2025 DHS H2 Chem Prelim Paper 3_Suggested Solutions

© DHS 2025 9729/03 [Turn over DUNMAN HIGH SCHOOL Preliminary Examination Year 6 H2 CHEMISTRY Paper 3 Free Response Questions Candidates answer on the Question Paper. Additional Materials: Data Booklet 9729/03 25 September 2025 2 hours READ THESE INSTRUCTIONS FIRST Write your centre number, index number, name and class at the top of this page. 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. A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. The number of marks is given in brackets [ ] at the end of each question or part question. Suggested Solutions For Examiner’s Use Section A 1 20 2 20 3 20 Section B 4 / 5 20 Total 80 This document consists of 22 printed pages.

2 © DHS 2025 9729/03 Section A Answer all the questions in this section. 1 (a) The order of increasing Br Ønsted-Lowry acidity of cyclohexanol, benzoic acid and 2−chlorobenzoic acid is shown. Explain this order. OH cyclohexanol CO2H benzoic acid CO2H Cl 2-chlorobenzoic acid < < [3] Cyclohexanol is the least acidic because its conjugate base, is destabilised by the electron-donating effect of the cyclohexyl/alkyl group, which intensifies the negative charge on the oxygen in the conjugate base. Both 2 -chlorobenzoic acid and benzoic acid are more acidic than cyclohexanol because their conjugate bases are resonance -stabilised by the delocalisation of negative charge over two highly electronegative oxygen atoms. In 2-chlorobenzoic acid, presence of electron withdrawing Cl atom helps to disperse the negative charge, making its conjugate base (2 -chlorobenzoate ion) more stable than benzoate. Hence, 2-chlorobenzoic dissociates to the largest extent and cyclohexanol to the least extent. (b) BF3 acts as a Lewis acid in its reaction with dimethylether, CH 3OCH3, to form product P. (i) Describe how BF3 acts as a Lewis acid in this reaction. [1] The electron deficient B atom in BF3 accepts the (lone) electron pair from O in CH3OCH3. (ii) Draw a structure for P to show the bonding present. [1] F BF F O CH3 CH3 (c) Dimethylether, CH 3OCH3, can be formed from the reaction between carbon monoxide, CO, and hydrogen, H2, as shown by equation 1. equation 1 3CO(g) + 3H2(g) ⇌ CH3OCH3(g) + CO2(g) A mixture of CO and H2 was introduced into a 10 m3 sealed vessel at 500 K. The initial total pressure was 40 atm.

3 © DHS 2025 9729/03 [Turn over A