VJC Prelim P3

2  VJC 2015 9647/03/PRELIM/15 [Turn over 1 Magnesium ethanoate is commonly used as a source of magnesium or ethanoate ions in chemistry experiments. One of the more prevalent uses of magnesium ethanoate is in the mixture called calcium magnesium ethanoate (CMA). It is a mixture of calcium ethanoate and magnesium ethanoate. CMA acts as a powerful SO 2, NO x, and toxic particulate emission control agent in coal combustion processes to reduce acid rain, and as an effective catalyst for the facilitation of coal combustion. (a) Samples of anhydrous magnesium ethanoate, Mg(CH 3COO)2 and barium ethanoa te, Ba(CH3COO)2, were heated to a temperature of 600 oC causing them to thermally decompose. White residues were formed in both cases and a common gaseous product, X, with the molecular formula of C 3H6O was also given off. X gives a yellow precipitate upon warming with aqueous alkaline iodine. In addition, the decomposition of magnesium ethanoate also produced a second gaseous product which formed a white precipitate with calcium hydroxide solution. (i) Identify the gaseous product, X. [1] (ii) Write a balanced equation with state symbols, for each decomposition reaction. [2] (iii) Account for the difference in the decomposition products. [2] (b) Magnesium, is an extremely important light wei ght structural metal which can be produced by the electrolysis of magnesium chloride . Magnesium chloride can be prepared from magnesium oxide w hich is obtained from sea -water containing a significant amount of Mg2+ and Ca2+. The steps involved are shown below: The numerical values of the relevant solubility products are given below. Magnesium carbonate 1.0 x 10-5 Calcium carbonate 8.7 x 10-9 Magnesium hydroxide 1.1 x 10-11 Calcium hydroxide 5.5 x 10-6 Sea water (containing Mg2+ and Ca2+) filtrate Step 1 Controlled addition of CO3 2- filter Step 2 addition of OH-(aq) Mg(OH)2 Step 3 heat MgO Step 4 MgCl2 Step 5 Mg

3  VJC 2015 9647/03/PRELIM/15 [Turn over (i) Calculate and compare the solubility between magnesium carbonate and magnesium hydroxide. [3] (ii) Assuming the concentration of magnesium ions in the filtrate in step 2 is 3.0 x 10 -5 mol dm-3, calculate the concentration of hydroxide ions present in the filtrate when trace of solid magnesium hydroxide first appears. [1] (iii) Explain why the addition of carbonate ions in step 1 has to be controlled. [1] (iv) Give a reason why the electrolysis of magnesium chloride is preferred to that of magnesium oxide. [1] (c) Some organic and inorganic compounds are class ified under a category called ‘non - existent compounds’. They are called non -existent because so far chemists had been unable to synthesise them. Some of the reasons why these compoun ds are unstable are:  unfavourable bond ene