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Chemistry 9701/31 May/June 2024: question paper and mark scheme

Cambridge AS & A Level Chemistry (9701), Paper 3 Advanced Practical Skills 1, May/June 2024. Read both free below, or download them. Done the paper? Get it marked by AI against this mark scheme.

9701_s24_qp_319701_s24_ms_319701/31/M/J/24

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Paper
Paper 3 Advanced Practical Skills 1
Total marks
40
Time
2 hours
Questions
3
Code on the paper
9701/31/M/J/24
9701_s24_ms_31.pdf

9701/31 mark scheme, question by question

Each question as it's printed, and the mark scheme's points for it, as text. Diagrams, tables and graphs are in the PDFs above.

Question 113 marks

Question

In this experiment you will determine the enthalpy change, ∆Hr , for the reaction shown. NaOH(aq) + CO2(g) NaHCO3(aq) enthalpy change = ΔHr You will react each of sodium hydroxide and sodium hydrogencarbonate with excess dilute sulfuric acid. You will determine the enthalpy change for each reaction, then use Hess’s law to calculate ΔHr . (a) Reaction of sodium hydroxide with sulfuric acid 2NaOH(aq) + H2SO4(aq) Na2SO4(aq) + 2H2O(l) enthalpy change = ΔH1 FA 1 is 2.00 mol dm–3 sodium hydroxide, NaOH. FA 2 is 2.00 mol dm–3 sulfuric acid, H2SO4. Method ● Support a cup in the 250 cm3 beaker. ● Use the 50 cm3 measuring cylinder to transfer 30.0 cm3 of FA 1 into the cup. ● Place the thermometer in FA 1 and tilt the cup, if necessary, so that the bulb of the thermometer is fully covered. Record the temperature of FA 1. ● Use the 25 cm3 measuring cylinder to add 20.0 cm3 of FA 2 to the FA 1 in the cup. ● Stir the mixture. ● Measure and record the maximum temperature reached. ● Calculate and record the change in temperature. Results I II [2] (b) Calculations (i) Calculate the energy change, in J, in your experiment. energy change = …… J [1] (ii) Calculate the amount, in mol, of sulfuric acid that reacted with FA 1 in your experiment. amount of H2SO4 = …… mol [1] (iii) Calculate the enthalpy change of reaction, ΔH1, in kJ mol–1 of sulfuric acid, for the neutralisation of NaOH(aq) with H2SO4(aq). Show your working. ΔH1 = …… …… kJ mol–1 of H2SO4 [1] sign value (c) Reaction of sodium hydrogencarbonate with sulfuric acid 2NaHCO3(s) + H2SO4(aq) Na2SO4(aq) + 2H2O(l) + 2CO2(g) enthalpy change = ΔH2 FA 2 is 2.00 mol dm–3 sulfuric acid, H2SO4. FA 3 is sodium hydrogencarbonate, NaHCO3. Method ● Support the other cup in the 250 cm3 beaker. ● Use the 25 cm3 measuring cylinder to transfer 25.0 cm3 of FA 2 into the cup. ● Place the thermometer in FA 2 and tilt the cup, if necessary, so that the bulb of the thermometer is fully covered. Record the temperature of FA 2. ● Weigh the container with FA 3. Record the mass. ● Adding small quantities at a time, tip all the FA 3 from the container into the FA 2 in the cup. ● Stir the mixture. ● Measure and record the minimum temperature reached. ● Calculate and record the change in temperature. ● Weigh the container with any residual FA 3. Record the mass. ● Calculate and record the mass of FA 3 added. Results I II III [3] (d) Calculations Use your data to calculate the enthalpy change, ΔH2, in kJ mol–1 of sulfuric acid, for the reaction of NaHCO3(s) with H2SO4(aq). Show your working. ΔH2 = …… …… kJ mol–1 of H2SO4 [3] sign value (e) The enthalpy change when one mole of sodium hydrogencarbonate dissolves in water is ΔH3. NaHCO3(s) + aq NaHCO3(aq) enthalpy change = ΔH3 Using the symbols ΔH1, ΔH2 and ΔH3 in your answer, use Hess’s law to deduce an expression for ΔHr . NaOH(aq) + CO2(g) NaHCO3(aq) enthalpy change = ΔHr ∆Hr = …… kJ mol–1 [1] (f) A student suggested that the experiment in (c) would be more accurate if 25.0 cm3 of 3.00 mol dm–3 sulfuric acid was used instead of 25.0 cm3 of 2.00 mol dm–3 sulfuric acid. State whether the student’s suggestion is correct. Explain your answer. …… [1] [Total: 13]

