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/