Cambridge A Level Chemistry 9701 — 2020 Feb/March Paper 3 · Variant 3

1 The concentrations of solutions of hydrogen peroxide are often represented in terms of ‘volume strength’. In this experiment you will determine the ‘volume strength’ of a solution of hydrogen peroxide by titration with acidified potassium manganate(VII). 2MnO4–(aq) + 5H2O2(aq) + 6H+(aq) 2Mn2+(aq) + 8H2O(l) + 5O2(g) FA 1 is 0.0300 mol dm–3 potassium manganate(VII), KMnO4. FA 2 is dilute sulfuric acid, H2SO4. FA 3 is aqueous hydrogen peroxide, H2O2. (a) Method Dilution of FA 3 ●● Pipette 25.0 cm3 of FA 3 into the 250 cm3 volumetric flask. ●● Make the solution up to the mark using distilled water. ●● Shake the flask thoroughly. ●● Label this diluted solution of hydrogen peroxide FA 4. Titration ●● Fill the burette with FA 1. ●● Rinse the pipette thoroughly with distilled water and then with a little FA 4. ●● Pipette 25.0 cm3 of FA 4 into a conical flask. ●● Use the 25 cm3 measuring cylinder to add 20 cm3 of FA 2 into the same conical flask. ●● Perform 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 any recorded results show the precision of your practical work. ●● Record all of your burette readings and the volume of FA 1 added in each accurate titration. Keep FA 1, FA 2 and FA 3 for use in Questions 2 and 3. I II III IV V VI I [7] (b) From your accurate titration results, obtain a suitable value for the volume of FA 1 to be used in your calculations. Show clearly how you obtained this value. 25.0 cm3 of FA 4 required .............................. cm3 of FA 1. [1] (c) Calculations (i) ive your answers to (ii), (iii), (iv) and (v) to the appropriate number of significant figures. [1] (ii) Calculate the number of moles of potassium manganate(VII) present in the volume calculated in (b). moles of KMnO4 = .............................. mol [1] (iii) The equation for the reaction of potassium manganate(VII) with hydrogen peroxide is shown. 2MnO4–(aq) + 5H2O2(aq) + 6H+(aq) 2Mn2+(aq) + 8H2O(l) + 5O2(g) Use your answer to (c)(ii) to calculate the number of moles of hydrogen peroxide used in each titration. moles of H2O2 = .............................. mol Hence calculate the concentration of H2O2 in FA 4, in mol dm–3. concentration of H2O2 in FA 4 = .............................. mol dm–3 [1] (iv) Calculate the concentration of H2O2 in FA 3, in mol dm–3. concentration of H2O2 in FA 3 = .............................. mol dm–3 [1] (v) When hydrogen peroxide decomposes in the presence of a catalyst, oxygen is produced. H2O2(aq) H2O(l) + 12O2(g) The ‘volume strength’ of hydrogen peroxide is equal to the volume of oxygen, in dm3, produced under room conditions, when 1.00 dm3 of the solution decomposes. Use your answer to (c)(iv) and the equation above to calculate the volume, in dm3, of oxygen produced when 1.00 dm3 of FA 3 decomposes. This is the ‘volume strength’, in vol, of FA 3. (Under room conditions 1.00 mol of gas occupies a volume of 24.0 dm3. If you were unable to calculate the concentration of H2O2 in FA 3, assume that it is 1.02 mol dm–3. This may not be the correct value.) ‘volume strength’ of FA 3 = .............................. vol [2] (d) The maximum error in reading a 25.0 cm3 pipette is ±0.06 cm3. Show by calculation that the pipette is more accurate than a burette for measuring 25.0 cm3 of solution. .................................................................................................................................................... .............................................................................................................................................. [1] [Total: 15]