Mark scheme

1(a) I Three pieces of data shown 2 Two thermometer readings recorded AND temperature rise correctly calculated, all listed in the space provided, with units. ‘Correct’ headings are not required for this mark, but it must be clear what the three figures refer to. Ignore precision of readings (but see II of 1(c)(i)). Do not award if either thermometer reading is below 10 °C. Temperature units: / °C OR in °C OR (°C) OR °C by every reading. Accuracy mark Calculate the difference δ between temperature rises for supervisor and candidate. Supervisor value ⩾ 10 °C, award mark II if δ⩽ 1.5 °C. Supervisor value ⩾ 5 °C and < 10 °C, award mark II if δ⩽ 1.0 °C. Supervisor value < 5 °C, award mark II if δ⩽ 0.5 °C. 1(b)(i) correctly calculates energy released 1 energy = 50 × 4.18 × temp rise (J) AND answer given to 2–4 significant figures 1(b)(ii) amount H2SO4 used = (0.030 × 2 × 0.5) = 0.03(000) (mol) 1 1(b)(iii) correct use of data to calculate ΔH1 1 energy ΔH1 = /1000 × (b)(ii) AND answer must have negative sign AND be expressed to 2–4 significant figures 1(c) I Six correct headings shown, with units, in list / table 3 (mass of) container + FA 3 / solid / NaHCO3 (mass of) container (empty or with residual FA 3) (mass of) FA 3 / solid / NaHCO3 OR mass used initial temperature / thermometer reading lowest / minimum / final temperature / thermometer reading temperature change/decrease Mass units: / g OR in g OR (g) OR g by every reading Temperature units: / °C OR in °C OR (°C) OR °C by every reading II All readings shown to appropriate precision all four balance / thermometer readings shown all four thermometer readings [including those in 1(a)] to .0 or .5 both masses to 2 decimal places OR both masses to 3 decimal places III Subtractions correct mass of solid correct AND between 5.00–7.00 g temperature fall correct AND between 8.0–12.0 °C 1(d) Correct use of data 3 expressions for M1 and M2 need not be evaluated to score M1 energy = 25 × 4.18 × temp decrease M2 amount of H2SO4 used = 0.5 × mass NaHCO3 used/84 (mol) M3 enthalpy change = energy / amount of H2SO4 used × 1000 AND answer has positive sign AND expressed to two or more significant figures 1(e) ΔHr = 0.5 ΔH1 – 0.5 ΔH2 + ΔH3 1 1(f) (student is not correct) because acid is used in excess (in candidate’s experiment (c)) (so temperature fall is unchanged) 1

