MI Prelim H2 Chem P2 QP_for exchange

Class Adm No Candidate Name: This question paper consists of 21 printed pages and 1 blank page. 2025 Preliminary Examination Pre-University 3 H2 CHEMISTRY 9729/02 Paper 2 Structured Questions 3 September 2025 2 hours Candidates answer on the Question paper. Additional materials: Data Booklet READ THESE INSTRUCTIONS FIRST Do not turn over this question paper until you are told to do so Write your name, class and admission number 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. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. 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. Question 1 2 3 4 5 Total Marks 13 14 12 12 24 75

2 1 The fire triangle (Fig 1.1), shows the three components necessary for combustion. Disrupting any one of these components can suppress or extinguish a fire. Fig 1.1 Fire retardants are substances added to materials to reduce flammability by disrupting the fire triangle. For Examiners’ Use (a) Magnesium hydroxide, Mg(OH)2, is an example of an inorganic fire retardant. It undergoes thermal decomposition upon heating and releases water vapour in the process. (i) Write a balanced equation for the thermal decomposition of magnesium hydroxide. State symbols are not required. ………………………………………………………………………………………………… [1] (ii) With reference to the components in Fig 1.1, suggest one way in which this decomposition reaction helps to suppress a fire. …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… ………………………………………………………………………………………………… [1] Aluminium hydroxide, A l(OH)3 is also a good fire retardant. It undergoes thermal decomposition in the same way as magnesium hydroxide. (iii) Explain why t he thermal decomposition temperature of Al(OH)3 is lower than that of Mg(OH)2. …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… ………………………………………………………………………………………………… [2]

3 [Turn over (iv) Hence, explain whether A l(OH)3 would be a better fire retardant compared to Mg(OH)2. …………………………………………………………………………………………………… …………………………………………………………………………………………………… ………………………………………………………………………………………………… [1] (b) Halogenated fire retardants often use bromine or chlorine derivatives and they release hydrogen halides or halogen radicals upon decomposition which disrupt free radic