2025 H2 Chem Prelim P2 Answers_VJC

@VJC 2025 9729/02/PRELIM/2025 [Turn over CANDIDATE NAME CT GROUP VICTORIA JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 ……………………………………………….………….. …………………………….. CHEMISTRY 9729/02 Paper 2 Structured Questions 29 August 2025 2 hours Additional Materials: Data Booklet ________________________________________________________________________________________________________________ READ THESE INSTRUCTIONS FIRST Write your name and CT group on this cover page. Write in dark blue or black pen on both sides of the paper. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. 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. For Examiner’s Use 1 / 18 2 / 16 3 / 17 4 / 9 5 / 15 Total / 75 This document consists of 18 printed pages and 2 blank pages

2 © VJC 2025 9729/02/PRELIM/2025 Answer all the questions in the spaces provided. 1 Alkynes are a class of organic compounds with the general formula CnH2n−2. (a) (i) With the aid of a labelled diagram, explain how the orbitals overlap to form the C≡C bond in ethyne, H–C≡C–H. [3] • correct diagram • correct description for σ bond • correct description for the two π bonds (ii) Table 1.1 shows the carbon-hydrogen bond length in ethane, ethene and ethyne. Table 1.1 Molecule Carbon-hydrogen bond length / nm ethane 0.114 ethene 0.109 ethyne 0.106 Use the concept of hybridisation to explain the difference in bond length of the carbon - hydrogen bond between the molecules as shown in Table 1.1. [2] Bond length decreases in the following manner: ethane, ethene, ethyne. Hence bond strength increases in the following manner: ethane, ethene, ethyne • The sp hybridised carbon atom in ethyne has the highest percentage s character , followed by the sp2 hybridised carbon atom in ethene and lastly, sp3 hybridised carbon atom in ethane. • Hence, the extent of orbital overlap between the sp hybridised carbon atom and the s orbital of H atom is the largest, resulting in a shortest bond length. (b) Table 1.2 contains data that is relevant for this question. Table 1.2 Equation Ho / kJ mol–1 3C(s) + H2O(l) → CO(g) + C2H2(g) +401

3 © VJC 2025 9729/02/PRELIM/2025 [Turn over 2C(s) + O2(g) → 2CO(g) −221 2H2O(l) → 2H2(g) + O2(g) +572 (i) Write an equation to represent the standard enthalpy change of formation of C2H2(g). [1] • 2C(s) + H2(g) → C2H2(g) (ii) Use data from Table 1.2 to calculate the standard enthalpy change of formation of C2H2(g). [