2025 DHS H2 Chem Prelim Paper 3 QP

© 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. Name: Centre/Index Number: Class: For Examiner’s Use Section A 1 20 2 20 3 20 Section B 4 / 5 20 Total 80 This document consists of 31 printed pages and 1 blank page.

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. [3] OH cyclohexanol CO2H benzoic acid CO2H Cl 2-chlorobenzoic acid < < …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. ………………………………………………………………………………………………….

3 © DHS 2025 9729/03 [Turn over (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] (ii) Draw a structure for P to show the bonding present. [1] …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. ………………………………………………………………………………………….

4 © DHS 2025 9729/03 (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. After dynamic equilibrium was established at 500 K, the total pressure in the vessel decreased to 28 atm. (i) Write an expression for the equilibrium constant, Kp, for this reaction, and state its units. [1] (ii) Use of the Data Booklet is relevant to this question. The amount of CH 3OCH3 at equilibrium was found to be 732 mol. Show that the equilibrium partial pressure of CH3OCH3 in the sealed vessel is 3 atm. [1] (iii) At equilibrium, it was found that 60% of the CO had been converted to the products. Calculate the equilibrium partial pressures of CO and H2 in atm. Hence, determine the value of Kp for the reaction. [3] (iv) The volume of the vessel is reduced from 10 m 3 to 5 m 3 and the system is allowed to reach equilibrium. Explain the effect this will have on the partial pressures of the individual gases and the composition of the reaction mixture. [2] …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. ………………………………………………………………………………………….

5 © DHS 2025 9729/03 [Turn over …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. ………………………………………………………………………………………….

6 © DHS 2025 9729/03 (d) NH3 is commonly used as a nucleophile in organic chemistry. In some instances, it adds to the C=C bond as shown in Fig. 1.1. NH3 + CH2=CH2 CH2(NH2)CH3 NH3 + CH2=CHCN CH2(NH2)CH2CN x Fig. 1.1 Suggest reasons to explain Fig. 1.1. Use concepts of electronegativity and electronic effects in your answer. [2] …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. (e) Ammonia or primary amines react with aldehydes and ketones to produce imines as shown in Fig. 1.2. R1, R2 and R3 represent alkyl groups or hydrogen atoms. R1 R2 O R1 R2 N R3 + H2O imine + R3NH2 Fig. 1.2 The reaction is carried out at carefully controlled pH of between 4 and 5. The mechanism for the formation of an imine between a primary amine and propanone is shown in Fig. 1.3.