2023 ACJC H2 Chem Prelim P4 Solutions

© ACJC 2023 9729/04/Prelim/2023 [Turn over Anglo-Chinese Junior College JC2 Preliminary Examination Higher 2 CANDIDATE NAME SUGGESTED SOLUTIONS FORM CLASS TUTORIAL CLASS INDEX NUMBER CHEMISTRY Paper 4 Practical Candidates answer on the Question Paper. 9729/04 1 August 2023 2 hours 30 minutes READ THESE INSTRUCTIONS FIRST Write your index number and name on all the work you hand in. Give details of the practical shift and laboratory, where appropriate, in the boxes provided. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. The use of an approved scientific calculator is expected, where appropriate. You may lose marks if you do not show your working or if you do not use appropriate units. Qualitative Analysis Notes are printed on pages 21 and 22. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use 1 / 16 2 / 19 3 / 20 Total / 55 This document consists of 20 printed pages and 2 blank pages. Shift Laboratory

© ACJC 2023 9729/04/Prelim/2023 [Turn over 2 1 Determination of the change in the oxidation number of a transition metal ion, X2+ FA 1 is 0.0125 mol dm−3 potassium manganate(VII), KMnO4 FA 2 is 0.0720 mol dm−3 transition metal salt, XSO4 FA 3 is 1.00 mol dm−3 sulfuric acid, H2SO4 You are to determine, by titration, the change in the oxidation number of a transition metal ion, X2+, upon oxidation by acidified potassium manganate(VII). (a) Procedure 1. Fill the burette with FA 1. 2. Using the pipette, transfer 25.0 cm3 of FA 2 into the conical flask. 3. Using a measuring cylinder, transfer 10.0 cm3 of FA 3 to the same conical flask. 4. Titrate the mixture in the conical flask with FA 1. The end-point is reached when the first permanent pale pink colour is seen. 5. Record your titration results, to an appropriate level of precision, in the space provided. 6. Repeat steps 2 to 5 to obtain consistent results. (i) Titration results Final burette reading / cm3 27.90 27.90 Initial burette reading / cm3 0.00 0.00 Volume of FA1 used / cm3 27.90 27.90 [2] (ii) From your titration results in (a)(i), obtain a suitable volume of FA 1, VFA 1, to be used in your calculations. Show clearly how you obtained this volume. Average titre volume = (27.90 + 27.90) / 2 = 27.90 cm3 VFA 1 = ....................................... [4]

© ACJC 2023 9729/04/Prelim/2023 [Turn over 3 (b) (i) Calculate the amount of MnO4– present in VFA 1 obtained in (a)(ii). Amount of MnO4– = 27.90/1000 x 0.0125 = 3.488 x 10–4 = 3.49 x 10–4 mol (3 s.f.) amount of MnO4– = ....................................... [1] (ii) Calculate the amount of X2+ in 25.0 cm3 of FA 2. Amount of X2+ = 25.0/1000 x 0.0720 = 0.00180 mol amount of X2+ = ....................................... [1] (iii) The half-equation for the reduction of MnO4– is shown. MnO4– + 8H+ + 5e– → Mn2+ + 4H2O It is known that X2+ is oxidised to either X3+ or X4+, but not both. Determine the stoichiometric ratio of: • MnO4– to X2+ if X3+ is formed, • MnO4– to X2+ if X4+ is formed. MnO4– + 8H+ + 5e– → Mn2+ + 4H2O (x1) X2+ → X3+ + e– (x5) MnO4– + 8H+ + 5e– → Mn2+ + 4H2O (x2) X2+ → X4+ + 2e– (x5) MnO4– : X2+ if X3+ is formed = 1 : 5 MnO4– : X2+ if X4+ is formed = 2 : 5 [1] (iv) Using your answers from (b)(i) to (iii), determine the change in the oxidation number of X2+ upon oxidation by acidified MnO4–. MnO4– : X2+ = 3.49 x 10–4: 0.00180 = 1 : 5.16 = 1 : 5 Hence, X3+ is formed. Change in oxidation number = +3 – (+2) = +1 change in oxidation number = ....................................... [2] (v) Hence, write a balanced equation, with state symbols, for the oxidation of X2+ by acidified MnO4–. ……………………………………………………………………………………….. [1] MnO4–(aq) + 8H+(aq) + 5X2+(aq) → 5X3+(aq) + Mn2+(aq) + 4H2O(l)