Question 214 marks

Question

Sodium sulfite is oxidised when it reacts with excess iodine. Na2SO3(aq) + I2(aq) + H2O(l) Na2SO4(aq) + 2HI(aq) The remaining iodine is then titrated using aqueous sodium thiosulfate. I2(aq) + 2Na2S2O3(aq) Na2S4O6(aq) + 2NaI(aq) You will determine the integer value of x in the formula of hydrated sodium sulfite, Na2SO3•xH2O, by titration. FA 4 is aqueous sodium thiosulfate containing 14.24 g of Na2S2O3 in 1.00 dm3. FA 5 is aqueous iodine, prepared as shown. ● 5.00 g of hydrated sodium sulfite is added to 600 cm3 of 0.100 mol dm–3 aqueous iodine. ● The mixture is allowed to stand to ensure that all the sodium sulfite has been oxidised. ● The mixture containing the remaining iodine is made up to 1.00 dm3 with distilled water. FA 6 is starch indicator. (a) Method ● Fill the burette with FA 4. ● Pipette 25.0 cm3 of FA 5 into a conical flask. ● Add FA 4 from the burette into the conical flask until the colour of the solution changes to yellow. ● Add 10 drops of FA 6 to the conical flask. Continue titrating until the blue-black colour just disappears. ● 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, in a suitable form in the space below, all your burette readings and the volume of FA 4 added in each accurate titration. I II III IV V VI VII [7] (b) From your accurate titration results, calculate a suitable mean value to use in your calculations. Show clearly how you obtain the mean value. 25.0 cm3 of FA 5 required …… cm3 of FA 4. [1] (c) Calculations (i) Give your answers to (c)(ii), (c)(iii), (c)(iv) and (c)(v) to an appropriate number of significant figures. [1] (ii) Calculate the amount, in mol, of sodium thiosulfate present in the volume of FA 4 in (b). amount of Na2S2O3 = …… mol [1] (iii) Calculate the amount, in mol, of iodine in 1.00 dm3 of FA 5. amount of I2 in 1.00 dm3 = …… mol [1] (iv) Use the information given and your answer to (c)(iii) to calculate the amount, in mol, of iodine that reacted with sodium sulfite when solution FA 5 was prepared. amount of I2 reacted with sodium sulfite = …… mol [1] (v) Use your answer to (c)(iv) to calculate the relative formula mass, Mr , of hydrated sodium sulfite. Mr of Na2SO3•xH2O = …… [1] (vi) Calculate the value of x. Show your working. x = …… [1] [Total: 14] 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. If a solid is heated, a hard-glass test-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 The following data are recorded: 7 two burette readings AND titre for the rough titration initial and final burette readings for two (or more) accurate titrations. II Titre values recorded for accurate titrations AND correct headings and units in the accurate titration table. initial / start AND (burette) reading / volume final / end AND (burette) reading / volume titre OR volume used / added / OR FA 4 used / added unit: / cm3 OR (cm3) OR in cm3 (for each heading) OR cm3 unit given for each volume recorded. III All accurate burette readings recorded to 0.05 cm3. IV The final accurate titre recorded must be within 0.10 cm3 of any other accurate titre. Accuracy marks Check and correct titre subtractions. Select the ‘best’ titres using the hierarchy (ignoring the rough titre value): 2 identical titres, 2 titres within 0.05 cm3, 2 titres within 0.10 cm3, etc. Calculate candidate’s mean titre to 2 decimal places. Calculate the difference δ between the candidate’s titre and the supervisor’s titre. V Award if δ ⩽ 0.60 cm3 VI Award if δ ⩽.0.40 cm3 VII Award if δ ⩽ 0.20 cm3 2(b) Candidate calculates mean titre correctly to 2 decimal places 1 Candidate must take the average of two (or more) titres that are within a total spread of not more than 0.20 cm3. Working/ explanation must be shown OR ticks must be put next to the two (or more) accurate readings selected. The mean should be quoted to 2 decimal places and be rounded to the nearest 0.01 cm3. 2(c)(i) All answers given for (c)(ii), (iii), (iv) and (v) given to 3 or 4 significant figures. 1 2(c)(ii) Correctly calculates 1 amount of sodium thiosulfate used = 14.24/158.2 × mean titre/1000 (mol) 2(c)(iii) Correct use of 2(c)(ii) 1 amount of iodine (in 1 dm3) = (c)(ii)/2 × 1000/25 (mol) 2(c)(iv) Correct use of 2(c)(iii) 1 amount of iodine reacting with sodium sulfite = 0.0600 – (c)(iii) 2(c)(v) Correct use of 2(c)(iv) 1 Mr of sodium sulfite = 5.0/(c)(iv) 2(c)(vi) Correct use of 2(c)(v) 1 x = [(c)(v) – 126.1]/18 answer must be correctly rounded to an integer FA 7 is AgNO3(aq), FA 8 is CH3COOH(aq), FA 9 is HCl (aq), FA 10 is MgSO4(aq) and FA 11 is Zn(NO3)2(aq).

