SRJC 2008 JC2 Prelim H2 P3

SERANGOON JUNIOR COLLEGE General Certificate of Education Advanced Level Higher 2 CHEMISTRY 9746/03 Preliminary Examination Paper 3 Free Response 19th August 2008 2 hours Candidates answer on the separate paper. Additional Materials: Answer Paper Data Booklet READ THESE INSTRUCTIONS FIRST Write your name and class 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 work. 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 the brackets [ ] at the end of each question or part question. At the end of the examination, fasten all your work securely together. This document consists of 11 printed pages and 1 blank page SRJC 2008 9746/03/Prelim/2008 [Turn over

2 SRJC 2008 9746/03/Prelim/2008 [Turn over Answer any four questions. 1. The Haber Process is the nitrogen fixation reaction of nitrogen and hydrogen over magnetite, iron oxide, to produce ammonia. This process is important because ammonia is difficult to be produced on an industrial scale, and ammonia is the precursor for many nitrogen-containing products such as nitrate fertilizers and munitions. N2(g) + 3H2(g) 2 NH3(g) ΔHo = – 92.4 kJ mol-1 (a) Write an expression for Kp for the above reaction, stating its units. [2] (b) When a 1:3 mixture by volume of N 2 and H2 at an initial pressure of 200 atm is allowed to reach equilibrium at 500 oC, it was found that the percentage yield of NH3 is 30 %. ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ×= % 100% amount l theoretica amountactualyield (i) Calculate the equilibrium partial pressures of N2 and H2. (ii) Hence calculate a value of Kp at 500 oC. (iii) Suggest, with a reason, whether or not a higher temperature would favour the formation of ammonia. (iv) When some H2 is added to the existing equilibrium mixture, the partial pressure of ammonia is increased to 50 atm. Calculate the new equilibrium partial pressure of H 2. (v) The value of Δ G is 0 kJ mol -1 at 196 °C. Use this information, and any other information to calculate the value of ΔS. (vi) Calculate ΔG for the forward reaction at 500 oC. (vii) Based on the answer in (vi), comment on the feasibility and relate it with the reaction conditions of 200 atm and 500 oC. [8] (c) Ammonia can be used as a reagent to make nitrogen-containing products. (i) State the reagents and conditions that is needed for the conversion of ammonia into I propanamine II propanamide (ii) It is noted that in the formation of propanamine, a by-product is formed which has a melting point of 270 °C and gives an immediate precipitate with aqueous silver nitrate. Suggest a structure for this by-product. [3]

3 SRJC 2008 9746/03/Prelim/2008 [Turn over (d) The compound propanamide, which is a product of ammonia, can be used to make other organic compounds. (i) Propanamide can undergo hydrolysis to form propanoic acid. How would you expect the rate of this reaction to compare to that of the hydrolysis of propanoyl chloride? Explain your answer using their respective reaction conditions. (ii) State the intermediates and suggest the reagents and conditions needed for the conversion of propanoic acid into butanoic acid. [5] (e) When propanamide, propanoyl chloride and propanoic acid are added into separate portions of water, solutions of pH 1.0, 2.8 and 7.0 are formed. (i) Suggest which pH value is associated with each compound. (ii) Give a reason for your choice of pH of propanamide. [2] [Total: 20]

4 SRJC 2008 9746/03/Prelim/2008 [Turn over 2 Trichloroethanoic acid (TCA), CC l3COOH, widely used in biochemistry for the precipitation of macromolecules such as proteins. Solutions co ntaining trichloroethanoic acid are also used in tattoo removal. (a) Trichloroethanoic acid is prepared by the reaction of chlorine with ethanoic acid. CH3COOH + 3Cl2 → CCl3COOH + 3HCl (i) State the type of reaction for the preparation of trichloroethanoic acid. (ii) Suggest a suitable condition for this reaction. (iii) A by-product of this synthesis has a molecular formula of C 4H4O4Cl2. Draw a suitable structure for this compound. [3] (b) In a laboratory experiment, the decomposition of CCl3COOH was conducted at 70 oC and the volume of carbon dioxide produced was monitored using 0.100 mol dm -3 and 0.200 mol dm-3 of trichloroethanoic acid separately. CCl3COOH → CO2 + CHCl3 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 0 . 00 . 51 . 01 . 52 . 02 . 53 . 03 . 5 Volume of CO2 / dm3 Time / hour Graph of volume of CO2 / dm3 against time / hour [TCA] = 0.100 mol dm-3 [TCA] = 0.200 mol dm-3

