MI_H2_CHEM_P3 Prelim

Class Adm No Candidate Name: This document consists of 9 printed pages and 1 blank page. [Turn over 2014 Preliminary Examination II Pre-university 3 H2 CHEMISTRY 9647/03 Paper 3 Free Response Tuesday, 23 Sep 2014 2 hours Candidates answer on separate paper. Additional Materials: Answer Paper Data Booklet Graph Paper READ THESE INSTRUCTIONS FIRST Write your name, class and index number on all the work you hand in. 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 any four questions. A Data Booklet is provided. You are reminded of the need for good English and clear presentation in your answers. The number of marks is given in brackets [ ] at the end of each question or part question. At the end of the examination, fasten all your work securely together. Begin each question on a fresh sheet of paper.

2 1. Lactic acid (2-hydroxypropanoic acid), also known as milk acid, is a chemical compound that plays a role in several biochemical processes. (a) (i) A is an isomer of lactic acid. It rotates plane -polarised light and reacts with Fehling’s solution to give a reddish brown precipitate. 0.01 mol of A reacts with sodium to give 0.24 dm 3 of hydrogen gas under room temperature and pressure conditions. Suggest a structural formula of A, giving reasons for your answer. (ii) Compare the acidity of isomer A and lactic acid. Explain your answer. [7] (b) Lactic acid can undergo a condensation reaction in the presence of hot concentrated sulfuric acid to form a cyclic diester B, C6H8O4. The system exists in dynamic equilibrium. (i) Draw the structure of diester B. (ii) Explain what is meant by the term dynamic equilibrium. (iii) Write an expression for the equilibrium constant, K C, for the reaction, stating its units. (iv) In an experiment, 2.0 mol of lactic acid was allowed to reach equilibrium. Given that 95% of lactic acid has reacted at equilibrium and the total volume of the mixture remained constant at 200 cm 3, calculate the K C for this reaction. (v) In the reverse reaction, i.e. the hydrolysis of the diester B, the rate of reaction gradually increases at first, but subsequently it decreases. Explain why this is so. [8]

3 [Turn Over (c) Lactic acid, along with ammonium hydrogen carbonate, (NH 4)HCO3, is used in mosquito attractant. Ammonium hydrogen carbonate is a solid which can decompose into ammonia, carbon dioxide and water. (i) Write an equation , with state symbols, for the decomposition of ammonium hydrogen carbonate. (ii) Using the enthalpy changes given in the table below, calculate the enthalpy change of reaction for the decomposition of (NH4)HCO3. Compound Enthalpy change of formation / kJ mol-1