Question 313 marks

Question

(a) FA 7, FA 8 and FA 9 are dilute ethanoic acid, dilute hydrochloric acid and aqueous silver nitrate but not necessarily in that order. The solutions of acids have equal concentrations. (i) You are supplied with strips of magnesium ribbon. You must not use any other reagents in this part of the question. Carry out tests to identify each of the three solutions, FA 7, FA 8 and FA 9. Obtain as much evidence as you can for your identifications. Use a 1 cm depth of solution in a test-tube for each test you carry out. Record all your observations. FA 7 is …… . FA 8 is …… . FA 9 is …… . [5] (ii) Give the ionic equation for the reaction of magnesium with FA 8. Include state symbols. …… [1] (b) FA 10 and FA 11 are both aqueous solutions of salts, each of which contains one cation and one anion listed in the Qualitative analysis notes. (i) Carry out the following tests and record your observations in Table 3.1. For Tests 1 and 2, use a 1 cm depth of FA 10 or FA 11 in a test-tube. For Test 3, use a 1 cm depth of FA 10 or FA 11 in a boiling tube. Table 3.1 observations test FA 10 FA 11 Test 1 Add aqueous ammonia. Test 2 Add a few drops of aqueous barium chloride or aqueous barium nitrate, then add dilute hydrochloric acid. Test 3 Add aqueous sodium hydroxide, then warm the mixture carefully, then add one piece of aluminium foil. [5] (ii) Use your observations in (b)(i) to complete Table 3.2 by identifying the formulae of the ions present in FA 10 and FA 11. If you cannot identify an ion write ‘unknown’. Table 3.2 cation anion FA 10 FA 11 [2] [Total: 13] Qualitative analysis notes 1 Reactions of cations cation reaction with NaOH(aq) NH3(aq) aluminium, Al 3+(aq) white ppt. soluble in excess white ppt. insoluble in excess ammonium, NH4+(aq) no ppt. – ammonia produced on warming barium, Ba2+(aq) faint white ppt. is observed unless no ppt. [Ba2+(aq)] is very low calcium, Ca2+(aq) white ppt. unless [Ca2+(aq)] is very no ppt. low chromium(III), Cr3+(aq) grey-green ppt. soluble in excess grey-green ppt. insoluble in excess giving dark green solution copper(II), Cu2+(aq) pale blue ppt. insoluble in excess pale blue ppt. soluble in excess giving dark blue solution iron(II), Fe2+(aq) green ppt. turning brown on green ppt. turning brown on contact with air contact with air insoluble in excess insoluble in excess iron(III), Fe3+(aq) red-brown ppt. insoluble in excess red-brown ppt. insoluble in excess magnesium, Mg2+(aq) white ppt. insoluble in excess white ppt. insoluble in excess manganese(II), off-white ppt. rapidly turning brown off-white ppt. rapidly turning brown Mn2+(aq) on contact with air on contact with air insoluble in excess insoluble in excess zinc, Zn2+(aq) white ppt. soluble in excess white ppt. soluble in excess 2 Reactions of anions anion reaction carbonate, CO32– CO2 liberated by dilute acids chloride, Cl –(aq) gives white ppt. with Ag+(aq) (soluble in NH3(aq)) bromide, Br –(aq) gives cream / off-white ppt. with Ag+(aq) (partially soluble in NH3(aq)) iodide, I–(aq) gives pale yellow ppt. with Ag+(aq) (insoluble in NH3(aq)) nitrate, NO3–(aq) NH3 liberated on heating with OH–(aq) and Al foil nitrite, NO2–(aq) NH3 liberated on heating with OH–(aq) and Al foil; decolourises acidified aqueous KMnO4 sulfate, SO42–(aq) gives white ppt. with Ba2+(aq) (insoluble in excess dilute strong acids); gives white ppt. with high [Ca2+(aq)] sulfite, SO32–(aq) gives white ppt. with Ba2+(aq) (soluble in excess dilute strong acids); decolourises acidified aqueous KMnO4 thiosulfate, S2O32–(aq) gives off-white / pale yellow ppt. slowly with H+ 3 Tests for gases gas test and test result ammonia, NH3 turns damp red litmus paper blue carbon dioxide, CO2 gives a white ppt. with limewater hydrogen, H2 ‘pops’ with a lighted splint oxygen, O2 relights a glowing splint 4 Tests for elements element test and test result iodine, I2 gives blue-black colour on addition of starch solution Important values, constants and standards molar gas constant R = 8.31 J K–1 mol–1 Faraday constant F = 9.65 × 104 C mol–1 Avogadro constant L = 6.022 × 1023 mol–1 electronic charge e = –1.60 × 10–19 C molar volume of gas Vm = 22.4 dm3 mol–1 at s.t.p. (101 kPa and 273 K) Vm = 24.0 dm3 mol–1 at room conditions ionic product of water Kw = 1.00 × 10–14 mol2 dm–6 (at 298 K (25 °C)) specific heat capacity of water c = 4.18 kJ kg–1 K–1 (4.18 J g–1 K–1) The Periodic Table of Elements Group 1 2 13 14 15 16 17 18 1 2 H He hydrogen helium Key 1.0 4.0 3 4 atomic number 5 6 7 8 9 10 Li Be atomic symbol B C N O F Ne lithium beryllium name boron carbon nitrogen oxygen fluorine neon 6.9 9.0 relative atomic mass 10.8 12.0 14.0 16.0 19.0 20.2 11 12 13 14 15 16 17 18 Na Mg Al Si P S Cl Ar sodium magnesium aluminium silicon phosphorus sulfur chlorine argon 23.0 24.3 3 4 5 6 7 8 9 10 11 12 27.0 28.1 31.0 32.1 35.5 39.9 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 at www.cambridgeinternational.org after the live examination series. K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr potassium calcium scandium titanium vanadium chromium manganese iron cobalt nickel copper zinc gallium germanium arsenic selenium bromine krypton 39.1 40.1 45.0 47.9 50.9 52.0 54.9 55.8 58.9 58.7 63.5 65.4 69.7 72.6 74.9 79.0 79.9 83.8 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 12 Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe rubidium strontium yttrium zirconium niobium molybdenum technetium ruthenium rhodium palladium silver cadmium indium tin antimony tellurium iodine xenon 85.5 87.6 88.9 91.2 92.9 95.9 – 101.1 102.9 106.4 107.9 112.4 114.8 118.7 121.8 127.6 126.9 131.3 55 56 57–71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 lanthanoids Cs Ba Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn caesium barium hafnium tantalum tungsten rhenium osmium iridium platinum gold mercury thallium lead bismuth polonium astatine radon 132.9 137.3 178.5 180.9 183.8 186.2 190.2 192.2 195.1 197.0 200.6 204.4 207.2 209.0 – – – 87 88 89–103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 actinoids Fr Ra Rf Db Sg Bh Hs Mt Ds Rg Cn Nh Fl Mc Lv Ts Og francium radium rutherfordium dubnium seaborgium bohrium hassium meitnerium darmstadtium roentgenium copernicium nihonium flerovium moscovium livermorium tennessine oganesson – – – – – – – – – – – – – – – – – 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 lanthanoids La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu lanthanum cerium praseodymium neodymium promethium samarium europium gadolinium terbium dysprosium holmium erbium thulium ytterbium lutetium 138.9 140.1 140.9 144.2 – 150.4 152.0 157.3 158.9 162.5 164.9 167.3 168.9 173.1 175.0 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 actinoids Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr actinium thorium protactinium uranium neptunium plutonium americium curium berkelium californium einsteinium fermium mendelevium nobelium lawrencium – 232.0 231.0 238.0 – – – – – – – – – – – Assessment International Education Copyright Acknowledgements Booklet. This is produced for each series of examinations and is freely available to download To avoid the issue of disclosure of answer-related information to candidates, all copyright acknowledgements are reproduced online in the Cambridge

