MI Prelim H2 Chem P2 Suggested answers_for exchange

Class Adm No Candidate Name: 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 Suggested answers

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] Mg(OH)2 → MgO + H2O (ii) With reference to the components in Fig 1.1, suggest two ways in which this decomposition reaction helps to suppress a fire. [1] The decomposition process is endothermic, which removes the heat component from the fire triangle. Water (vapour) released dilutes the oxygen that is available, reducing the amount of oxygen in the fire triangle. Magnesium oxide forms a protective barrier over the fuel, keeping oxygen and the fuel separated. Any 1 for [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] Al3+ has a higher charge density than Mg 2+ and polarises the O–H bond and weakens it to a larger extent, hence it decomposes at a lower temperature. [1] higher charge density [1] polarises O–H bond and weakens to a larger extent

3 [Turn over (iv) Hence, explain whether A l(OH)3 would be a better fire retardant compared to Mg(OH)2. [1] Better since the decomposition will occur at the lower temperature and release water vapour earlier in a fire. (b) Halogenated fire retardants often use bromine or chlorine derivatives and they release hydrogen halides or halogen radicals upon decomposition which disrupt free radical chain reactions that sustain combustions. (i) One method of preparing brominated organic compounds is free radical substitution. Draw the mechanism for the formation of bromomethane from methane. [3] Initiation √ √ Propagation √ CH4 + Br• → •CH3 + HBr √ •CH3 + Br2 → CH3Br + Br• √ Termination √ 2 •CH3 → C2H6 √ •CH3 + Br• → CH3Br √ 8 ticks – [3] 5 – 7 ticks – [2] 3 – 4 ticks – [1] Tetrabromobisphenol A (TBBPA) is a common brominated flame retardant. It can be made from bisphenol A (BPA) in a single step reacti