PJC H2 CHEM P2 (Answers)

PIONEER JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATION HIGHER 2 CANDIDATE NAME CT INDEX GROUP NUMBER CHEMISTRY 9647/02 Paper 2 Structured 23 September 2014 2 hours Candidates answer on Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, CT group and index number 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 all questions. The use of an approved scientific calculator is expected where appropriate. A Data Booklet is provided At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 16 printed pages. FOR EXAMINER’S USE Paper 2 1 / 12 5 / 12 2 / 15 3 / 10 Penalty sf units 4 / 23 Total / 72 1 3

© PJC 2014 9647/02/JC2 Prelim/2014 2 Answer all the questions in the spaces provided. 1 Planning The label of a bottle of an aqueous acid, represented by H xA, had been damaged. Only the concentration of the acid, 0.50 mol dm3 was known. A student was tasked to determine (1) if t he acid is monobasic or dibasic, and (2) if the acid is a strong acid or a weak acid. (a) The student was given a 0.50 mol dm 3 sodium hydroxide solution. She proposed to first determine the basicity of the acid in the bottle by mixing different volumes of the acid and sodium hydroxide and measure the maximum temperature change, T, for each of the constant volume mixtures. The following experiments were performed: Experiment 1: 50 cm3 of HxA(aq) was added to 100 cm3 of NaOH(aq) Experiment 2: 100 cm3 of HxA(aq) was added to 50 cm3 of NaOH(aq) The changes in temperature of the mixture were measured for Experiment 1 and Experiment 2 as T1 and T2 respectively. (i) Write an equation to represent the standard enthalpy change of neutralisation between sodium hydroxide and HxA. 1 x HxA(aq) + NaOH(aq)  1 x NaxA(aq) + H2O(l) (ii) Explain how the basicity of the acid can be determined by comparing the T1 and T2 values. The basicity of the un known acid can be determined by finding out the number of moles of water formed during neutralisation. Since heat evolved = nHneu θ = mcT, by keeping the total volume of the mixture constant, the value of n is directly proportional to the value of the T. If x = 1 HA + NaOH  NaA + H2O Experiment HxA NaOH nH2O 1 50 (limiting) 100 (excess) y 2 100 (excess) 50 (limiting) y Both experiments will produce same amount of water. Hence ∆T1 = ∆T2 will be observed if HxA is monobasic (since total volume of mixture is fixed). If x = 2 H2A + 2NaOH  Na2A + 2H2O

© PJC 2014 9647/02/JC2 Prelim/2014 3 Experiment HxA NaOH nH2O 1 50 (limiting) 100 (limiting) 2y 2 100 (excess) 50 (limiting) y Experiment 1 will produce twice the amount of H2O. Hence ∆T1 = 2∆T2 will be observed if HxA is dibasic (as total volume of mixture kept constant). [4] (b) It was determined that the acid in the bottle is a monobasic acid, HA. The student proceeded to determine if HA is a strong or weak acid using the following:  FA 1, a solution of 0.50 mol dm–3 sodium hydroxide.  FA 2, a solution of 0.50 mol dm–3 HA.  Thermometers  Styrofoam cups  Apparatus normally found in a college laboratory She performed a series of experiments by mixing different volumes of FA 1 with FA 2 and measuring the temperature changes of each of these mixtures. (i) Write a plan to determine the temperature changes, ∆T, for the following series of reactions between FA 1 and FA 2: Mixture Volume of FA 1 / cm3 Volume of FA 2 / cm3 1 15 45 2 25 35 3 35 25 4 45 15 5 50 10 Your plan should include the apparatus for measurement of volume. Indicate clearly the measurements that will be made during the experiment and show how these measurements can be used to obtain the temperature change, T. 1. Use a burette / measuring cylinder to transfer 15 cm3 of FA 1 into the Styrofoam cup labelled FA 1 . Place the cup in a 25 0 cm 3 beaker to prevent it from tipping over. 2. Use another burette / measuring cylinder to transfer 45 cm3 of FA 2 into the Styrofoam cup labelled FA 2. 3. Use thermometer to stir and measure the temperature of the FA 1 and FA 2 solution, TFA1 and TFA2 respectively.

© PJC 2014 9647/02/JC2 Prelim/2014 4 4. Calculate the average temperature, Taverage of the solutions by Taverage = [(TFA1 VFA1) + (TFA2 VFA2)] / Vtotal. 5. Add the contents of FA 2 cup to the FA 1 cup. Use the thermometer to stir the mixture and measure the maximum temperature , Tmax of the mixture. 6. Calculate the temperature change, ∆T, by taking ∆T = Tmax - Taverage 7. Wash and carefully dry both the FA 1 and FA 2 styrofoam cups. 8. Repeat steps 1 to 7 for Mixtures 2 to 5. (ii) The student performed the experiment for the series of reactions between FA 1 and FA 2 and the temperature changes were plotted as shown below. By means of two straight lines , determine the ∆Tmax for the series of experiments. Draw 2 straight lines for each side of each graph. From the graph, ∆Tmax = 3.2 °C.

© PJC 2014 9647/02/JC2 Prelim/2014 5 (iii) Using your graph in (b)(ii), calculate the enthalpy change of neutralisation for the reaction between HA and NaOH . [Assume that the heat capacity of the solution = 4.2 J K–1 cm–3.] Heat evolved = msolutionc∆Tmax = 60 cm3 × 4.2 J K–1 cm–3 × 3.2 K = 806.4 J HA + NaOH → NaA + H2O ∆Tmax occurs at end-point, when 30 cm3 of FA 2 is added. Since 1 mol of HA ≡ 1 mol of H2O Therefore, (0.50)(0.030) mol of HA ≡ (0.50)(0.030) mol of H2O ≡ 0.015 mol of H2O nH2O × ∆Hneu = –806.4 J 0.015 mol × ∆Hneu = –806.4 J ∆Hneu = –53760 J mol–1 ≈ –53.8 kJ mol–1 (iv) Given that the enthalpy change of neutralis ation for reaction between HC l and NaOH is –57.3 kJ mol –1, deduce whether the unknown ac id HA is a strong acid or a weak acid. Explain your answer. ∆Hneu is much less exothermic than ∆Hneu involving a strong acid and strong base. This means that some of the energy must have been absorbed to fully dissociate the acid HA . Hence HA must be a weak acid which does not dissociate completely in water. [8] [Total: 12] 2 Magnesium metal can be manufactured by the electrolysis of molten magnesium salt, Z using the setup shown below. During the electrolysis process, 0.912 g of unknown gas Y was produced which occupied 500 cm3 volume at 200 °C and 1 atm.

© PJC 2014 9647/02/JC2 Prelim/2014 6 (a) (i) Determine the Mr of gas Y and hence identify it. pV = nRT Mr = )1000(101000)(5 200)31)(273(0.912)(8. 6  = 71.0 Therefore, the gas produced is chlorine gas. (ii) Explain why you would expect the behavio ur of g as Y to be less ideal at low temperature. At low temperature, the gas molecules do not sufficient energy to overcome intermolecular forces of attraction and hence it is less ideal. (iii) Write ion-electron equations for the reaction occurring at the cathode and anode. Cathode: Mg2+(l) + 2e– → Mg(l) Anode: 2Cl–(l) → Cl2(g) + 2e– (iv) Given that the electrolysis took place for 40 minutes, calculate the current used for the process. Amount of Cl2 produced = 71.0 0.912 = 0.1285 mol 0.1285 = 965002 60)(40I   I = 1.03 A (v) Using relevant Eo data from the Data Booklet , explain why magnesium metal cannot be obtained by the electrolysis of aqueous magnesium salt. M