2025 H2 Chem Prelim P4 Answers VJC

1 © VJC 2025 9729/04/PRELIM/25 [Turn over CANDIDATE NAME CT GROUP VICTORIA JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 ……………………………………………….………….. …………………………….. CHEMISTRY 9729/04 Paper 4 Practical Candidates answer on the Question Paper. 27 August 2025 2 hours 30 minutes Additional Materials: As listed in the instructions below READ THESE INSTRUCTIONS FIRST Write your name and CT group on all the work you hand in. Give details of the shift and laboratory in the boxes provided. Write in dark blue or black pen. You may use a 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. Shift Laboratory For Examiner’s Use 1 / 14 2 / 17 3 / 12 4 / 12 Total / 55 This document consists of 18 printed pages.

2 © VJC 2025 9729/04/PRELIM/25 Answer all the questions in the spaces provided. 1 Determination of the molar enthalpy change of reaction When zinc powder is added to an aqueous solution of copper(II) ions, a displacement reaction occurs. Cu2+ + Zn → Zn2+ + Cu In this question, you will perform an experiment to determine a value for ΔH. You are provided with: • solid FA 1, zinc • FA 2, containing 0.80 mol dm−3 copper(II) ions (a) (i) You will measure the temperature of the contents in a polystyrene cup at regular time intervals, before and after zinc is added. You will analyse your results graphically to obtain an accurate value of the temperature change caused by the reaction. You will use this value to calculate the heat change, q, for the experiment and hence determine a value for the molar enthalpy change of the reaction, ΔH. In the space provided on page 4 , prepares tables in which to record for your experiment: • all weighings to an appropriate level of precision, • all values of temperature, T, to an appropriate level of precision, • all values of time, t, recorded to 0.5 min. It is important that you measure each temperature at the specified time. Procedure 1. Weigh the capped container containing solid FA 1. 2. Place one polystyrene cup inside another polystyrene cup and place both in a glass beaker. 3. Use the measuring cylinder to transfer 30 cm3 of FA 2 into the cup. 4. Carefully stir the FA 2 in the polystyrene cup with the thermometer. Read and record the temperature, T. Start the stopwatch (t = 0.0 min). The stopwatch must be left to run for the rest of the experiment. 5. Continue to stir FA 2. Read and record T every 0.5 minutes for two minutes. 6. At exactly 2.5 minutes, transfer all the solid FA 1 to the polystyrene cup. Stir the mixture but do not read T. 7. Continue to stir the mixture. Read and record T at t = 3.0 min and every 0.5 minutes until t = 8.0 min. 8. Reweigh the empty capped container. Note: You should keep FA 2 for use in Question 2.

3 © VJC 2025 9729/04/PRELIM/25 [Turn over Question 1 continues on the next page.

4 © VJC 2025 9729/04/PRELIM/25 Results Mass of container and FA 1 / g Mass of container and residual FA 1 / g Mass of FA 1 used / g time / min temperature / oC time / min temperature / oC 0.0 5.0 0.5 5.5 1.0 6.0 1.5 6.5 2.0 7.0 2.5 - 7.5 3.0 8.0 3.5 8.5 4.0 9.0 4.5 [2] • 1m for correct headers and units • 1m for 3 d.p. for mass and correct mass of FA1, 1 d.p. for temperature and no temperature reading at 3 min (ii) Plot a graph of temperature, T, on the y-axis, against time, t, on the x-axis, on the grid in Fig. 1.1. Draw a best-fit straight line taking into account all of the points before t = 2.5 min. Draw another best -fit straight line taking into account all of the points after the temperature of the mixture has started to fall steadily. Extrapolate both lines to t = 2.5 min. [3]

5 © VJC 2025 9729/04/PRELIM/25 [Turn over Fig. 1.1 • 1m for axes correctly labelled and appropriate scales • 1m for all points correctly plotted • 1m for 2 lines of best fit (iii) From your graph, read the minimum temperature, Tmin, and the maximum temperature, Tmax, at t = 2.5 min. Record these values in the spaces below. Calculate the temperature change, ΔT, at t = 2.5 min.

6 © VJC 2025 9729/04/PRELIM/25 Tmin = ……………………………………… Tmax = ……………………………………… ΔT = ……………………………………… [3] • 1m for correct ΔT from the graph • 1m for ΔT ±0.5 oC of supervisor value • 1m for ΔT1 (ΔT1 = Tmax – Tt=2) ±2.0 oC of supervisor value (iv) Calculate the heat change, q, for your experiment using the value you calculated in (a)(iii). Assume that the specific heat capacity of the reaction mixture is 4.18 J g−1 K−1, and that the density of the reaction mixture is 1.00 g cm−3. • Heat change = 30 × 4.18 × ans (a)(iii) J q = …………………………… [1] (v) Determine the molar enthalpy change of reaction, ΔH, for the reaction between zinc and copper(II) ions. Include the sign of ΔH in your answer. [Ar: Zn, 65.4, Cu, 63.5] Amount of Zn used = mass of FA1 used ÷ 65.4 mol Amount of Cu2+ used = 0.8 × 0.03 = 0.0240 mol • Cu2+ is the limiting reagent. • H = –ans (a)(iv) ÷ 0.0240 J mol–1 = convert to kJ kJ mol–1 Must include sign + units ΔH = ……………………………. [3] (b) A student followed the procedure in (a)(i) to perform the experiment, except that he used 60 cm3 of 0.40 mol dm−3 copper(II) ions. State and explain the effect on the temperature change as compared to the value in (a)(iii). .…………………………………………………………………………………………………….... ……….……………………………………………………………………………………………… ……...…………………………………………………………………………………………….. [2] [Total: 14] ● Amount of Cu2+ ions remained the same so heat change will be the same. ● Since vol. of solution doubled, temperature change will half.

7 © VJC 2025 9729/04/PRELIM/25 [Turn over 2 Qualitative analysis FA 2, which you used in Question 1, contains one anion. FA 3 is a solid metal oxide. FA 4 is an aqueous solution of an organic compound, C 6H12O6, containing a functional group which is either an alcohol, an aldehyde or a carboxylic acid. You will carry out tests to identify the anion in FA 2, cation in FA 3 and whether an alcohol, an aldehyde or a carboxylic acid functional group is present