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