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 ∆Hfo (N2H4) = +50.6 kJ mol−1 ∆Hdecompo (N2H4) = −50.6 kJ mol−1

3 ACJC2025 9729/03/Prelim/2025 (c) The first ever rocket-powered fighter plane, the Messerschmitt Me 163, was powered by the reaction between a hydrazine -methanol fuel mixture, known as 'C-Stoff', and hydrogen peroxide, known as 'T-Stoff'. (i) Hydrazine reacts with hydrogen peroxide as shown in equation 2. equation 2 N2H4(l) + 2H2O2(l) → N2(g) + 4H2O(l) State the oxidation number of nitrogen and oxygen in the reactants and products. [1] (ii) Methanol reacts with hydrogen peroxide to form carbon dioxide and water. Write a balanced equation for this reaction. [1] (iii) The fighter plane would hold 225 dm3 of hydrazine and 862 dm3 of methanol. Table 1.2 shows the standard enthalpy change of combustion, ∆Hco, and densities of hydrazine and methanol. Table 1.2 compound ∆Hco / kJ mol−1 density / g cm−3 N2H4 −622.2 1.021 CH3OH −726.0 0.7918 Use data in Table 1.2 to calculate the heat energy evolved during the combustion of this quantity of rocket fuel at standard conditions. Assume that hydrazine and methanol are fully combusted. [2] ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. (i) N: –2 in N2H4 and 0 in N2; O: –1 in H2O2 and –2 in H2O (ii) CH3OH + 3H2O2 → CO2 + 5H2O (iii) Amount of hydrazine = (225000 × 1.021) / 32.0 = 7179 mol Amount of methanol = (862000 × 0.7918) / 32.0 = 21329 mol Heat energy evolved from hydrazine = 7179 × 622.2 = 4.467 x 106 kJ Heat energy evolved from methanol = 21329 × 726.0 = 15.485 x 106 kJ Total heat energy evolved from oxidation of rocket fuel = 19.9 x 106 kJ

4 ACJC2025 9729/03/Prelim/2025 (d) Hydrazine is also commonly combined with dinitrogen tetroxide, N2O4, in rocket fuels. (i) Reactions used in rocketry produce chemically stable gaseous products. Suggest the products that are formed in the reaction between N 2H4 and N2O4. [1] (ii) Pure N2O4, when warmed, does not immediately decompose into its elements , but instead forms a brown gas. Suggest the identity of this brown gas. [1] (iii) N2H4 does not exhibit ideal gas behaviour. State and explain two reasons for its deviation from ideal gas behaviour. [2] ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. e) A derivative of hydrazine with formula C 2H8N2 was used as rocket fuel in the Apollo missions. Draw two isomers of C2H8N2 containing an N–N bond. [1] ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. (i) N2 and H2O (ii) NO2 (iii) There are significant intermolecular forces of attraction / hydrogen bonding between molecules and is not negligible. The particles of N2H4 take up space relative to the volume of the gas and are not negligible. Collisions are inelastic. 2 of 3 (CH3)2N−NH2, (CH3)NH−NH(CH3), or CH3CH2NH–NH2. 2 of 3

5 ACJC2025 9729/03/Prelim/2025 (f) Phosphorus is one element below nitrogen in the Periodic Table. Describe the reactions of the oxide and chloride of phosphorus with water . Write equations where appropriate and suggest the pH of the solutions formed. [3] ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. …………………………………………………………