2023_DHS_H2_Chem_Prelim_Paper_2_Suggested_Solutions (1)

© DHS 2023 9729/02 [Turn over Suggested Solutions DUNMAN HIGH SCHOOL Preliminary Examination Year 6 H2 CHEMISTRY Paper 2 Structured Questions Candidates answer on the Question Paper. Additional Materials: Data Booklet 9729/02 14 September 2023 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. The use of an approved scientific calculator is expected, where appropriate. You may lose marks if you do not show your working or if you do not use appropriate units. A Data Booklet is provided. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use 1 10 2 12 3 14 4 14 5 25 Total 75 This document consists of 25 printed pages and 1 blank page.

2 © DHS 2023 9729/02 Answer all the questions in the spaces provided. 1 (a) A fluorophore is a fluorescent chemical compound that can re -emit light upon light excitation. Fluorophores typically contain several combined aromatic groups, or planar or cyclic molecules with several π bonds. Derivatives of trans -stilbene ha ve been recently studied due to their well -known fluorophore properties and they are generally more stable than cis -stilbene. The structure of cis-stilbene is shown in Fig. 1.1. Fig. 1.1 (i) Explain why the melting point of trans -stilbene is higher than that of cis-stilbene. [1] Trans-isomer has higher symmetry and packs more closely into a crystal lattice. More energy is required to break the stronger intermolecular forces of attraction in crystal lattice in the trans–isomer than the cis–isomer. (ii) Based on the structure of stilbene, suggest a reason why trans -stilbene is more stable than cis-stilbene. [1] It has two bulky phenyl groups far from each other, making this compound more stable than cis isomers due to less steric hindrance. (b) Aluminium chloride, A lCl3, is used as a halogen carrier in order for chlorination of benzene to take place in stilbene. AlCl3 + Cl2 ⇌ AlCl4 – + Cl + (i) Name the type of bond formed when AlCl3 reacts with C l2. Explain how this bond is formed. [2] Dative bond. Al atom has a vacant low-lying orbital in its valence shell so it can accept an electron pair from Cl2. (ii) Draw the dot-and-cross diagram for the AlCl4 – ion. Use the VSEPR theory to state and explain the shape of the ion and the bond angle present. [3] x x x AlCl Cl Cl Cl x _

3 © DHS 2023 9729/02 [Turn over There are 4 bond pairs and 0 lone pairs around the central aluminium atom. As the repulsion between the 4 bond pairs are similar in s