ACJC H2 Chem 2012 Prelim P3 QP

1 © ACJC 2012 9647/03/Prelim/12 [Turn over ANGLO-CHINESE JUNIOR COLLEGE DEPARTMENT OF CHEMISTRY Preliminary Examination CHEMISTRY H2 9647/03 Paper 3 Free Response 29 August 2012 2 hours Candidates answer on separate paper. Additional Materials: Data Booklet Answer Paper Cover Page READ THESE INSTRUCTIONS FIRST Write your index number, name, form class and tutorial class on all the work you hand in. Write in dark blue or black pen. You may use a pencil for any diagrams, graphs, or rough working. Do not use staples, paper clips, highlighters, glue or correction fluids. 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 brackets [ ] at the end of each question of part question. At the end of the examination, fasten all your work securely together. This document consists of 12 printed pages and 1 blank page. 9647/Prelim/12 ANGLO-CHINESE JUNIOR COLLEGE © ACJC 2012 Department of Chemistry [Turn over

2 © ACJC 2012 9647/03/Prelim/12 [Turn over Answer any four questions. 1 (a) When hydrochloric acid is added to sodium thiosulfa te, Na 2S2O3, a fine precipitate of sulfur appears. This gradually makes the solution opaque. Na 2S2O3(aq) + 2 HC l(aq) S(s) + SO 2(g) + 2 NaC l (aq) + H 2O( l) The rate of the reaction may be determined by measuring how long it takes for the solution to become opaque such that a cross marked on a piece of paper placed under the beaker containing the reaction mixture just becomes obscured. A series of experiments were carried out to study t he order of reaction with respect to Na 2S2O3 and the following results were obtained. Experiment Number Volume / cm 3 Time / s HC l Na 2S2O3 H 2O 1 20.0 40.0 40.0 39 2 20.0 30.0 50.0 50 3 20.0 20.0 60.0 83 4 20.0 10.0 70.0 170 (i) Explain why the total volume of the reaction mixture is kept constant for all the four experiments. (ii) Calculate the values of 1s / )Time 1( − for the various volumes of Na 2S2O3 used. (iii) Hence or otherwise, state how the rate changes with the volume of Na 2S2O3 used. (iv) Explain why the rate changes with the volume of Na 2S2O3 used. [5] (b) Sulfur reacts with chlorine gas to form a compound A containing 47.4% sulfur by mass. When compound A is reacted with water, a yellow precipitate was fo rmed together with a solution containing a mixture of su lfurous acid, H 2SO 3 and hydrochloric acid, HC l. (i) Given that the relative molecular mass of A is 135, determine the empirical