TJC Prelim P3_Qn_&_Ans

1 9729/03/TJC Prelims/2018 [Turn over CHEMISTRY 9729/03 Paper 3 Free Response 14th September 2018 2 hours Candidates answer on separate paper. Additional Materials: Answer Paper Data Booklet READ THESE INSTRUCTIONS FIRST Write your Centre number, index number, name and civics group on all the work you hand in. Write in dark blue or black pen. You may use a HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Section A Answer all questions. Section B Answer one question. 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.

2 9729/03/TJC Prelims/2018 [Turn over Section A Answer all the questions in this section. 1 (a) There are several stages in the industrial production of methanol from methane. The first stage involves a gaseous equilibrium between the reactants (methane and steam), and some gaseous products. Figures 1.1 and 1.2 show the percentage conversion of methane into the gaseous products under different conditions at equilibrium. (i) Use the information from the graphs above to deduce the sign of the • enthalpy change, ΔH and • entropy change, ΔS for the first stage industrial conversion reaction of methane and steam into the gaseous products. [2] • the enthalpy change, ΔH As temperature increases from 600 0C to 880 0C, % conversion of methane increases. This indicates that the forward reaction is favoured as it absorbs energy. Hence, forward reaction is endothermic, ΔH is positive. • entropy change, ΔS As pressure increases from 1000 kPa to 5000 kPa, % conversion of methane decreases. Forward reaction is not favoured and this indicates that the number of moles of gaseous products particles is more than the gaseous reactants particles. Hence, ΔS for the forward reaction is positive. (ii) The optimum temperature for the industrial conversion of methane and steam into the gaseous products is between 780-880oC. Explain why this is so. [2] • Lower than 780 0C, rate of reaction is slow / % conversion is low, hence not cost effective. • Higher than 880 0C, results in high energy costs / expensive & also after a certain temperature, yield does not increase significantly. Therefore, there is no gain in using a higher temperature. (b) The equation shows t