Cambridge A Level Chemistry 9701 — 2024 May/June Paper 3 · Variant 2

1 Students are told to plan and carry out an experiment to determine the enthalpy change, ΔH1, when one mole of anhydrous sodium thiosulfate, Na2S2O3, is hydrated. Na2S2O3(s) + 5H2O(l) Na2S2O3•5H2O(s) One student suggested adding five moles of water to one mole of anhydrous sodium thiosulfate and measuring the temperature change. Their teacher said that method would not work and suggested another method using Hess’s law. You will carry out the teacher’s method to determine the enthalpy change, ΔH2, when one mole of hydrated sodium thiosulfate, Na2S2O3•5H2O, is dissolved in water. Na2S2O3•5H2O(s) + water Na2S2O3(aq) You will do this by adding a known mass of hydrated sodium thiosulfate to a known volume of water and measuring the temperature change when the solid dissolves. (a) Explain why the student’s suggestion to add five moles of water to one mole of anhydrous sodium thiosulfate would not be a suitable method to determine ΔH1. ................................................................................................................................................... ............................................................................................................................................. [1] (b) Teacher’s method FB 1 is hydrated sodium thiosulfate, Na2S2O3•5H2O. ● Support the cup in the 250 cm3 beaker. ● Use the 25 cm3 measuring cylinder to transfer 25.0 cm3 of distilled water into the cup. ● Place the thermometer in the water and tilt the cup, if necessary, so that the bulb of the thermometer is fully covered. Record the temperature of the water in the space for results. ● Weigh the container with FB 1. Record the mass. ● Add all of the FB 1 to the water in the cup. ● Stir the mixture. Measure and record the minimum temperature reached. ● Reweigh the container and any residual FB 1. Record the mass. ● Calculate and record the mass of FB 1 added. ● Calculate and record the change in temperature. Results I II III IV [4] (c) Calculations (i) Calculate the energy change, in J, when FB 1 is added to water. energy change = .............................. J [1] (ii) Calculate the enthalpy change, ΔH2, in kJ mol–1, when one mole of hydrated sodium thiosulfate, FB 1, dissolves in water. ΔH2 = ...... ..................... kJ mol–1 [2] sign value (iii) The enthalpy change, ΔH3, when one mole of anhydrous sodium thiosulfate is dissolved in water is –8.1 kJ mol–1. Na2S2O3(s) + water Na2S2O3(aq) ΔH3 = –8.1 kJ mol–1 Use your answer to (c)(ii) and the information given to construct a Hess’s cycle to calculate ΔH1 in kJ mol–1. Show clearly how you used the data. (If you were unable to calculate an answer in (c)(ii), assume a value of 31.6 kJ mol–1. Note this may not be the correct value and the sign has been omitted.) Na2S2O3(s) + 5H2O(l) Na2S2O3•5H2O(s) ΔH1 ΔH1 = ...... ..................... kJ mol–1 [2] sign value (d) A sample of FB 1 was contaminated with anhydrous sodium thiosulfate. State what effect this would have on the temperature change in (b). Explain your answer. ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [1] [Total: 11]

Mark scheme: 1(a) One of:  would not all form pentahydrate  some would dissolve  cannot (accurately) measure (change in) temperature of a solid. 1 1(b) I Unambiguous headings and units covering all entries  (mass of) container + solid / FB 1  (mass of) container (+ residue)  initial thermometer reading / temperature (of water)  final / lowest thermometer reading / temperature (of solution)  (mass of) solid (used / added) / FB 1  change in temperature / T Units: / g, (g), in g and / °C, (°C), in °C ll All balance readings consistent to either 2 or 3 dp AND thermometer readings to .0 °C or.5 °C AND written in their table AND all subtractions correct 4 Accuracy marks Calculate T / mass to 2 decimal places for candidate and for supervisor. lll Award if the candidate ratio is within 30% of the supervisor ratio lV Award if the candidate ratio is within 15% of the supervisor ratio 1(c)(i) Correctly calculates energy change = 4.18  temperature change  25 (J) AND answer given to 2–4 significant figures (ignore sign) 1 Question Answer Marks 1(c)(ii) M1 Correctly uses amount of thiosulfate = candidate mass / 248.2 mol M2 Correct use of H2 = + (c)(i) / (moles of thiosulfate  1000) (kJ mol−1) AND answer given to 2–4 significant figures 2 1(c)(iii) M1 diagram with two downward arrows AND correct labels Na2S2O3(s) + 5H2O(l) → Na2S2O3.5H2O(s) ΔH3 or –8.1 ΔH2 or (c)(ii) or (+)31.6 (Na2S2O3(aq)) M2 H1 = –8.1 – (answer to 1(c)(ii)) (default answer = –39.7) 2 1(d) (temperature change is) less / decreased because reaction of anhydrous (sodium thiosulfate) is exothermic / causes a temperature increase 1

