Prelim Paper 3 Solutions_ACJC H2 Chem

1 ACJC2025 9729/03/Prelim/2025 Anglo-Chinese Junior College JC2 Preliminary Examination Higher 2 CANDIDATE NAME Solutions FORM CLASS TUTORIAL CLASS INDEX NUMBER CHEMISTRY Paper 3 Free Response Candidates answer on the Question Paper. Additional Materials: Data Booklet 9729/03 1 September 2025 2 hours READ THESE INSTRUCTIONS FIRST Write your index number and name 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. If additional space is required, you should use the pages at the end of this booklet. The question number must be clearly shown. Section A Answer all questions. Section B Answer one question. Circle the number of the question you have attempted. A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. 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. For Examiners’ use only Section A 1 / 15 2 / 24 3 / 21 Section B 4 / 5 / 20 Presentation Total / 80 This document consists of 28 printed pages.

2 ACJC2025 9729/03/Prelim/2025 Section A Answer all the questions in this section. 1 The Mars Curiosity rover’s landing in August 2012 was achieved using hydrazine fuelled rocket thrusters. The rapid decomposition of hydrazine , N 2H4, over a suitable catalyst to produce hot gaseous elements as products provides the thrust. Ammonia can be formed as an intermediate during the decomposition. (a) Write a balanced equation for hydrazine decomposing to ammonia and nitrogen gas. [1] ………………………………………………………………………………………………….. (b) Hydrazine may be obtained from the reaction between ammonia and hydrogen peroxide as shown in equation 1. equation 1 2NH3(g) + H2O2(l) → N2H4(l) + 2H2O(l) ∆Hro = −241.0 kJ mol−1 Table 1.1 shows the standard enthalpy change of formation, ∆Hfo, for some compounds in equation 1. Table 1.1 compound ∆Hfo / kJ mol−1 NH3 −46.1 H2O2 −187.8 H2O −285.8 Calculate the standard enthalpy change for the decomposition of hydrazine to its elements. [2] ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. 3N2H4 → 4NH3 + N2 ∆Hro = –(2 × –46.1) – (–187.8) + ∆Hfo (N2H4) + (2 × –285.8) = −241.0