RI_H2_CHEM_P3 Prelim

RAFFLES INSTITUTION 2011 YEAR 6 PRELIMINARY EXAMINATION Higher 2 CHEMISTRY 9647/03 Paper 3 Free Response 13 September 2011 Candidates answer on separate paper. 2 hours Additional Materials: Writing Paper Data Booklet READ THESE INSTRUCTIONS FIRST DO NOT open this question booklet until you are told to do so. Write your name, class and index number in the spaces provided on the cover page. 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 English and c lear presentation 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 securely together, with the cover page on top. ________________________________________________________________________ This document consists of 11 printed pages and 1 blank page.

2 Answer any four questions. Begin each question on a fresh sheet of paper . 1 This question is about the chemistry of halogens and their compounds. (a) Many compounds of chlorine are manufactured from br ine, concentrated NaC l(aq). The electrolysis of brine produces C l2(g) and NaOH(aq). In some industrial electrolytic cells, these two substances are allowed to react further. The products formed in this second reaction depend on the operating conditions used. (i) Write balanced equations for the reaction between Cl2(g) and • cold aqueous NaOH, • hot aqueous NaOH. (ii) Suggest why F 2(g) cannot be manufactured in a similar way using concentrated NaF(aq). [3] (b) The standard electrode potentials, E/ring2, and standard Gibbs free energy changes, ∆ G/ring2, of different chlorine-containing species are tabulated below. Half-equation E /ring2/ring2 /ring2/ring2 / V ∆∆ ∆∆ G/ring2/ring2 /ring2/ring2 / kJ mol −− −− 1 1 C lO4 − + 2H + + 2e − ⇌ ClO3 − + H 2O +1.19 − 230 2 2C lO3 − + 12H + + 10e − ⇌ Cl2 + 6H 2O +1.47 ? 3 Cl2 + 2e − ⇌ 2C l− +1.36 − 262 These electrode potentials can be summarised using the Latimer diagram shown below. In a Latimer diagram, the most highly oxidis ed form of the element is on the left, with successively lower oxidation states to the right. The electrode potentials are shown on the arrows. ∆ G /ring2 and E/ring2 are related by the following equation. ∆ G/ring2 = − zFE /ring2 where ∆ G/ring2 is the standard Gibbs free energy change in joules per mole , z is the number of moles of electrons tr