5 SRJC 2008 9746/03/Prelim/2008 [Turn over (i) From the graph, determine the order of reaction and the rate constant, k. (ii) Determine the half life of reaction using rate constant that you have calculated in (i). (iii) Explain, using Maxwell Boltzmann distribution curve, the effect of lowering temperature to 60 0C. [7] (c) A student collected the CO 2 produced by delivering the gas into a measuring cylinder originally filled with water. Some of the carbon dioxide dissolved in water to form carbonic acid, H 2CO3. The following equilibria were established. CO 2(g) + H2O(l) H2CO3(aq) H2CO3(aq) H+(aq) + HCO3 -(aq) K a = 4.27 x 10-7 mol dm-3 A 20.0 cm 3 of 0.20 mol dm -3 H2CO3 was titrated with 0.20 mol dm -3 of aqueous sodium hydroxide, NaOH. 20.00 cm 3 of NaOH was required for complete neutralisation of the following reaction. H2CO3(aq) + NaOH(aq) → NaHCO3(aq) + H2O(l) (i) What do you understand by the term weak acid? (ii) Calculate the initial pH of the solution. (iii) Calculate the pH of the solution after adding 10.00 cm3 of NaOH. (iv) The pH at equivalence point is 8.35. Using an appropriate equation, account for this observation. (v) An acid-base indicator (HIn) has a Ka value of 3.00 x 10-5 mol dm-3. HIn(aq) H+(aq) + In-(aq) The acidic form of the indicator (H In) is red and the basic form ( In-) is blue. Given that the indicator appears red when it contains at least 75% of H In, and appears blue when it contains at least 75% of In-. What is the pH range for colour change of indicator? Based on your answer, suggest if this indicator is suitable for the H 2CO3 titration. (vi) Suggest a better method of collecting the CO2 gas. [10] [Total: 20]

6 SRJC 2008 9746/03/Prelim/2008 [Turn over 3 This question is about physical and chemical properties of Group II compounds. (a) 400 cm3 of 0.015 mol dm-3 of magnesium nitrate was mixed with 600 cm3 of 0.2 mol dm-3 of aqueous ammonia [Kb of NH3 = 1.74 x 10-5 mol dm-3]. (i) Write an expression for the solubility product, Ksp, of magnesium hydroxide. (ii) Calculate the amount of Mg2+ and NH3 added. (iii) Given that the K sp of Mg(OH) 2 is 1.65 x 10 -10 mol 3dm-9, prove that there is precipitate of Mg(OH)2 formed in the above resulting mixture. (iv) The pH of this resulting solution is measured at 11.1. Calculate the concentration of Mg2+ ions in the solution. [5] (b) Magnesium nitrate decomposes at 330oC. (i) Write a balanced chemical equation, with state symbols, for the decomposition of magnesium nitrate. (ii) How would you expect the decomposition temperature of barium nitrate to differ from that of magnesium nitrate? Explain your answer. [3] (c) Solubility of Group II sulphates vary down the group. (i) With an appropriate equation, define the enthalpy change of solution of calcium sulphate. (ii) Given: ΔHo lattice (CaSO4) = -2704 kJmol -1 ΔHo hydration (Ca2+) = -1562 kJmol -1 ΔHo hydration (SO4 2-) = -1160 kJmol -1 Using Hess’ Law, calculate the enthalpy change of solution of calcium sulphate. Ba