2025 H2 Chem Prelim Paper 4 (Solutions) ACJC

© ACJC 2025 9729/04/Prelim/2025 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 Final burette reading / cm3 24.70 24.70 Initial burette reading / cm3 0.00 0.00 Volume of FA 1 used / cm3 24.70 24.70 [2]

© ACJC 2025 9729/04/Prelim/2025 [Turn over (ii) From your titration results in (b)(i), obtain a suitable volume of FA 1, VFA 1, to be used in your calculations. Show clearly how you obtained this volume. VFA1 = (24.70 + 24.70) / 2 = 24.70 cm3 VFA 1 = ....................................... [4] (b) (i) Calculate the amount of MnO4– present in VFA 1 obtained in (a)(ii). 24.70/1000 x 0.0150 = 0.0003705 = 3.71 x 10–4 mol (3 s.f.) 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- 5 x 0.0003705 = 0.0018525 mol = 0.00185 mol (3 s.f.) 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. Mr of NH2OH.HCl = 69.5 Amount of NH2OH.HCl in 1.0 dm3 = 3.3 / 69.5 = 0.04748 mol Amount of NH2OH•HCl in 25.0 cm3 = 25/1000 x 0.04748 = 0.001187 = 0.00119 mol (3 s.f.) amount of NH2OH.HCl = ....................................... [2] assumption …………………………………………………………………………….. …………………………………………………………………………………………… ……………………………………………………………………………………….. [1] There was no significant water loss through boiling, such that the total volume of FA 2 remained at 1.00 dm3. OR Any water lost through boiling was replaced by topping up with deionised water to 1.00 dm3 after cooling. OR NH2OH.HCl is fully soluble in aqueous solution, such that 1 mol of the solid gives 1 mol of NH2OH.

© ACJC 2025 9729/04/Prelim/2025 (iv) Determine which equation represents the reaction between hydroxylamine and iron(III) ion. Show your working. Amount of NH2OH : Amount of Fe2+ 0.001187 : 0.001845 1 : 1.55 ≈ 1 : 2 Hence, it is equation 1.2. 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] NH2OH(aq) + 2Fe3+(aq) → 2Fe2+(aq) + ½N2O(g) + 2H+(aq) + ½H2O(l) eqn 1.1 NH2OH(aq) + Fe3+(aq) → Fe2+(aq) + ½N2(g) + H+(aq) + H2O(l) eqn 1.3 NH2OH(aq) + 3Fe3+(aq) → 3Fe2+(aq) + NO(g) + 3H+(aq) Hydroxylamine acts as a reducing agent. It is oxidised as the oxidation number of nitrogen increases from –1 in hydroxylamine to +1 in N2O. With impurities, the actual mass and therefore the amount of NH2OH.HCl and, in turn, the amount of Fe2+ would be lower than expected. As such, a lower amount MnO4– is required to react completely with Fe2+, resulting in a lower average titre than expected.

© ACJC 2025 9729/04/Prelim/2025 [Turn over 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) ΔHreaction 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, ΔHreaction, may be determined. (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: • FA4 (approximately 120 cm3 of 0.100 mol dm-3 of citric acid solution) • FA5 (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 bicarbonate (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] [9]

© ACJC 2025 9729/04/Prelim/2025 Pre-calculation Let the volume of citric acid used be 50.0 cm3. Amount of citric acid = 50.0 1000 × 0.100 = 0.00500 mol Amount of NaHCO3 needed for complete reaction = 0.005 × 3 = 0.0150 mol Mass of NaHCO3 for complete reaction = 0,0150 × 84.0 = 1.26 g Therefore, for citric acid to be in excess, mass of NaHCO3 should be less than 1.26 g. Procedure 1. Using a 50.0 cm 3 measuring cylinder, transfer 50.0 cm³ of citric acid solution into a polystyrene cup. Note that t