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2 Iodate ions contain iodine and oxygen. They have the formula IOx– where x is an integer. In this experiment you will determine the value of x in an iodate. You will first react IOx– with an excess of iodide ions, I–, to form iodine, I2. The amount of iodine produced is then determined by titration with thiosulfate ions, S2O32–. I2(aq) + 2S2O32–(aq) 2I–(aq) + S4O62–(aq) FB 2 is 0.100 mol dm–3 sodium thiosulfate, Na2S2O3. FB 3 is a solution containing 0.0140 mol dm–3 IOx– ions. FB 4 is dilute sulfuric acid, H2SO4. FB 5 is 0.500 mol dm–3 potassium iodide, KI. FB 6 is starch indicator. (a) Method ● Fill the burette with FB 2. ● Pipette 25.0 cm3 of FB 3 into a conical flask. ● Use the 25 cm3 measuring cylinder to add 10 cm3 of FB 4 to the conical flask. ● Use the same measuring cylinder to add 10 cm3 of FB 5 to the conical flask. ● Add FB 2 from the burette until the solution turns yellow. ● Add 10–15 drops of FB 6 to the solution in the conical flask. ● Continue to add more FB 2 from the burette 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 FB 2 added in each accurate titration. Keep the remaining FB 2 for use in Question 3. 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 FB 3 required .............................. cm3 of FB 2. [1] (c) Calculations (i) Give your answers to (c)(ii), (c)(iii) and (c)(iv) to an appropriate number of significant figures. [1] (ii) Use your answer to (b) and the information given to calculate the amount, in mol, of iodine formed when 25.0 cm3 of FB 3 reacts with 10 cm3 of FB 5. amount of I2 = .............................. mol [1] (iii) Calculate the amount, in mol, of IOx– ions in 25.0 cm3 of FB 3. amount of IOx– ions = .............................. mol [1] (iv) An unbalanced equation for the reaction of IOx– ions with iodide ions, I–, and hydrogen ions, H+, is shown. IO…. – + ..…… I– + ..…… H+ …….. I2 + …..…. H2O Use the ratio of your answers to (c)(ii) and (c)(iii) to balance this equation and determine the value of x. Show your working. ratio IOx– : I2 = 1 : .............................. x = .............................. [2] (d) A student carries out the same experiment as in (a) but uses 0.0140 mol dm–3 IO2– ions in place of FB 3. Tick the correct box in Table 2.1. Explain your answer. (If you were unable to determine the value of x in (c)(iv) or you determined the value of x to be 2, assume x = 4. Note that this may not be the correct value). Table 2.1 Volume of FB 2 will be smaller. Volume of FB 2 will be unchanged. Volume of FB 2 will be larger. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... [2] [Total: 15] 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 All the following data are recorded  two burette readings AND titre for the rough titration  initial and final burette readings for two (or more) accurate titrations ll Appropriate headings and units shown in the accurate titration table AND titre values recorded for accurate titrations.  initial / start AND (burette) reading / volume  final / end AND (burette) reading / volume  titre OR volume / FB 2 AND used / added  unit: / cm3 OR (cm3) OR in cm3 (for each heading) OR cm3 unit given for each volume recorded lll All accurate burette readings are recorded to the nearest 0.05 cm3. IV The final accurate titre recorded is within 0.10 cm3 of any other accurate titre. 7 Accuracy marks Round all burette readings to the nearest 0.05 cm3. Check and correct subtractions. Then select the ‘best’ titres using the hierarchy:  two (or more) accurate identical titres (ignoring any that are labelled ‘rough’), then  two (or more) accurate titres within 0.05 cm3, then  two (or more) accurate titres within 0.10 cm3, etc These ‘best’ titres should be used to calculate the mean titre, expressed to nearest 0.01 cm3. Calculate the Supervisor’s mean titre to two decimal places. Calculate the candidate’s mean titre to two decimal places. Calculate the difference () between the candidate’s titre and the supervisor’s titre. Award V if  ⩽ 0.50 cm3 Award Vl if  ⩽ 0.30 cm3 Award VII if  ⩽ 0.20 cm3 If supervisor’s titre ⩽ 10.00 cm3 then halve the tolerances. If supervisor’s titre ⩽ 5.00 cm3 then award for 0.15, 0.10 and 0.05 cm3. Question Answer Marks 2(b) Correctly calculates the mean titre  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 shown next to the two (or more) accurate readings selected.  The mean should be quoted to 2 decimal places AND be rounded correctly to nearest 0.01 cm3. 1 2(c)(i) answers for (c)(ii) and (c)(iii) are given to 3 or 4 significant figures AND answer to (c)(iv) value of x is an integer 1 2(c)(ii) Correctly calculates amount of I2 = ½ (b) x 0.1 / 1000 (mol) 1 2(c)(iii) Correctly calculates amount of IOx− = (25 x 0.0140) / 1000 = 3.50 x 10−4 (mol) 1 2(c)(iv) M1 Display of (c)(ii) / (c)(iii) ratio AND correctly uses (c)(ii) and (c)(iii) ratio giving integer value for x M2 Equation balanced for value of x from M1 x = 5; IO5+ 9I + 10H+ → 5I2 + 5H2O x = 4; IO4+ 7I + 8H+ → 4I2 + 4H2O x = 3; IO3+ 5I + 6H+ → 3I2 + 3H2O x = 2; IO2+ 3I + 4H+ → 2I2 + 2H2O x = 1; IO+ I + 2H+ → I2 + H2O 2 2(d) M1 volume of FB 2 will be smaller AND as less iodine produced / less yield M2 less change in oxidation state OR balanced equation of IO2 (IO2+3I + 4H+→ 2I2 + 2H2O) 2