Mark scheme

3(a)(i) M1 adds Mg to FA7: records dark grey / black AND solid / precipitate / deposit / coating / layer 5 M2 adds Mg to FA8 and FA9: records fizzing / effervescence in both reactions M3 clear indication that the reaction with magnesium of FA9 is faster / more vigorous than the reaction with magnesium of FA8 (ORA) M4 Any one of the following: (gas) pops with lighted splint in either the Mg / FA8 or Mg / FA9 reactions Mg dissolves / disappears AND colourless solution formed / heat produced in either the Mg / FA8 or Mg / FA9 reactions white precipitate forms when FA7 and FA9 are mixed. M5 all three identities correct: FA7 = AgNO3, FA8 = CH3COOH and FA9 = HCl 3(a)(ii) Mg(s) + 2H+(aq) → Mg2+(aq) + H2(g) 1 3(b)(i) 5 FA10 FA11 Test 1 white ppt AND insoluble in excess white precipitate NH3 soluble in excess Test 2 white precipitate no reaction / no change / no ppt Ba2+ AND AND HCl (white ppt) insoluble / no change no reaction / no change no ppt Test 3 white ppt AND insoluble in excess white precipitate NaOH soluble in excess Ignore anything written in either of these boxes, unless any positive gas test is recorded Heat (including “litmus goes red”), in which case it contradicts the gas test bullet in the box below the gas test – mark by column. fizzing / effervescence fizzing / effervescence + Al (gas) pops with a lighted splint gas turns (red) litmus blue. OR gas does not turn (red) litmus blue Two points needed for each mark. 3(b)(ii) 2 cation anion FA 10 Mg2+ SO42– FA 11 Zn2+ unknown Two points needed for each mark.

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Chemistry 9701/31 for May/June 2024 is out of 40 marks, with 2 hours to answer it, across 3 questions.

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