2021 SAJC Prelim P3 Answer

ST ANDREW’S JUNIOR COLLEGE JC2 Preliminary Examinations Higher 2 CANDIDATE NAME CLASS 2 0 S CHEMISTRY Paper 3 Free Response Candidate answer on the Question Paper. Additional Materials: Data Booklet 9729/03 15 September 2021 2 hours READ THESE INSTRUCTIONS FIRST Write your name and class 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. Answer all questions in the spaces provided on the Question Paper. If additional space is required, you should use the pages at the end of this booklet. The question number must be clearly shown. Section A Answer all the 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 39 printed pages. For Examiner’s Use Q1 24 Q2 17 Q3 19 Q4/5 20 Total 80

Section A Answer all the questions in this section. 1 (a) Iodates are compounds that contain the IO3 anion. (i) Draw the dot-and-cross diagram of IO3. [1] (ii) Use your knowledge of VSEPR theory to name the shape of and state the bond angle for IO3. Explain your reasoning. [3] Electron pairs repel each other and arrange themselves as far apart as possible to maximise stability and minimize (electrostatic) repulsion Lone pair-lone pair repu lsion are strong er than lone pair -bond pair rep ulsion which are stronger than bond pair-bond-pair repulsion. IO3 has trigonal pyramidal[√] shape and 107o since there are 3 bond pairs and 1 lone pair of electrons around the central atom I. (iii) Explain why BrO3 has a larger bond angle than IO3. [1] Br is more electronegative than I, hence it draws the bond pair of electrons closer to itself, resulting in greater bond pair-bond pair repulsion, and a larger bond angle. (b) The decomposition of hydrogen peroxide, H2O2, can be catalysed by acidified IO3. 2H2O2 → 2 H2O + O2 With the aid of relevant data from the Data Booklet and the information below, show that IO3 is a suitable catalyst for the decomposition of H2O2 under standard conditions. IO3 + 6H+ + 5e⇌  1 2I2 + 3H2O Eo = +1.19 In your answer, give relevant equations for the reactions that occur. [3] Step 1 O2 + 2H+ + 2e– → H2O2 Eo = +0.68 5H2O2 + 2H+ + 2IO3– → I2 + 5O2 + 6H2O [1/2 for either half-eqn or full eqn] Eocell = +1.19 – (+0.68) = +0.51 V IO O O x x x x x x x x xx x x x x x xx xx

Step 2 H2O2 + 2H+ + 2e– → 2H2O Eo = +1.77 5H2O2 + I2 → 4H2O + 2IO3 + 2H+ [1/2 for either half-eqn or full eqn] Eocell = +1.77 – (+1.19) = +0.58 V (c) The kinetics of the catalytic decomposition of H2O2 by IO3 can b