RJC_Chemistry H2 P3 with Ans Prelims

RAFFLES JUNIOR COLLEGE PRELIMINARY EXAMINATION 2008 © RJC 2008 [Turn over 1 HIGHER 2 CHEMISTRY 9746/03 Paper 3 Free Response 12 September 2008 2 hours Candidates answer on separate paper. Additional Materials: Writing Papers Data Booklet READ THESE INSTRUCTIONS FIRST DO NOT open this question booklet until you are told to do so. Write your name, civics tutorial group and index number in the spaces provided on the cover page on page 10 and the writing papers. Write in dark blue or black pen on both sides of 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 any four questions. Begin each question on a fresh sheet of paper. A Data Booklet is provided. Do not write anything on it. You are reminded of the need for good Eng lish and clear present ation in your answers. The number of marks is given in brackets [ ] at the end of each question or part question. At the end of the examination, fasten all your work secu rely together, with the cover page on top. ________________________________________________________________________ This question booklet consists of 9 printed pages and one cover page.

Answer any four questions. Begin each question on a fresh sheet of paper. 1 This question involves Period 3 elements and their compounds. (a) An aqueous mixture of sodium hydrogencarbonate and sodium carbonate can act as a buffer solution. (i) Define the term buffer solution. (ii) With the aid of two equations, explain how a solution of sodium hydrogencarbonate and sodium carbonate can act as a buffer. (iii) Calculate the pH range in which the buffer is most effective. Take the K a for HCO3 − to be 5.61 x 10−11 mol dm−3 at 25 °C. (iv) Calculate the change in pH of the solution if the ratio 2 3 3 [CO ] [HCO ] − − was increased from 0.30 to 0.96. [7] (b) Explain, with the aid of equations, why (i) a solution of aluminium sulphate turns blue litmus red. (ii) aluminium oxide dissolves on additi on of excess aqueous sodium hydroxide. [ 4 ] (c) Sketch the graph of the pH of the resulting mixture when the oxides of Na to S are separately added to water. [1] © RJC 2008 [Turn over 2

(d) Deuterium (D or 2H) is a heavy isotope of hydrogen. Chlorine can react with deuterium to form deuterium chloride, DC l. When DCl is dissolved in methylbenzene, no reaction occurs. However, if an anhydrous catalyst W is added to the solution, a reaction takes place, and a compound X is one of the products. If more DC l is present, a compound Y is formed. CH3 D CH3 DD D D D X Y (i) Suggest a suitable identity for the catalyst W. Explain why W must be anhydrous and why it can act as the catalyst in the reaction.