TJC 08 TJC Chem H2 Prelim P3

TEMASEK JUNIOR COLLEGE CHEMISTRY 9746/03 Higher 2 Paper 3 Free Response Thursday 11th SEPTEMBER 2008 2 hours Candidates answer on separate paper. Additional materials: Answer paper Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, centre/index number & CG in the spaces provided on the cover page provided and 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. You may use a calculator. A Data Booklet is provided. At the end of the examination, fasten all your work securely together 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. This question paper consists of 12 printed pages. [Turn over

2 [Turn over [Turn over Answer any four questions. 1. ) ome data on three nitrogen-containing compounds are given in the table below: Compound Molecular formula Boiling point/ C (a S o Dinitrogen pentoxide N 2O5 decomposes nitric acid HNO3 83 nitrosyl chloride NOCl -6.4 (i) symmetrical molecule) and NOCl. Indicate the relevant bond angles in each case. (ii) ference in the boiling points of HNO3 and NOCl in terms of structure and bonding. [6] (b) oxide and chlorine is an endothermic process hich takes ing to the equation: 2NOCl (g) 2NO (g) + Cl2 (g) ∆H > 0 3 vessel at 400 K, 0.5 mol of NOCl is injected and the equilibrium pressure is 101 kPa. (i) e the total number of moles of gas at equilibrium, assuming the gases behave ideally. (ii) ence calculate the percentage of the nitrosyl chloride that has dissociated. Draw diagrams to illustrate the shapes of N2O5 (a Explain the dif The dissociation of nitrosyl chloride into nitric w place accord In an evacuated 20 dm Calculat H

3 [Turn over [Turn over A mixture of NOCl, NO and Cl2 is injected into a second evacuated vessel of a fixed volume at 400 K. It takes 10 minutes for the system to reach equilibrium and the equilibrium pressure is 200 kPa. The initial and final compositions of the mixture of gases are shown in the table below. Initial number of moles at 400 K Equilibrium number of moles at 400 K NOCl 0.5 0.3 NO 0.1 0.3 Cl2 0.1 0.2 (iii) Write an expression for Kp for the reaction and calculate Kp, stating its units at 400 K. (iv) The temperature of the system in the second vessel is increased to 500 K at time, t = 25 minutes. Suggest, with an explanation, how the position of equilibrium might change. Give a sketch to show how the number of moles of nitrosyl chloride gas might change with time from t = 0 min to t = 40 min. [9] (c) OH CH3 OH Cl CH2OH Cl A B C (i) Arrange compounds A, B and C in order of increasing pKa. Explain your reasoning. (ii) Explain why compound C is insoluble in water but soluble in aqueous sodium hydroxide. [5] [Total: 20]

4 [Turn over [Turn over 2 (a) Magnesium and its compounds are very useful in industrial processes. Magnesium is generally obtained by the electrolysis of magnesium halides. In the production of magnesium from seawater, magnesium is precipitated as the hydroxide and converted to the chloride by reaction with hydrochloric acid. A dilute magnesium chloride solution is obtained at this point. The magnesium chloride is then recovered by evaporation of the solution, and magnesium metal is obtained by electrolysis of the molten salt. The chlorine gas by-product is then pumped into reinforced cylinders each with a capacity of 2.0 dm 3. The cylinders are maintained at a pressure of 1240 kPa and 25 oC. (i) During the electrolysis of molten magnesium chloride using inert electrodes, a current of 8.0 A was passed through the cell for 67 hours. Calculate the mass of magnesium extracted, and the number of cylinders that were filled in this duration. (ii) By quoting relevant Eo values from the Data Booklet, predict what would be the products obtained if electrolysis was done on the dilute magnesium chloride solution instead of the molten salt. [8] (b) Magnesium oxide is a useful compound of magnesium, and can be used as a refractory material for lining furnaces operating at high temperatures. One method of obtaining magnesium oxide is through the decomposition of magnesium nitrate as shown below: Mg(NO 3)2(s) MgO(s) + 2NO2(g) + ½O2(g) The standard change in entropy for this reaction is +273 J K-1 mol-1. (i) Explain, in terms of structure and bonding, why magnesium oxide can be used to line furnaces. (ii) Suggest why the standard change in entropy for this reaction is positive. (iii) With reference to the following information, calculate the standard enthalpy change of the decomposition of magnesium nitrate. Hence calculate the minimum temperature for the decomposition to be spontaneous. Standard enthalpy change of formation of magnesium nitrate -790 kJ mol -1 Standard enthalpy change of formation of magnesium oxide -602 kJ mol-1 Standard enthalpy change of formation of nitrogen dioxide +33.9 kJ mol-1 (iv) Explain why calcium nitrate has a higher decomposition temperature than magnesium nitrate. [8]

5 [Turn over [Turn over (c) Grignard reagents, which are alkyl- or aryl-magnesium halides, are often used for many organic synthesis reactions. In the preparation of a Grignard reagent, a halogenoalkane is added to small bits of magnesium in a flask containing dry ether as solvent, fitted with a reflux condenser, and the mixture is warmed over a water bath for 20 - 30 minutes. The reaction is as follows: CH3Br + Mg → CH3MgBr This Grignard reagent can be used to synthesize propan-2-ol from ethanal. The reaction takes place in two stages as shown below: CO H H3C C CH3 H O MgBrCH3 C CH 3 H O MgBrCH3 C CH3 H O HCH3 (i) State the type of reaction taking place in stage 1. (ii) Suggest a suitable Grignard reagent and carbonyl compound that can be used to synthesize propan-1-ol using the Grignard reaction. (iii) State the structural formula of the organic product formed when CH3MgBr reacts with instead. [4] [Total: 20] Stage 1: CH3MgBr + Stage 2: H2O + Mg(OH)Br CH2Br

6 [Turn over [Turn over 3 (a) Element X forms a series of coloured ions and complexes. Electrode potentials of X and its ions in acidic aqueous solution can be represented on a voltage/oxidation state diagram as shown below. The electrode potential for a process can be calculated from the formula Eθ = ___Change in voltage_____ Change in oxidation state Some examples of the ion-electron half equations and electrode potentials, Eθ, that can be derived using the formula and the data obtained from the graph are shown below. Ion-electron half equation Eθ/V X3+ + e X2+ -0.30 XO2 + + 2H+ + e XO2+ + H2O +1.00 (i) Explain why the complex ions of element X are coloured. (ii) Write the ion-electron equation for XO2+/X3+ half-cell. Use the formula and the graph to calculate the electrode potential fo