2025 H2 Chem Prelim P2 QP CJC

[Turn over 9729/02 CJC JC2 Preliminary Examination 2025 CANDIDATE NAME CLASS 2T INDEX NUMBER CHEMISTRY 9729/02 Paper 2 Structured Questions 29 August 2025 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name and class on all the work you hand in. 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 calculat or is expected, where appropriate. A Data Booklet is provided. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 21 printed pages and 1 blank page. For Examiner’s Use Paper 1 30 Paper 2 Q1 /16 Q2 / 7 Q3 /10 Q4 /18 Q5 /13 Q6 /11 75 Paper 3 80 Paper 4 55 OVERALL (100%) GRADE Catholic Junior College JC2 Preliminary Examination Higher 2

2 9729/02 CJC JC2 Preliminary Examination 2025 Answer all the questions in the space provided. 1 (a) Many chemical compounds used for rocket fuel and propellants are highly reactive and hazardous. One of these compounds, diborane, B 2H6, can be formed from its elements according to the following equation: 2B(s) + 3H2(g) → B2H6(g) Given the following data, 2B(s) + 3 2O2(g) → B2O3(s) ΔH1 ⦵ = −1273 kJ mol−1 B2H6(g) + 3O2(g) → B2O3(s) + 3H2O(l) ΔH2 ⦵ = −2035 kJ mol−1 H2(g) + 1 2O2(g) → H2O(l) ΔH3 ⦵ = −242 kJ mol−1 (i) Name the enthalpy change represented by ΔH1 ⦵. ……..…………………………………………………………………………………….. [1] (ii) Construct a suitable energy cycle and calculate the enthalpy change of formation of diborane, B2H6(g). [3] (b) The reaction of 2,2-dimethylhydrazine, (CH3)2N2H2, with dinitrogen tetroxide, N2O4, is another energy source for rockets. (CH3)2N2H2(l) + 2N2O4(l) → 2CO2(g) + 3N2(g) + 4H2O(g) ΔS⦵ = +1141.2 J K‒1 mol‒1 (i) One of the reactants in the above reaction , dinitrogen tetroxide, N2O4, may be obtained from the more commonly available NO2. State and explain the effect on the entropy, S, of the chemical system during the conversion of NO2(g) to N2O4(g). …………………………………………………………………………………… ..….…..... …………………………………………………………………………………… ..….…..... ……………………… …………………………………………………………… ..….….... …………………………………………………………………………… ………………[1]

3 9729/02 CJC JC2 Preliminary Examination 2025 [Turn over (ii) By using the standard enthalpy change of formation, ΔHf ⦵, values given below, calculate the standard Gibbs free energy change, ∆G⦵, in kJ mol‒1 for the reaction of (CH3)2N2H2 with N2O4 at 298 K. substance (CH3)2N2H2(l) N2O4(l) CO2(g) H2O(g) ΔHf ⦵ / kJ mol‒1 +48.9 ‒19.6 ‒393.5 ‒241.8 ……………………………………………………………………………………..….……... ……………………………………………………………………………………..…..…..... ……………………………………………………………………………………..………… …………………………………………………………………………..….….....…….…… …………………………………………………………………………..….….....……….… …………………………………………………………………………..….…....………..… ……………………………………………………………………………………………. [3] (c) (i) The chlorides of Period 2 elements behave similarly to chlorides of Period 3 elements. Based on your knowledge of chlorides of Period 3 elements, sketch a graph of pH against the chlorides of lithium to nitrogen. In your sketch, consider the chloride of carbon is immiscible with water. [3] (ii) Write an equation with state symbols to account for the pH of liquid NCl3 when dissolved in water. Aqueous HNO 2 and steamy white fumes are formed in the reaction. ..……......................................................................................................................[2] pH LiCl BeCl 2 BCl 3 CCl 4 NCl 3 7

4 9729/02 CJC JC2 Preliminary Examination 2025 (d) Describe and explain how the thermal stability of the hydrogen halides varies down Group 17. Include an equation for the thermal decomposition reaction in your answer. ……………………………………………………………………………………..….….....…..… ………………………………………………………………………………..….….....………..… …………………………………………………………………………..……………..………...… ……………………………………………………………..….….....…………………………..… ………………………………………………………..….….....………………………………..… …………………………………………………..….…....………………………………………… …………………………………………………………………..……………………………....[3] [Total: 16]

5 9729/02 CJC JC2 Preliminary Examination 2025 [Turn over 2 Ammonia gas, NH3, is used in industrial refrigeration systems. A refrigeration chamber contains 0.686 mol of ammonia gas at a temperature of 360K. The chamber volume is 2.25 dm3. (a) State two main assumptions of kinetic theory of gases. …………………..……………………………………………………………………………….… …………………..…………………………………………………………………………...…..… …….…….………………………………………………………………………….......…….……. ……………………………………………………………………………………………............... ..........................................................................................................................................[2] (b) Calculate the pressure of ammonia, in kPa, in the chamber using the ideal gas equation. [1] (c) To determine the pressure of ammonia in the chamber more precisely, the van der waals’ equation shown below is used. (p + n2a V2 )(V – nb) = nRT (where a and b are constants) Given that the pressure obtained using van der waals’ equation is lower than your answer in (b), explain the difference. ……….…….………………………………………………………………………….......…….…. .………………………………………………………………………………………..................... .....................................................................................................................……………..[1] (d) Under what conditions of temperature and pressure would you expect ammonia to be most like that of an ideal gas? ……..………………………………………………………………………………………..….. [1]

6 9729/02 CJC JC2 Preliminary Examination 2025 (e) The graph below shows the compressibility curve of 1 mole of ammonia gas. On the same axes, draw the compressibility curve for 1 mole of ammonia at a lower temperature in the graph below and explain the shape of your graph. Your answer should include reference to intermolecular forces. ……….…….……………………………………………………………….………….......……….. ……….…….………………………………………………………………………….......…….….. ……………………………………………………………………………………………................ ..........................................................................................................................................[2] [Total: 7] Ideal gas pV RT NH3 p

7 9729/02 CJC JC2 Preliminary Examination 2025 [Turn over 3 (a) A 2.67 g sample of a Period 3 chloride is heated to 227 °C in a sealed flask. At this temperature, the chloride is a gas of volume 250 cm 3 and the pressure in the flask is 323 kPa. Using the general gas equation, calculate the Mr of the Period 3 chloride. Deduce its formula. [3] (b) Triphenylphosphine (represented as PPh3) is used in a type of reaction known as a Wittig reaction. In the Wittig reaction, a carbonyl compound reacts with a halogenoalkane to form an alkene. The conversion is shown in the following unbalanced equation. (i) Draw the structure of the product formed from the Wittig reaction of the following compounds.