Mark scheme: 1(a) I Titration data shown • two burette readings for the rough titration • titre for rough titration • initial and final burette readings for two (or more) accurate titrations 1 II Titre values for accurate titrations recorded and appropriate headings and units in accurate titration table • initial / start and (burette) reading / volume • final / end and (burette) reading / volume • titre or volume / FA 1 and used / added • unit: / cm3 or (cm3) or in cm3 (for each heading) or cm3 unit given for each volume recorded 1 III All accurate burette readings are recorded to the nearest 0.05 cm3. 1 IV The final accurate titre recorded is within 0.10 cm3 of any other accurate titre. 1 Award V, VI and VII if δ ⩽ 0.20 (cm3) Award V and VI if 0.20 < δ ⩽ 0.40 Award V only if 0.40 < δ ⩽ 0.60 3 1(b) Correctly calculates mean titre from two (or more) accurate titres where the total spread is ⩽ 0.20 cm3 AND Answer is given to 2 dp AND Working must be shown or ticks must be put next to the two (or more) accurate titres selected 1 1(c)(i) All final answers in 1(c) are quoted to 3 or 4 significant figures Minimum of four answers attempted 1 1(c)(ii) Correctly calculates number of moles of KMnO4 used = 0.03(00) × mean titre 1000 1 1(c)(iii) Two correct multiplying factors shown • answer (ii) × 2.5 • (subsequent answer) (mol of H2O2) × 40 (× 1000 25 ) 1 Question Answer Marks 1(c)(iv) Correctly calculates concentration of H2O2 = final answer in (iii) × 10 1 1(c)(v) Correctly uses (iv) to find moles of O2 = answer (iv) × 0.5 1 Correctly uses (iv) to find ‘volume strength’ = moles of O2 × 24 Answer for default value = 12.24 vol 1 1(d) % error pipette = 0.24 and % error burette = 0.4(0) OR 2 × 0.05 (cm3) is greater (than 0.06 / pipette error) Working must be shown 1

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2 In this experiment you will determine the enthalpy change, ΔH, for the catalytic decomposition of hydrogen peroxide into water and oxygen. H2O2(aq) H2O(l) + 12O2(g) FA 3 is aqueous hydrogen peroxide, H2O2. FA 5 is manganese(IV) oxide, MnO2. (a) Method Experiment 1 ●● Support one of the plastic cups inside the 250 cm3 beaker. ●● Use the 50 cm3 measuring cylinder to add 30 cm3 of FA 3 into the plastic cup. ●● Measure and record the initial temperature of the solution. ●● Add a heaped spatula measure of FA 5 to the solution in the plastic cup. ●● Stir constantly until the maximum temperature is reached and record this temperature. ●● Calculate and record the temperature rise. ●● Rinse and dry the thermometer. Experiment 2 ●● Support the second plastic cup inside the 250 cm3 beaker. ●● Use the 50 cm3 measuring cylinder to add 40 cm3 of FA 3 into the plastic cup. ●● Measure and record the initial temperature of the solution. ●● Add a heaped spatula measure of FA 5 to the solution in the plastic cup. ●● Stir constantly until the maximum temperature is reached and record this temperature. ●● Calculate and record the temperature rise. I II III IV V [5] (b) Calculation (i) Calculate the energy released in Experiment 1. [Assume that 4.2 J changes the temperature of 1.0 cm3 of solution by 1.0 °C.] energy released = .............................. J [1] (ii) Use your answer to 1(c)(iv) to calculate the number of moles of hydrogen peroxide used in Experiment 1. (If you were unable to calculate the concentration of H2O2 in FA 3, assume that it is 1.02 mol dm–3. This may not be the correct value.) moles of H2O2 = .............................. mol [1] (iii) Calculate the enthalpy change, in kJ mol–1, for the decomposition of 1 mole of hydrogen peroxide into water and oxygen. enthalpy change = ...... ............................. kJ mol–1 [1] sign value (c) (i) A student suggested that the experiment would be more accurate if the same mass of FA 5, manganese(IV) oxide, had been weighed out for each experiment. State and explain whether you agree with the student’s suggestion. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [2] (ii) The student also suggested that Experiments 1 and 2 should give the same temperature rise, even though a greater volume of FA 3 was used in Experiment 2. State and explain whether you agree with the student’s suggestion. ............................................................................................................................................. ............................................................................................................................................. ....................................................................................................................................... [1] [Total: 11] 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 (or two lists) showing unambiguous headings and data for both experiments in the space provided: • two initial thermometer readings + values • two final / highest thermometer readings + values • two temperature rises / changes + values 1 II Recording of data • correct units ‘covering’ all thermometer readings • all four readings recorded to .0 or .5 °C • both rises in temperature correctly calculated 1 III Award this mark based on the table below 1 IV Award this mark based on the table below 1 ΔTsup (ºC) ΔT ⩾ 15.0 15 > ΔT ⩾10.0 10 > ΔT ⩾ 5.0 ΔT < 5.0 δ 2.0 1.5 1.0 0.5 V Award mark if both the candidate’s corrected temperature rises are within 1.0 °C of each other 1 2(b)(i) Correctly calculates heat produced = 30 × 4.2 × temp rise for expt 1 AND answer given to 2–4 sf 1 2(b)(ii) Correctly uses number of moles H2O2 = 0.03 × answer to 1(c)(iv) AND answer given to 2–4 sf Answer for default value = 0.0306 / 0.031 mol 1 2(b)(iii) Correct expression for enthalpy change • Enthalpy change = ans (i) ans (ii) × 1 1000 • Negative sign must be shown • Answer is shown to 2–4 sf 1 Question Answer Marks 2(c)(i) (FA 5 / MnO2 is a) catalyst 1 (Student is wrong:) the mass of MnO2 used does not alter the heat produced / enthalpy change / temperature change (for decomposition of H2O2) 1 2(c)(ii) Student is correct: energy / heat released is greater because more / greater (moles / molecules of) hydrogen peroxide / FA 3 is used in Experiment 2 AND more moles / molecules / greater amount of water / solution / FA 3 / hydrogen peroxide heated is greater (in the same proportion) 1

