSO4 is in excess 2(b)(iii) M1 Correctly uses: 2 H = (b)(i) / (1000  n(FA 4) from 2(b)(ii)) M2 sign is negative AND answer to 2–4 sf 2(c) M1 Measure the temperature of FA 3 (at regular intervals and) before adding FA 4 3 AND (Stir the mixture and) measure the temperature of the mixture at regular intervals after adding FA 4 until temperature is steady or decreasing M2 Plot a graph of temperature against time AND point of addition of FA 4 must be given in text or clearly shown/labelled on sketch graph M3 extrapolate lines of best fit to find T(max) or T at time of addition (of FA 4)

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3 (a) Each of the solutions FA 5, FA 6 and FA 7 has an anion containing sulfur. All the anions are listed in the Qualitative analysis notes. None of the anions is present in more than one compound. None of the solutions contain a cation listed in the Qualitative analysis notes. Use 1 cm depth of each solution in a test-tube for each test. Record your observations in Table 3.1. Table 3.1 observations test FA 5 FA 6 FA 7 Test 1 Add a few drops of aqueous acidified potassium manganate(VII) then leave it to stand for 2 minutes. Test 2 Add a piece of magnesium ribbon. Test 3 Add aqueous barium chloride or aqueous barium nitrate. [5] (b) (i) Use your observations from (a) to identify the formula of each of the anions present in FA 5, FA 6 and FA 7. FA 5 FA 6 FA 7 [2] (ii) Use your observations from (a), to suggest the identity of the cation present in FA 6. The cation in FA 6 is ............ . Carry out a further test to check whether your suggestion is correct. Record your test and observations. State the identity of the cation in FA 6. The cation in FA 6 is ............ . [2] (c) Write an ionic equation for one of the reactions in either Test 2 or Test 3 in (a). Include state symbols. ............................................................................................................................................. [1] (d) FA 8 is a solid compound. (i) Gently warm (do not boil) a 4 cm depth of FA 6 in a boiling tube. Stop warming the FA 6, add all the FA 8 and shake the boiling tube. Filter the mixture into a second boiling tube. The filtrate will be used in (d)(ii). Describe the appearance of the residue and the filtrate. residue .............................................................................................................................. filtrate ................................................................................................................................ [1] (ii) To a 2 cm depth of the filtrate from (d)(i) in a test-tube, add an equal volume of aqueous potassium iodide. Record your observations. Filter the mixture into a test-tube for use in (d)(iii). ........................................................................................................................................... ..................................................................................................................................... [1] (iii) To a 1 cm depth of the filtrate from (d)(ii), add aqueous sodium hydroxide. Record your observations. ........................................................................................................................................... ..................................................................................................................................... [1] [Total: 13]

Mark scheme: 3(a) FA 5 is Na2S2O3 ; FA 6 is H2SO4 FA 7 is Na2SO3 FA 8 is Cu2O 11 * observations 5 2* = 1 mark (round down) observations test FA 5 FA 6 FA 7 Test 1 *purple (solution) / MnO4−/ *no change / (MnO4–) stays *purple (solution) / MnO4− / KMnO4 purple / does not decolourise KMnO4 and (KMnO4) and turns colourless / decolourised turns colourless / decolourised *white / off-white / cream / pale *no change no change yellow ppt forms and Test 2 *no change *effervescence / bubbling / *no change and fizzing *(gas) pops with a lighted splint Test 3 no change *white ppt *white ppt 3(b)(i) FA 5 is S2O32− ; FA 6 is SO42− FA 7 is SO32− 2 2 correct = 1 mark 3 correct = 2 marks 3(b)(ii) M1 Cation is H+ 2 AND chooses Na2CO3 / sodium carbonate / other named carbonate or named indicator M2 effervescence / fizzing / bubbling or suitable indicator colour AND gas / CO2 gives a white ppt with limewater / suitable indicator colours before and after use, e.g. blue litmus goes red 3(c) Mg(s) + 2H+(aq) → Mg2+(aq) + H2(g) 1 or Ba2+(aq) + SO42−(aq) → BaSO4(s) or Ba2+(aq) + SO32−(aq) → BaSO3(s) 3(d)(i) Residue: red-brown / brown / pink / pink-brown (solid) 1 AND filtrate: (pale) blue (solution) 3(d)(ii) Brown / yellow-brown / orange-brown / red-brown (mixture) 1 3(d)(iii) (On addition of NaOH) blue ppt insoluble in excess 1

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