Cambridge A Level Chemistry 9701 — 2020 May/June Paper 3 · Variant 2

1 In this experiment you will determine the formula of the ion, IOx–. To do this you will first react IOx– ions with an excess of iodide ions, I– , to form iodine, I2. The equation for this reaction is: – 1 + y z IOx + yI– + zH+ I2 + 2 H2O 2 where x, y and z are all integers. The amount of iodine produced will then be determined by titration with thiosulfate ions, S2O32–. I2 + 2S2O32– 2I– + S4O62– FB 1 is a solution containing 0.0150 mol dm–3 IOx– ions. FB 2 is dilute sulfuric acid, H2SO4. FB 3 is 0.500 mol dm–3 potassium iodide, KI. FB 4 is 0.100 mol dm–3 sodium thiosulfate, Na2S2O3. starch indicator (a) Method ●● Pipette 25.0 cm3 of FB 1 into a conical flask. ●● Use the measuring cylinder to add 25 cm3 of FB 2 to the conical flask. ●● Use the measuring cylinder to add 10 cm3 of FB 3 to the conical flask. The solution will turn brown as iodine is produced. ●● Fill the burette with FB 4. ●● Add FB 4 from the burette until the solution in the conical flask turns yellow. ●● Add 10–15 drops of starch indicator to the conical flask. The solution will turn blue-black. ●● Continue to add more FB 4 from the burette until the blue-black colour just disappears. This is the end-point of the titration. ●● Carry out a rough titration and record your burette readings in the space below. The rough titre is .............................. cm3. ●● Carry out as many accurate titrations as you think necessary to obtain consistent results. ●● Make sure that your recorded results show the precision of your practical work. ●● Record in a suitable form in the space below all of your burette readings and the volume of FB 4 added in each accurate titration. Keep FB 3 and FB 4 for use in Question 3. I II III IV V VI VII [7] (b) From your accurate titration results, obtain a value for the volume of FB 4 to be used in your calculations. Show clearly how you obtained this value. 25.0 cm3 of FB 1 required .............................. cm3 of FB 4. [1] (c) Calculations (i) Give your answers to (c)(ii), (c)(iii) and (c)(iv) to the appropriate number of significant figures. [1] (ii) Use your answer to (b) and the relevant equation on page 2 to calculate the number of moles of iodine that form when 25.0 cm3 of FB 1 react with 10 cm3 of FB 3. moles of I2 = .............................. mol [1] (iii) Calculate the number of moles of IOx– ions in 25.0 cm3 of FB 1. moles of IOx– ions = .............................. mol [1] (iv) Use the ratio of your answers to (c)(ii) and (c)(iii) along with the relevant equation given on page 2 to calculate the value of y. (Note that y is an odd integer such as 1, 3, 5, 7 etc.) Show your working. y = .............................. [2] (v) Use your value of y to determine the formula of the IOx– ion. formula = .............................. [1] (d) (i) The maximum error in the volume dispensed by the pipette is ±0.06 cm3. Calculate the maximum percentage error in the volume of FB 1 used. maximum percentage error = ..............................% [1] (ii) A student suggested that a more accurate value of x could be obtained if a 10 cm3 pipette is used to measure FB 3 rather than the measuring cylinder. State whether you agree with the student. Explain your answer. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [1] [Total: 16]

Mark scheme: 1(a) I The following data must be shown • burette readings and titre for rough titration • 2 × 2 ‘box’ showing both accurate burette readings 1 II Headings and units correct for accurate titration table and headings match readings. • initial / start (burette) reading / volume + unit (allow vol but not V) • final / end (burette) reading / volume + unit (allow value for reading) • titre or volume / FA 4 used / added (not ‘difference’ or ‘total’) + unit Units: (cm3) or / cm3 or in cm3 or cm3 by every entry 1 III All accurate burette readings to 0.05 cm3 1 IV The final accurate titre recorded is within 0.10 cm3 of any other accurate titre. 1 1(b) Candidate must average two (or more) titres that are all within 0.20 cm3. Working must be shown or ticks must be put next to the two (or more) accurate titres selected. 1 1(c)(i) Answers for (ii), (iii) to 3–4 sf and answer to (iv) is an integer. 1 1(c)(ii) ½ ((b) × 0.1 / 1000) 1 1(c)(iii) (25 × 0.015) / 1000 = 3.75 × 10–4 1 1(c)(iv) Display of (ii) (iii) = + 1 2 y 1 Correctly calculates y to the nearest odd integer 1 Question Answer Marks 1(c)(v) Identifies ion as IO3– Allow ecf from (iv) as below. IO4– + 7I– + 8H+ → 4 I2 + 4 H2O IO3– + 5I– + 6H+ → I2 + 3 H2O IO2– + 3I– + 4H+ → I2 + 2 H2O IO– + I– + 2H+ → I2 + H2O 1 1(d)(i) (0.06/25) × 100 = 0.24% 1 1(d)(ii) Not correct as KI is in excess (so volume does not matter) 1

