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 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]

4 Tetrabromobisphenol A (TBBPA) is a common brominated flame retardant. It can be made from bisphenol A (BPA) in a single step reaction, as shown in Fig. 1.2. Fig 1.2 (ii) State the reagent and conditions required to convert BPA to TBBPA. ………………………………………………………………………………………………… [1] Tetrachlorobisphenol (TCBPA) is the chlorinated analogue of TBBPA, where the bromine atoms are replaced with chlorine. (iii) Using relevant data from the Data Booklet, explain the difference in strength between C–Cl and C–Br. …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………[2]

5 [Turn over (iv) Hence, explain whether TCBPA would be a better flame retardant than TBBPA. …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………[1] (c) Despite their effectiveness, halogenated fire retardants such at TBBPA are being phased out due to concerns over the release of toxic persistent organic pollutants during fires. Suggest one reason why Mg(OH) 2 is considered a safer alternative to halogenated fire retardants. ………………………………………………………………………………………….……………... ………………………………………………………………………………………….……………... ……………………… ………………………………………………………………….………… [1] [Total: 13]

6 2 (a) Lead(II) sulfate, PbSO 4, is a sparingly soluble white solid that was historically used as a pigment in paints. For Examiners’ Use (i) Write an expression for the solubility product, Ksp, of PbSO4, stating its units. ………………………………………………………………………………………………… [1] (ii) Calculate the solubility of PbSO4 in pure water, given the value of Ksp is 1.60 × 10–8. [1] (iii) A common method used by early artists to prepare PbSO 4 was to mix solutions of Na2SO4 and Pb(NO3)2. A solution of 0.0200 mol dm–3 Pb(NO3)2 was mixed with an equal volume of Na2SO4 to prepare PbSO4. Calculate the minimum concentration of Na2SO4 required for PbSO4 to be formed. [2]

7 [Turn over (b) Due to the increased awareness of the toxicity of lead compounds, titanium dioxide, TiO 2, is now widely used as a non -toxic alternative to PbSO 4 in white pigments by modern artists. (i) Write the full electronic configuration of the titanium ion in TiO2. ………………………………………………………………………………………………… [1] (ii) Explain what is meant by the term transition element. …………………………………………………………………………………………………… ………………………………………………………………………………………………… [1] (iii) Explain why TiO2 appears white despite containing a transition element. …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………[2] (c) Copper obtained from the extraction of copper ores is often impure and contains small amounts of other metals such as zinc, silver and lead. To obtain high purity copper for electrical use, impure copper is placed at the anode and a pure copper rod is placed at the cathode in an electrolytic cell. The electrolyte used is aqueous copper(II) sulfate. (i) Write the half equation for the reaction occurring at the anode for copper. ………………………………………………………………………………………………… [1]

8 (ii) Using relevant data from the Data Booklet, explain how zinc and silver impurities are removed during the purification process. …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… ………………………………………………………………………………………………… [3] While the lead impurity was expected to dissolve into the electrolyte as Pb 2+ ions, it was instead found in the anodic sludge as a compound. (iii) State the identity of the compound and explain how it was formed. …………………………………………………………………………………………………… …………………………………………………………………………………………………… ………………………………………………………………………………………………… [2] [Total: 14]

9 [Turn over 3 Advanced Oxidation Processes (AOPs) are used to degrade organic pollutants in wastewater through the formation of highly reactive hydroxyl radicals (•OH). Two examples of AOP s are the Fenton reaction and the UV/H 2O2 process which generate hydroxyl radicals (•OH) from H2O2 that is mixed in with