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3 (a) FA 3 is aqueous hydrogen peroxide, H2O2. FA 6 is a solution containing two cations and one anion from those listed in the Qualitative analysis notes. (i) To a 1 cm depth of FA 6 in a boiling tube, add aqueous sodium hydroxide until it is in excess. Then heat the tube, gently and carefully. Keep the mixture obtained in the boiling tube for the test in (a)(ii). Record all your observations. Identify the cations in FA 6. observations ........................................................................................................................ ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. FA 6: cations are ..................................................... and .................................................... [4] (ii) To the mixture obtained from (a)(i), carefully add a 1 cm depth of FA 3. Record your observations. ............................................................................................................................................. ....................................................................................................................................... [1] (iii) One reaction taking place in (a)(ii) involves oxidation of one of the cations in FA 6. Give the half-equation to show this oxidation reaction. State symbols are not required. ....................................................................................................................................... [1] (b) FA 1 is aqueous potassium manganate(VII). FA 2 is dilute sulfuric acid. FA 7 and FA 8 are solutions, each containing one cation and one anion. (i) Carry out the following tests and record your observations in the table. observations test FA 7 FA 8 Test 1 To a 1 cm depth of solution in a test‑tube, add a small spatula measure of solid sodium carbonate. Test 2 To a 1 cm depth of solution in a test‑tube, add an equal volume of FA 2 and a few drops of FA 1, then add a few drops of aqueous starch. Test 3 To a 1 cm depth of solution in a test‑tube, add a few drops of aqueous silver nitrate, then add aqueous ammonia. [5] (ii) Identify the anion in FA 7. anion ............................................................................................................................. [1] (iii) Identify FA 8. FA 8 is ........................................................................................................................... [1] (iv) Carry out one further test to confirm the identity of the cation in FA 8. State the name of the reagent you used and record the observation(s) you made. reagent ................................................................................................................................ observation(s) ..................................................................................................................... ............................................................................................................................................. [1] [Total: 14]

Mark scheme: 3(a)(i) (Green) precipitate darkens and / or goes brown 1 (When mixture heated) gas / ammonia turns (red) litmus blue 1 Both cations in FA 6 identified • Fe2+ ions / iron(II) • Ammonium / NH4+ 1 3(a)(ii) Goes brown / rust / red-brown / orange-brown AND bubbles / fizzing / effervescence 1 3(a)(iii) Fe2+ → Fe3+ + e−/ Fe2+ − e− → Fe3+ 1 Question Answer Marks 3(b)(i) Award one mark for every two correct observations (*) as shown in table below 5 test observations FA 7 FA 8 Test 1 + Na2CO3(s) no (visible) reaction / no change / no precipitate / solid (carbonate) dissolves / no effervescence * effervescence / fizzing / bubbles * gas / CO2 gives a white ppt with limewater / turns limewater milky / cloudy white / chalky * Test 2 + H+/KMnO4(aq) solution turns yellow / brown / orange-brown / red-brown / yellow-brown * no (visible) reaction / no change / KMnO4 / solution stays purple / colourless solution turns purple / purple solution formed * + starch(aq) (turns) dark blue / deep blue / blue-black / black * ignore Test 3 + AgNO3(aq) (pale) yellow ppt (formed) * white ppt (formed) * + NH3(aq) (ppt) insoluble / does not dissolve / no change * (ppt) dissolves / soluble / gives a colourless solution * 3(b)(ii) Anion in FA 7 is iodide (ion) / I− must be concluded from a (pale) yellow precipitate 1 3(b)(iii) FA 8 is hydrochloric acid / HCl 1 3(b)(iv) One suitable test for H+ (reagent and observation) in any acid identified in (b)(iii) • named pH indicator and correct final colour OR • add magnesium and fizzes or gas / H2 pops with a lighted splint 1

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