Cambridge A Level Chemistry 9701 — 2023 Oct/Nov Paper 3 · Variant 4

1 Iron(III) ions, Fe3+, can oxidise iodide ions, I–, to iodine, I2. 2Fe3+(aq) + 2I–(aq) 2Fe2+(aq) + I2(aq) It is possible to determine the rate of this reaction by measuring the time to produce a certain amount of iodine. To do this, thiosulfate ions, S2O32–, are added to the reaction mixture. The thiosulfate ions react immediately with the iodine produced by the reaction and convert it back to iodide ions. I2(aq) + 2S2O32–(aq) 2I–(aq) + S4O62–(aq) Iodine remains in the solution when all the thiosulfate ions have reacted. The remaining iodine is detected by starch indicator in the reaction mixture, which causes the solution to turn blue-black. In this experiment you will investigate how the rate of the reaction between iron(III) ions and iodide ions is affected by the concentration of the iron(III) ions. FB 1 is 0.0500 mol dm–3 acidified iron(III) chloride, FeCl 3. FB 2 is 0.0500 mol dm–3 potassium iodide, KI. FB 3 is 0.00500 mol dm–3 sodium thiosulfate, Na2S2O3. FB 4 is starch indicator. (a) Method Experiment 1 ● Fill the burette labelled FB 1 with FB 1. ● Run 20.00 cm3 of FB 1 into a 100 cm3 beaker. ● Use the 25 cm3 measuring cylinder to add the following to the other 100 cm3 beaker: ○ 10.0 cm3 of FB 2 ○ 20.0 cm3 of FB 3 ○ 10.0 cm3 of FB 4. ● Add the contents of the first beaker to the second beaker and start timing immediately. Ignore any initial colouration on mixing. ● Stir the mixture once and place the beaker on the white tile. ● Stop timing as soon as the solution turns blue-black. ● Record this reaction time to the nearest second. ● Rinse both beakers and shake dry. ● Rinse and dry the glass rod. Experiment 2 ● Fill the other burette with distilled water. ● Run 10.00 cm3 of FB 1 into a 100 cm3 beaker. ● Run 10.00 cm3 of distilled water into the same beaker containing FB 1. ● Use the 25 cm3 measuring cylinder to add the following to the other 100 cm3 beaker: ○ 10.0 cm3 of FB 2 ○ 20.0 cm3 of FB 3 ○ 10.0 cm3 of FB 4. ● Add the contents of the first beaker to the second beaker and start timing immediately. Ignore any initial colouration on mixing. ● Stir the mixture once and place the beaker on a white tile. ● Stop timing as soon as the solution turns blue-black. ● Record this reaction time to the nearest second. ● Rinse both beakers and shake dry. ● Rinse and dry the glass rod. Experiments 3 to 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 20.00 cm3. Do not use a volume of FB 1 that is less than 5.00 cm3. The rate of reaction is given by the following expression. 1000 rate = reaction time in seconds Use this expression to calculate the rate for each of your experiments. Record all your results in a single table. You should include the volume of FB 1, the volume of distilled water, the reaction time and the rate of reaction. Results I II III IV V VI VII VIII IX [9] (b) On the grid opposite, plot the rate (on the y-axis) against the volume of FB 1 (on the x-axis). Include the origin in your plot. Label any points that you consider to be anomalous. Draw the line of best fit. (c) In these experiments, the volume of FB 1 is directly related to the concentration of iron(III) ions. Using your graph, state what conclusion can be drawn about the relationship between the rate of reaction and the concentration of the iron(III) ions. ................................................................................................................................................... ............................................................................................................................................. [1] (d) A student wants to increase the concentration of the sodium thiosulfate solution while keeping the rest of the experiment the same. The student realises that the amount of thiosulfate ions must not be too high otherwise there will be no remaining iodine. You will calculate the concentration of thiosulfate ions that will react with all the iodine produced in Experiment 1. (i) Calculate the amount, in mol, of iron(III) ions in the solution at the start of Experiment 1. amount of Fe3+ = .............................. mol [1] (ii) Calculate the amount, in mol, of iodide ions in the solution at the start of Experiment 1. amount of I– = .............................. mol [1] (iii) Use the equation to determine the maximum amount, in mol, of iodine that can be made during this reaction. 2Fe3+(aq) + 2I–(aq) 2Fe2+(aq) + I2(aq) amount of I2 = .............................. mol [1] (iv) Use the equation to determine the concentration, in mol dm–3, of sodium thiosulfate solution that will react with all the iodine produced in Experiment 1. Show your working. I2(aq) + 2S2O32–(aq) 2I–(aq) + S4O62–(aq) concentration = .............................. mol dm–3 [2] [Total: 19]

