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 energy terms  a redox incompatibility of the ions making up the compound  reaction occurring between the ions For the following cases, suggest an explanation for each observation. You may use data from the Data Booklet to assist you in your answers. Give relevant equations to support your answers. (i) When aqueous sodium carbonate is added to aqueous aluminium chloride, a colourless gas is liberated and the precipitate formed is not aluminium carbonate, Al2(CO3)3. [4] (ii) Caprolactum, a monomer of the polymer nylon 6, exists as an alicyclic ring structure and not as 6 -aminohexanoic acid, a straight chain aliphatic structure as most monomers do. O H C N H2NCH2CH2CH2CH2CH2CO2H Caprolactum 6-aminohexanoic acid [3] (iii) It is possible to find MnCl2 in the laboratory but not MnCl3. [2] [Total: 20]

4  VJC 2015 9647/03/PRELIM/15 [Turn over e– 2 (a) The Contact Process is an industrial process for manufacturing sulfuric acid. The key stage in this process is the reaction between sulfur dioxide and oxygen. 2SO2(g) + O2(g) ⇌ 2SO3(g) H = –197 kJ mol–1 Vanadium(V) oxide , V 2O5, is used as a heterogeneous catalyst for this pro cess. Describe the mode of action by which V2O5 fulfils this role. [3] (b) In an experiment to determine the Kp of the above equilibrium, a mixture containing 0.200 mol of SO 2 and 0.100 mol of O 2 was heated in a closed flask and allowed to reach equilibrium at 550oC and 3.5 atm. The flask was then rapidly cooled to liquefy the SO 3 so that it can be separated from the gaseous SO 2 and O 2. Excess water was carefully added to the liquid SO 3, causing the following reaction to occur: SO3(l) + H2O(l)  H2SO4(aq) The resulting solution was made up to 250 cm 3 in a standard volumetric flask. 25.0 cm 3 of this solution was titrated with 1.00 mol dm −3 NaOH and required 36.0 cm3 for complete neutralisation. (i) Calculate the equilibrium amount of SO2, O2 and SO3 at 550oC. [2] (ii) Hence, calculate a value for Kp, including its correct units. [1] (iii) In liquefying the SO 3, it was mentioned that the cooling was performed rapidly. Explain how the titration volume wou ld be affected if the cooling had been performed slowly. [2] (c) One important use of vanadium compounds is the v anadium redox battery, a rechargeable battery which utilizes graphite electrodes. The electrolytes are pumped from separate sto rage tanks A and B into compartments X and Y of the cell respectively, where they are separated by a semi –permeable proton exchange membrane which allows only passage of protons. The direction of the electron flow during the discharging process is indicated in the diagram of the battery above. Tank A acidified VO2 +(aq) and VO2+(aq) Tank B acidified V3+(aq) and V2+(aq) electrical load membrane X Y

5  VJC 2015 9647/03/PRELIM/15 [Turn over (i) By means of the letters X and Y, state and briefly justify the direction of proton flow across the membrane during the discharging process. [1] In a particular setup, compartment X contained 5 dm 3 of the electrolyte, with VO 2 + and VO2+ each having an original concentration of 1.5 mol dm -3. The cell discharged until 9 0% of the reactants was consumed, after which the cell was recharged by connecting an electricity source across the electrodes. (ii) With the aid of the Data Booklet, w rite half–equations for the reactions that occur at the electrodes in X and Y during the charging process. [1] (iii) If the current used for the charging process is 14 A, calculate the time taken, in hours, needed to restore the concentration of the vanadium -containing ions in compartment X to their original value. [2] (d) Chromium is another transition metal adjacent to vanadium in the Periodic Table. Some of its reactions are described below: When chromium metal is dissolved in dilute sulfuric acid, it produces a blue solution containing Cr2+(aq). The blue solution slowly turns green even in the absence of air. This mixture is then filtered to remove the unreacted metal. To the green filtrate V, sodium hydroxide solution was added , forming a grey−green precipitate, W. In excess sodium hydroxide, the precipitate dissolves to form a dark green solution containing an anion, X. The dark green solution turns into a yellow solution, Y, upon warming with hydrogen peroxide solution. (i) By quoting and using relevant Eo values from the Data Booklet , explain why the blue solution turns green. [2] (ii) Identify the precipitate W and the anion X and suggest ionic equations, with state symbols, for the formation of W, and for the formation of X. [2] (iii) State the r