2025 H2 Chem Prelim Paper 4 (QP) ACJC

Anglo-Chinese Junior College JC2 Preliminary Examination Higher 2 CANDIDATE NAME FORM CLASS TUTORIAL CLASS INDEX NUMBER CHEMISTRY Paper 4 Practical Candidates answer on the Question Paper. 9729/04 5 August 2025 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 17 and 18. 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 / 17 2 / 21 3 / 17 Total / 55 This document consists of 18 printed pages and 1 blank page. Shift Laboratory

© ACJC 2025 9729/04/Prelim/2025 2 1 Determination of the stoichiometry of the reaction between hydroxylamine and iron(III) ion A redox reaction takes place between hydroxylamine, NH 2OH, and iron(III) ion, Fe 3+, in an acidic medium. The iron(III) ion is converted to iron(II) ion, Fe2+. The reaction is slow at room temperature, but is completed in a few minutes when heated to 90 oC. The iron(II) ions formed can then be oxidised by manganate(VII) ions for quantitative analysis. You are to determine, by titration, which of the following unbalanced equations represents the reaction between hydroxylamine and iron(III) ion. equation 1.1 NH2OH + Fe3+ → Fe2+ + …N2 + …H+ + …H2O equation 1.2 NH2OH + 2Fe3+ → 2Fe2+ + …N2O + …H+ + …H2O equation 1.3 NH2OH + 3Fe3+ → 3Fe2+ + …NO + …H+ FA 1 is 0.0150 mol dm−3 potassium manganate(VII), KMnO4. FA 2 is a solution prepared by boiling a 1.00 dm3 aqueous mixture of 3.30 g of hydroxylamine hydrochloride, NH2OH.HCl, excess iron(III) chloride, FeCl3, and excess sulfuric acid, H2SO4. FA 3 is 1.00 mol dm−3 sulfuric acid, H2SO4. (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 [2]

© ACJC 2025 9729/04/Prelim/2025 [Turn over 3 (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. VFA 1 = ....................................... [4] (b) (i) Calculate the amount of MnO4– present in VFA 1 obtained in (a)(ii). amount of MnO4– = ....................................... [1] (ii) Calculate the amount of Fe2+ in 25.0 cm3 of FA 2. MnO4–(aq) + 8H+(aq) + 5e– → Mn2+(aq) + 4H2O(l) Fe2+(aq) → Fe3+(aq) + e– amount of Fe2+ = ....................................... [1] (iii) Calculate the amount of NH2OH.HCl that reacted in the FA 2 pipetted into the conical flask. [Ar: H, 1.0; N, 14.0; O, 16.0; Cl, 35.5] Other than the fact that the reaction to prepare FA 2 was complete and there were no impurities in the chemicals used, state an assumption made in your calculation. amount of NH2OH.HCl = ....................................... [2] assumption …………………………………………………………………………….. …………………………………………………………………………………………… ……………………………………………………………………………………….. [1]

© ACJC 2025 9729/04/Prelim/2025 4 (iv) Hence, d etermine which equation represents the reaction between hydroxylamine and iron(III) ion. Show your working. equation …………. [1] (v) From your answer to (b)(iv), write the balanced chemical equation , with state symbols, for the reaction between hydroxylamine and iron(III) ion. ……………………………………………………………………………………….. [2] (vi) Hence, with reference to the oxidation number of nitrogen, explain whether hydroxylamine acts as an oxidising agent or a reducing agent in the reaction with iron(III) ion. …………………………………………………………………………………………… …………………………………………………………………………………………… ……………………………………………………………………………………….. [1] (d) Another student prepared FA 2 in a similar way, but found out later that the 3.30 g of hydroxylamine hydrochloride, NH2OH.HCl, contained a small amount of inert impurities. Suggest what effect this would have on the average titre. Explain your answer. …………………………………………………………………………………………………... …………………………………………………………………………………………………... …………………………………………………………………………………………………... …………………………………………………………………………………………………... ……………………………………………………………………………………………….. [2] [Total: 17]

© ACJC 2025 9729/04/Prelim/2025 [Turn over 5 BLANK PAGE

© ACJC 2025 9729/04/Prelim/2025 6 2 Determination of a value for the molar enthalpy change of reaction for citric acid and sodium hydrogen carbonate Citric acid is a weak acid that is found naturally in all citrus fruits. Citric acid reacts with sodium hydrogen carbonate according to equation 2.1 and produces temperature change. equation 2.1 C6H8O7(aq) + 3NaHCO3(s) → Na3C6H5O7(aq) + 3CO2(g) + 3H2O(l) ∆Hreaction The maximum temperature change, ∆ Tmax, can be determined by direct measurement of the initial temperature and the final temperature reached. In this question, you are to plan a procedure that would provide sufficient data to allow you to determine an accurate and reliable value for the molar enthalpy change of reaction, ∆Hreaction. (a) Plan an investigation to d etermine the maximum temperature change, ∆Tmax, for the reaction between an aqueous citric acid solution and solid sodium hydrogen carbonate. Measurements should be taken: • before the reaction starts, • during the reaction, • for some time after the reaction is complete. You are provided with: • FA 4 (approximately 120 cm3 of 0.100 mol dm–3 of citric acid solution) • FA 5 (approximately 8 g of solid sodium hydrogen carbonate) • 250 cm3 beaker • 2 polystyrene cups • 0.2 oC division thermometer • 50 cm3 measuring cylinder • glass rod • weighing bottle • weighing balance In your plan you should include brief details of how you would: • prepare and set up the apparatus to minimise heat loss, • measure the initial temperature of the citric acid solution, • add the sodium hydrogen carbonate (do not exceed 3 g) and monitor the temperature change, • ensure the temperature is recorded accurately, • repeat the experiment for reliability, • record and process your data, including how to calculate the average temperature change, ∆Taverage. [Ar: C, 12.0; H, 1.0; O, 16.0; Na, 23.0]

© ACJC 2025 9729/04/Prelim/2025 [Turn over 7 ……………………………………………………………………………………………………………… ……………………………………………………………………………………………………………… ……………………………………………………………………………………………………………… ……………………………………………………………………………………………………………… ……………………………………………………………………………………………………………… ……………………………………………………………………………………………………………… ……………………………………………………………………………………………………………… ……………………………………………………………………………………………………………… ……………………………………………………………………………………………………………… ……………………………………………………………………………………………………………… ……………………………………………………………………………………………………………… ……………………………………………………………………………………………………………… ……………………………………………………………………………………………………………… …………………