Mark scheme: Question Answer Marks 1(a) I Constructs a single table on page 4 for results of experiments to include the 4 items of required data 9 and results to show at least 2 experiments completed. II Correct headings and units for recorded data given: • volume of FB 1 given as (cm3), / cm3, in cm3 or cm3 after each entry • volume of water given as (cm3), / cm3, in cm3 or cm3 after each entry • time given as (s), / s, in seconds or s after each entry • rate given as (s–1), / s–1 or s–1 after each entry III All times recorded to the nearest second (minimum of 3 experiments) and volumes of FB 1 and water recorded to .#0 or .#5 cm3 IV Three additional volumes chosen with intervals not less than 2.00 cm3 and all volumes of FB 1 ⩾ 5.00 cm3 V In all three additional experiments water is added to make the same total volume of FB 1 + H2O (20 cm3). VI All rates correctly calculated using 1000 / time and given to 2–4 significant figures and either to the same number of significant figures or decimal places. VII All 5 times increase with decrease in volume of FB 1. Examiner calculates the ratio of (time for expt 2 / time for expt 1) to 2 dp for candidate. VIII Award if the ratio is between 1.20 and 1.80 IX Award if the ratio is between 1.30 and 1.60 1(b) I Rate on y-axis and volume (of FB 1) on x-axis 4 and axes clearly labelled with unambiguous names or units and some numbers for scales. II Linear scales based on 1, 2 or 5 and include the origin and scale chosen so that the point plotted for 20 cm3 FB 1 is more than halfway along each axis. III All recorded points plotted correctly to within half a small square and in the correct square and minimum of 4 experiment points plotted. IV Draws a smooth curved line of best fit. 1(c) Curved line: 1 Rate is not proportional to concentration (of FB 1) or as concentration of FB 1 increases the rate increases 1(d)(i) Correctly calculates amount of Fe3+ 1 = (0.020  0.0500) = 1.00  10–3 mol and answer to 2–4 significant figures. 1(d)(ii) Correctly calculates amount of I− 1 = (0.010  0.0500 ) = 5.00  10–4 mol and answer to 2 – 4 significant figures. 1(d)(iii) Correct use of (d)(i) and (d)(ii) to calculate amount of I2 1 = (d)(ii) / 2 (= 2.50  10–4 mol) and answer to 2–4 significant figures. 1(d)(iv) Correct use of (d)(iii) to calculate concentration of Na2S2O3 2 M1 (d)(iii)  2 (= 5.00  10–4) M2 ((d)(iii)  2 / 0.020) (= 0.0250 mol dm–3) and answer to 2–4 significant figures.

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2 In this experiment you will determine the relative formula mass of an unknown ion, A2–. The ion is present in a salt with the formula Na2A•5H2O. You will first determine the enthalpy change of solution, ∆Hsol , for Na2A•5H2O. To do this you will measure the temperature change when a known mass of the salt dissolves in a known volume of water. You will then use a literature value for the enthalpy change of solution to calculate the relative formula mass of A2–. FB 5 is the salt, Na2A•5H2O. (a) Method ● Support the cup in the 250 cm3 beaker. ● Rinse the 25 cm3 measuring cylinder with distilled water. ● Use the 25 cm3 measuring cylinder to transfer 20.0 cm3 of distilled water into the cup. ● Weigh the stoppered container with the FB 5. Record the mass. ● Measure and record the initial temperature of the water in the cup. You may need to tilt the cup to make sure that the bulb of the thermometer is covered. ● Carefully add all the sample of FB 5 to the water in the cup. ● Stir the mixture and record the minimum temperature that is reached. ● Weigh the stoppered container and any residual FB 5. Record the mass. ● Calculate and record the mass of FB 5 added to the water and the change in temperature. I II III IV [4] (b) Calculations (i) Calculate the energy change in the reaction. energy change = .............................. J [1] (ii) A literature value for the enthalpy change of solution, ∆Hsol , for Na2A•5H2O is +47.4 kJ mol–1. Calculate the amount, in mol, of Na2A•5H2O in your sample of FB 5. Assume that the literature value was measured under the same conditions as your experiment. amount of Na2A•5H2O = .............................. mol [1] (iii) Calculate the relative formula mass of A2–. relative formula mass of A2– = .............................. [1] (c) A student suggests that the experiment can be made more accurate by increasing the volume of water used. State whether the student is correct. Explain your answer. ................................................................................................................................................... ............................................................................................................................................. [1] [Total: 8] 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 Six unambiguous headings recorded, with correctly displayed units and in the space provided. 4 Units: (°C), / g, in g, or °C or g against each value • (Mass of) container + FB 5 / contents / solid / g • (Mass of) (empty) container (+ residue) / g • (Mass of) FB 5 / Na2A•5H2O / solid used / g • (Initial) temperature (of water) / °C • minimum / final temperature (of solution) / °C • Temperature change / decrease / T / °C II Precision of readings. Two weighings to same number of decimal places (2 or 3) and two thermometer readings to .0 or .5 °C and appropriate to the headings given. III Correct subtractions for mass of FB 5 and T and Mass of FB 5 between 3.50 and 4.50 g IV Accuracy mark Examiner calculates T / mass to 2 dp for candidate. IV Award if T / mass is between 1.40 and 1.85. 2(b)(i) Correct calculation of energy change 1 Energy change = 20  4.18  T (= 83.6  T) and answer correctly rounded to 2–4 sf 2(b)(ii) Correctly uses ans (b)(i) / 47400 1 and answer given to 2–4 sf 2(b)(iii) Correct display of 1 Mr = mass in (a) / ans (b)(ii) and [mass in (a) / ans (b)(ii)] –136 and a final answer shown. 2(c) Student is incorrect as temp change/ decrease is smaller. 1

