HCI_H2 Chem Prelim P3

HWA CHONG INSTITUTION C2 PRELIMINARY EXAMINATION 9746 H2 CHEMISTRY Paper 3 Free Response 10 September 2008 2 hours Do not open this booklet until you are told to do so. READ THESE INSTRUCTIONS FIRST Write your name and class on all the work you hand in. Write in dark blue or black pen. You may use a soft pencil for any diagrams, graphs or rough working. Answer any four questions. Begin each question on a new piece of paper. A Data Booklet is provided. You may use a calculator. 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. Circle the question number that you have answered on the cover page provided. This question booklet consists of 8 printed pages.

2 Answer any four questions. 1 (a) (i) What do you understand by the term dynamic equilibrium? The ester, ethyl ethanoate, can be formed in the following reaction: CH3CO2H(l) + C2H5OH(l) = CH3CO2C2H5(l) + H2O(l) In an experiment, 0.5 mol of ethanoic acid and 1.0 mol of ethanol were shaken for a long time and allowed to reach dynamic equilibrium at room temperature. 0.42 mol of ethyl ethanoate was found to be present in the equilibrium mixture. (ii) Write an expression for the equilibrium constant, Kc, and calculate its value. (iii) Predict, with reasoning, the effect on the equilibrium when some aqueous sodium hydroxide is added to the equilibrium mixture. (iv) Give one other compound that can be used in place of CH 3CO2H to produce the same ester. [6] (b) Ethanoic acid is the main component in vinegar. When a 25.0 cm 3 sample of vinegar was titrated against 0.6 mol dm −3 aqueous potassium hydroxide using a pH meter, the following graph was obtained. (i) Using the information provided, show by calculations that the acid dissociation constant, Ka, of ethanoic acid has a numerical value of 1.84 × 10−5. Solution A was prepared by adding 10.0 cm 3 of the KOH solution to the 25.0 cm 3 vinegar sample. (ii) When a small amount of acid or base was added to solution A, its pH remained relatively constant. Explain qualitatively, with the aid of equations, why this is so. (iii) Calculate the pH of solution A. [8] 2.44 Volume of KOH / cm3 30.0 9746/03/HCI/C2 Prelim/2008

3 (c) Succinic acid, historically known as spirit of amber, plays an important biochemical role in the citric acid cycle. Its structure is shown below: HO2CCH2CH2CO2H succinic acid In no more than four steps, propose a reaction scheme to obtai