2025 H2 Chem Prelim P2 Worked Solutions 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. 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 WORKED SOLUTIONS

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) ∆H1θ = − 1273 kJmol−1 B2H6(g) + 3O2(g) → B2O3(s) + 3H2O(l) ∆H2θ = − 2035 kJmol−1 H2(g) + 1 2O2(g) → H2O(l) ∆H3θ = − 242 kJmol−1 (i) Name the enthalpy change represented by ∆H1θ. ……..…………………………………………………………………………………….. [1] (ii) By drawing an energy cycle diagram, calculate the enthalpy change of formation of diborane, B2H6(g). [3] Using Hess’s Law, ∆Hf (B2H6) = -1273 + 3 x (-242) – (-2035) = +36 kJ mol-1 (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) ∆Sθ = +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] Standard enthalpy change of formation of B2O3(s) B2O3(s) + 3H2O(l) 2B (s) + 3H2 (g) B2H6 (g) -1273 + 3 x (-242) -2035 + 3 O2(g) + 3 O2(g) ∆Hf (B2H6) 2 NO2(g) → N2O4(g). When NO2(g) is converted to N2O4(g), entropy decreases because there is a decrease in the number of gaseous molecules (from 2 molecules of NO 2 to 1 molecule of N 2O4). Hence there are lesser ways of arranging the particles resulting in a decrease in disorder of the system.

3 9729/02 CJC JC2 Preliminary Examination 2025 [Turn over (ii) By using the standard enthalpy change of formation, ∆H𝑓 o, values given below, calculate the standard Gibbs free energy change, ∆Gθ, 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) ∆H𝑓 o / 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 ∆Ho = ∆Hfo(products) – ∆Hfo(reactants) = [2(−393.5) + 0 + 4(−241.8)] – [(+48.9 + 2(−19.6)] = −1763.9 kJ mol−1 Now, ∆Go = ∆Ho − T∆So = –1763.9 – [298(+1141.2 x 10–3)] = – 2.10 x 103 kJ mol–1 NCl3(l) + 2H2O(l) → HNO2(aq) + 3HCl(g)

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] Describe: • Hydrogen halides decompose on heating to give hydrogen gas and halogens. 2HX → H2 + X2 where X is a halogen. • The thermal stability decreases down from HCl, HBr and HI. Explain: • Down the group, as the size of the halogen atom increases. • The H–X bond length increases and is weaker due to less effective orbital overlap. • Hence, the bond energy of H–X decreases, and thermal stability decreases down the group.

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] pV = nRT p (2.25 / 1000) = 0.686 (8.31)(360) p = 912 kPa The pressure is lower due to the presence of hydrogen bonding between ammonia molecules. high temperature and low pressure. The individual gas particles have negligible volume as compared to the overall gas volume. There ar