© ACJC 2023 9729/04/Prelim/2023 [Turn over 4 (c) A student claimed that using a burette to measure 25.0 cm 3 of FA 2 in Step 2 would give a more accurate result than using a pipette. The maximum error in each volume reading on a burette is ±0.05 cm3. The percentage error incurred using a 25.0 cm3 pipette is 0.24%. By calculating the percentage error incurred using a burette, deduce if the claim by the student is correct. …………………………………………………………………………………………………... …………………………………………………………………………………………………... ……………………………………………………………………………………………….. [1] (d) Explain whether using a burette instead of a measuring cylinder to measure 10.0 cm3 of FA 3 in Step 3 would affect the accuracy of the experimental results. …………………………………………………………………………………………………... …………………………………………………………………………………………………... ……………………………………………………………………………………………….. [1] (e) Table 1.1 shows electrode reactions with their standard electrode potential values. Table 1.1 electrode reaction Eo / V X3+ + e– ⇌ X2+ +0.77 X4+ + 2e– ⇌ X2+ +0.86 MnO4– + 8H+ + 5e– ⇌ Mn2+ + 4H2O +1.52 NO3– + 3H+ + 2e– ⇌ HNO2 + H2O +0.94 Use the relevant data to explain, with reference to the expected titre readings, whether nitric acid can be used in place of sulfuric acid in the oxidation of X2+ by MnO4–. …………………………………………………………………………………………………... …………………………………………………………………………………………………... …………………………………………………………………………………………………... …………………………………………………………………………………………………... ……………………………………………………………………………………………….. [2] [Total: 16] % error incurred using a burette = [2(0.05) / 25.0] x 100% = 0.40% As the % error incurred using a burette is higher than that of a pipette, the claim by the student is incorrect. As sulfuric acid is used in excess to provide the acidic medium for the oxidation, using a burette would not affect the accuracy of the experimental results. Since Eo of NO3–/HNO2 is more positive than Eo of X3+/X2+, (OR Eocell = +0.94 – (+0.77) = +0.17 V > 0) X2+ can also be oxidised by nitric acid on top of MnO4–. As such, nitric acid cannot be used in place of sulfuric acid as the titre readings obtained would be lower than expected.

© ACJC 2023 9729/04/Prelim/2023 [Turn over 5 BLANK PAGE

© ACJC 2023 9729/04/Prelim/2023 [Turn over 6 2 Determination of the kinetics of the iodination of propanone The rate of a chemical reaction is affected by the concentration of reactants according to the rate equation which has to be determined experimentally. In this experiment, propanone is reacted with iodine in the presence of an acid catalyst. CH3COCH3 + I2 CH3COCH2I + HI The order of reaction with respect to iodine can be found by titrating the concentration of iodine remaining at regular time intervals with sodium thiosulfate solution. FA 3 is 1.00 mol dm–3 sulfuric acid, H2SO4 FA 4 is 1.00 mol dm–3 propanone, CH3COCH3 FA 5 is a solution of iodine dissolved in potassium iodide solution FA 6 is 0.010 mol dm–3 sodium thiosulfate, Na2S2O3 FA 7 is 0.05 mol dm–3 sodium hydrogencarbonate, NaHCO3 starch indicator (a) Procedure: Notes: You will perform each titration once only. Great care must be taken that you do not overshoot the end-point. Once you have started the stopwatch, it must continue running for the duration of the experiment. You must not stop it until you have finished this experiment. 1. Fill the burette labelled FA 6, with FA 6. 2. Using a measuring cylinder, add 25.0 cm3 of FA 3 to the conical flask labelled reaction mixture. Using the same measuring cylinder, add 25.0 cm3 of FA 4 into the same conical flask. 3. Using a measuring cylinder, add 50.0 cm3 of FA 5