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2 In this experiment you will determine the enthalpy change of solution, ΔHsol, for hydrated sodium thiosulfate, Na2S2O3•5H2O. To do this you will measure the temperature change when a known mass of hydrated sodium thiosulfate is dissolved in a known volume of water. FB 5 is hydrated sodium thiosulfate, Na2S2O3•5H2O. (a) Method ●● Support the cup in the 250 cm3 beaker. ●● Use the 25 cm3 measuring cylinder to transfer 20.0 cm3 of distilled water into the cup. ●● Weigh the stoppered container of FB 5 and record the mass. ●● Measure and record the initial temperature of the water in the cup. ●● Add all the FB 5 to the water in the cup. ●● Stir the mixture and record the minimum temperature that is reached. ●● Reweigh the stoppered container. 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 of the reaction. (Assume that 4.2 J of heat energy changes the temperature of 1.0 cm3 of solution by 1.0 °C.) Show your working. energy change = .............................. J [1] (ii) Calculate the enthalpy change of solution, ΔHsol, for hydrated sodium thiosulfate. ΔHsol for Na2S2O3•5H2O = ...... ............................. kJ mol–1 sign value [2] (iii) Assume that under the same conditions, the enthalpy change of solution, ΔHsol, for anhydrous sodium thiosulfate, Na2S2O3, is –7.7 kJ mol–1. Construct a Hess’s cycle and determine the enthalpy change for the following reaction. (If you were unable to calculate an answer to (b)(ii), assume a value of +32.2 kJ mol–1. Note this is not the correct value.) Na2S2O3(s) + 5H2O(l) Na2S2O3•5H2O(s) ΔH = ...... ............................. kJ mol–1 sign value [2] (c) How would your temperature change in (a) be affected if your sample of FB 5 contained a small amount of anhydrous sodium thiosulfate? Explain your answer. .................................................................................................................................................... .................................................................................................................................................... .............................................................................................................................................. [1] [Total: 10] 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) I Table to include initial and final mass of stoppered container with FB 5, initial and final temperature, mass added and temperature change. All with correct units. 1 II All masses recorded to the same precision and all temperatures recorded to .0 or .5 °C. 1 Examiner calculates ΔT / mass for candidate and for supervisor III Award if the difference in the ratio between candidate and Supervisor is within 0.20 °C g–1 1 IV Award if the difference in the ratio between candidate and Supervisor is within 0.40 °C g–1 1 2(b)(i) correctly calculates 4.2 × temp change × 20 1 2(b)(ii) correct expression showing 248.2 moles of thiosulfate = candidate mass / 248.2 answer to 2–4 sf 1 ΔH = + ans (i) / moles of thiosulfate answer to min 2 sf penalise 1 Question Answer Marks 2(b)(iii) 2 arrows linking to Na2S2O3 (aq) with correct values indicated for each step 1 ΔH = –7.7 – (ans (b)(ii)) default value +39.9 1 2(c) anhydrous is exothermic, therefore the temperature drop would be less / final temp would be higher / temperature change would be less 1

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3 (a) FB 6 is an aqueous solution containing one cation and one anion, both of which are listed in the Qualitative Analysis Notes. (i) Carry out tests to identify the cation in FB 6. Record your tests and observations in the space below. [2] (ii) Carry out the following tests and record your observations. test observations Test 1 To a 2 cm depth of FB 6 in a test‑tube, add a few drops of nitric acid, followed by a few drops of aqueous silver nitrate. Pour approximately half the contents of the test‑tube into a clean test‑tube. Test 2 To one of the test‑tubes add aqueous ammonia. Test 3 To the other test‑tube add FB 4, Na2S2O3(aq). [2] (iii) Deduce the formula of FB 6. ....................................................................................................................................... [1] (b) FB 7 is acidified aqueous iron(III) chloride, FeCl 3. (i) Carry out the following tests and record your observations. test observations Test 1 To a 1 cm depth of FB 7 in a test‑tube, add a 1 cm depth of FB 3, KΙ(aq), then add starch indicator. [1] (ii) Carry out the following tests and record your observations. test observations Test 1 To a 1 cm depth of FB 7 in a test‑tube, add a 1 cm depth of FB 4, Na2S2O3(aq). Leave to stand until there is no further change, then add aqueous sodium hydroxide. [2] (iii) Explain your observation in (b)(ii) when aqueous sodium hydroxide is added. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (c) FB 8 is acidified aqueous iron(II) sulfate, FeSO4. (i) Carry out the following tests and record your observations and conclusions. test observations conclusions Test 1 To a 1 cm depth of FB 8 in a boiling tube, add a 1 cm depth of hydrogen peroxide, then add aqueous sodium hydroxide. [3] (ii) Write an ionic equation for the reaction that occurs on addition of sodium hydroxide in (c)(i). ....................................................................................................................................... [1] [Total: 14]

Mark scheme: 3(a)(i) Selects NaOH and warms. For co-ord: If get ppt with cold reagent is this a CON? 1 Fizzing / bubbling and gas given off / NH3 turns (damp) red litmus blue 1 3(a)(ii) Add AgNO3 gives a cream ppt 1 Add NH3(aq) ppt is insol / partially soluble AND Add FB 4 ppt is sol 1 3(a)(iii) NH4Br 1 3(b)(i) Add FB 3 gives a red-brown / orange-brown / yellow-brown / yellow solution AND Add starch the solution goes blue / black. 1 3(b)(ii) Add FB 4 turns the solution purple AND On standing becomes yellow / colourless / allow faint ppt. 1 Add NaOH gives a (dirty/dark) green ppt 1 3(b)(iii) Fe2+ is present 1 Fe3+ has been reduced (by S2O32–) mark on obs in (ii) 1 Question Answer Marks 3(c)(i) Add H2O2 solution turns yellow 1 Add NaOH gives red / brown ppt AND effervescence 1 Gas given off relights a glowing splint and gas is oxygen test obs conc H2O2 solution pale yellow* NaOH red-brown ppt* fizzes* relights glowing splint* Fe3+ ions formed* oxygen* 1 3(c)(ii) Fe3+(aq) + 3OH–(aq) → Fe(OH)3(s) 1

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