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3 FB 6 is a salt which contains a cation and an anion from those listed in the Qualitative analysis notes. FB 7 is a mixture of two acids. The possible acids are hydrochloric acid, nitric acid and sulfuric acid. By carrying out the following tests you will identify the ions in FB 6 and determine which acids are present in FB 7. (a) Carry out the following tests and record your observations in Table 3.1. Table 3.1 test observations Test 1 Place approximately one third of the sample of FB 6 in a hard-glass test-tube and heat strongly, then leave to cool. Test 2 To an approximately 2 cm depth of FB 7 in a boiling tube add carefully the remaining sample of FB 6. Keep this solution for use in 3(b). [4] (b) (i) Choose reagents and carry out tests to identify the cation in FB 6. Record your tests and observations. [2] (ii) State the formulae of the ions present in FB 6. FB 6 contains ........................ and ........................ . [1] (c) (i) Choose reagents and carry out tests to identify which of the following acids are present in FB 7. Use a 1 cm3 depth of FB 7 for each test. Record your observations. test for hydrochloric acid ................................................................................................... observations ...................................................................................................................... ........................................................................................................................................... test for nitric acid ............................................................................................................... observations ...................................................................................................................... ........................................................................................................................................... test for sulfuric acid ........................................................................................................... observations ...................................................................................................................... ........................................................................................................................................... [4] (ii) State the formulae of the acids present in FB 7. FB 7 contains ........................ and ........................ . [1] (iii) Give an ionic equation for any of the reactions you observed in (c)(i). Include state symbols. ..................................................................................................................................... [1] [Total: 13]

Mark scheme: 3(a) Observations: 2* = 1 mark (round down). 4 Test 1 condensation / droplets of water / steam / water vapour* (solid) goes yellow (on heating)* (solid returns to) white on cooling* Test 2 fizzing / effervescence / bubbles* solid dissolves or (colourless) solution formed* vigorous / strong / rapid reaction / fizzing or (tube / solution) becomes warm* In either Test 1 or Test 2: gas / CO2 tested with limewater* (either on heating or adding acid.) gives white ppt* 3(b)(i) M1 uses NH3 / ammonia. 2 M2 white ppt and soluble in excess 3(b)(ii) Zn2+ and CO32– 1 3(c)(i) M1 HCl : 4 AgNO3 and white ppt M2 HNO3: NaOH and Al and warms / heats M3 fizzing / effervescence / gas / NH3 / fumes turns litmus blue M4 H2SO4: BaCl2 / Ba(NO3)2 and no change / no ppt 3(c)(ii) HCl and HNO3 1 3(c)(iii) One from: 1 Ag+(aq) + Cl–(aq) → AgCl(s) 3NO3–(aq) + 8Al(s) + 5OH–(aq) + 18H2O(l) → 8Al(OH)4–(aq) + 3NH3(g) Alternative MS for 3(a) and 3(b) if candidates have been supplied with CaCO3 rather than ZnCO3 because a centre could not get hold of ZnCO3. This part of the MS would apply to a complete centre, not individual candidates. Question Answer Marks FB 6 = CaCO3; FB 7 =HCl and HNO3 Check for capital and lower case letters in formulae; reagents must be complete (not just ions); check for missing charges and state symbols in ionic equation and charges within formulae of solid compounds. 3(a) Observations: 2* = 1 mark (round down). 4 Test 1 no change when heated (owtte) / (solid) remains white* white residue / solid (after cooling)* Test 2 fizzing / effervescence / bubbles* solid dissolves or (colourless) solution formed* vigorous / strong / rapid reaction / fizzing or (tube / solution) becomes warm* Gas test in Test 2 only: gas / CO2 tested with limewater* (either on heating or adding acid) gives white ppt* 3(b)(i) M1 uses NH3 / ammonia and NaOH 2 M2 NaOH: white ppt and insoluble in excess And NH3: no change / no reaction / no ppt 3(b)(ii) Ca2+ and CO32– 1

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