SAJC 2022 P2 ANS

1 [TURN OVER ST ANDREW’S JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATIONS HIGHER 2 CANDIDATE NAME CLASS 2 1 S CHEMISTRY Paper 2 Structured Questions Candidates answer on the Question Paper. Additional Materials: Data Booklet 9729/02 31 August 2022 2 hours READ THESE INSTRUCTIONS FIRST Write your name and class on all the work that you hand in. Write in dark blue or black pen. You may use a 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. 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. This document consists of XX printed pages (including this cover page). For Examiner’s Use Q1 12 Q2 14 Q3 14 Q4 10 Q5 25 Total 75

2 [TURN OVER 1 Hydrazine, N2H4, is a colourless liquid with an ammonia-like odour. It is an important precursor in the pharmaceuticals industry. (a) Hydrazine exists as a liquid while ammonia exists as a gas at room temperature and pressure. State two reasons to explain this difference in physical state. [2] Both ammonia and hydrazine are polar simple covalent molecules, capable of forming intermolecular hydrogen bonds. Hydrazine has more hydrogen bonding sites and therefore, it is able to form more extensive intermolecular hydrogen bonds. At the same time, it has more electrons / larger electron cloud size and this leads to stronger intermolecular instantaneous dipole -induced dipole interactions . Hence, both of these require more energy to overcome. (b) The Kb values of hydrazine, ethylamine, and phenylamine are shown in Table 1.1. Table 1.1 base Kb / mol dm–3 Hydrazine 1.7 x 10–6 (for Kb1) Ethylamine 4.5 x 10–4 Phenylamine 7.4 x 10–10 (i) Explain what is meant by the term Bronsted-Lowry base. [1] A Bronsted-Lowry base refers to a proton acceptor. (ii) Explain the relative magnitudes of the Kb values in Table 1.1. [2] (most basic) ethylamine > hydrazine > phenylamine Unlike hydrazine, ethylamine has an electron donating alkyl/ethyl group which makes the lone pair of electrons on N more available to accept H+. Phenylamine is weaker base than hydrazine as the lone pair of electrons on N is delocalised into the benzene ring , making it less available to accept H+. (iii) The Kb values of diethylamine and triethylamine are shown in Table 1.2.

3 [TURN OVER Table 1.2 base Kb / mol dm–3 Diethylamine 6.9 x 10–4 Triethylamine 6.5 x 10–5 Suggest why the Kb value of triethylamine is significantly smaller than the Kb values of ethylamine and diethylamine. [1] Triethylamine is a tertiary amine and the presence of (one) more R/al