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3 FB 7 is an aqueous solution of a salt containing one cation and one anion, both of which are listed in the Qualitative analysis notes. (a) (i) Carry out the following tests using a 1 cm depth of FB 7 in a test-tube for each test. Record your observations in Table 3.1. Table 3.1 test observations Test 1 Add aqueous barium chloride or aqueous barium nitrate. Test 2 Add aqueous sodium hydroxide, then transfer the mixture to a boiling tube and warm gently, then add a small piece of aluminium foil. Test 3 Add aqueous ammonia. [4] (ii) The results of the tests in Table 3.1 allow you to deduce one cation and two possible anions in FB 7. Deduce which ions may be present in FB 7. Give the formulae of the ions. cation .................... anion ..................... or ..................... [2] (iii) Describe a test to identify which of the possible anions is present in FB 7. Carry out your test. Record your observations and conclusion. test .................................................................................................................................... ........................................................................................................................................... observations ...................................................................................................................... conclusion ......................................................................................................................... [2] Question 3 continues on page 12. (b) (i) FB 8 and FB 9 are solutions of Group 1 salts. Each contains one anion, both of which are listed in the Qualitative analysis notes. One of the anions contains oxygen but not nitrogen. The other anion is a halide. Carry out tests to identify the two anions. Record your tests and observations in a suitable form in the space below. [4] (ii) Use your observations in (b)(i) to complete Table 3.2 by identifying the formulae of the anions in FB 8 and FB 9. Table 3.2 FB 8 FB 9 anion [1] (iii) Write an ionic equation for a reaction observed in (b)(i) for one of the anions tested. Include state symbols. ..................................................................................................................................... [1] [Total: 14]

Mark scheme: 3(a)(i) Test 1  no (visible) change / no reaction / no ppt Test 2  white ppt  soluble in excess  negative litmus test / litmus stays red / no change / no reaction  fizzing  gas / NH3 turns (damp red) litmus blue Test 3  white ppt  soluble in excess Two points needed for each mark. 4 3(a)(ii) M1 cation: Zn2+ after correct observations in Test 3 M2 anions: NO2− or NO3− after correct ammonia gas observations in Test 2 with Al 2 3(a)(iii) EITHER M1 add (a few drops of) (acidified aqueous) KMnO4 / potassium manganate(VII) M2 no change or solution stays purple / pink AND anion is NO3− OR M1 add named mineral acid M2 no change/ no bubbling / no brown gas / no blue solution AND anion is NO3− 2 Question Answer Marks 3(b)(i) M1 table of results drawn with clear sections for test and for observations with each salt e.g. test as row-header and FB 8 and FB 9 as column-headers or vice versa M2 reagents are (aqueous) silver nitrate AND a named acid M3 + AgNO3(aq) FB 8: (pale) yellow ppt AND insoluble in (aqueous) NH3 AND white to brown ppt with FB 9 if tested M4 + named acid FB 9: effervescence / bubbling / fizzing AND gas / CO2 gives white ppt with limewater 4 3(b)(ii) FB 8: I – AND FB 9: CO32− 1 3(b)(iii) One of: Ag+(aq) + I –(aq) → AgI(s) 2H+(aq) + CO32−(aq) → H2O(l) + CO2(g) equation must match observation from 3(b)(i) 1

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