SAJC Prelim P3_QP

1 [Please Turn Over NAME Class ST ANDREW’S JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATIONS Chemistry (9729) Paper 3 Free Response 14 September 2018 2 hours Additional Materials: Data Booklet, Writing Paper READ THESE INSTRUCTIONS: 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. Section A Answer all questions. Marks [60] Section B Answer one question. Marks [20] 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 16 printed pages including this page.

2 Section A Answer all the questions in this section. 1 Electrosynthesis in organic chemistry is the synthesis of compounds in an electrolytic cell using inert electrodes. One important process is the Kolbe reaction. The Kolbe reaction is the decarboxylative dimerization of two carboxylate ions. Alkanes can be formed at the anode by the following reaction. (a) Explain why alkanes are generally unreactive. [1] (b) (i) Calculate the volume of carbon dioxide produced when a steady current of 5A is passed through the electrolyte for 32 minutes at standard temperature and pressure. [2] (ii) Methanoate ions undergo the Kolbe reaction to form only 2 gases. Using the axes given below, sketch the variation of pV/RT against p for one mole of each gas at the same temperature. Briefly explain your answer. [2] pV/RT p 0 1.0 ideal gas

3 [Please Turn Over 1 (c) The cathodic reaction for the Kolbe reaction can be represented by the following half equation: 2H2O + 2e  H2 + 2OH-. Write an overall equation for the Kolbe reaction involving ethanoate ions. Hence, calculate ΔGƟ for the Kolbe reaction given that the E Ɵcell value for the reaction is +2.28 V. [2] (d) The reaction mechanism of the Kolbe reaction involves a 4-step reaction mechanism as described below: Step 1: Dissociation of carboxylic acid to form a carboxylate ion. Step 2: Oxidation of carboxylate ion to form a carboxyl radical, RCOO•. Step 3: Decomposition of the carboxyl radical to form carbon dioxide gas and an alkyl radical, R•. Step 4: Formation of a covalent bond between two alkyl radicals. (i) Using ethanoic acid as a starting reagent, suggest suitable equations for Steps 1, 2 and 4 on how ethane can be produced using the Kolbe reaction. [3] (ii) The decomposition of the carboxyl radical to form carbon dioxide gas and an alkyl radical, R •, in Step 3 can be represented by the following equation: RCOO•  CO2 + R• Outline the mechanism of this step by copying the diagram below and include relevant curly half arrows. [1] (e) In another separate experiment, a new alkane A, C6H12, was prod