HCI H2 CHEM P2 Prelim

This document consists of 18 printed pages and 2 blank pages. HWA CHONG INSTITUTION C2 Preliminary Examinations Higher 2 CANDIDATE NAME CT GROUP 13S CENTRE NUMBER INDEX NUMBER CHEMISTRY Paper 2 Structured Questions Candidates answer on the Question Paper Additional Materials: Data Booklet 9647/02 15 September 2014 2 hours INSTRUCTIONS TO CANDIDATES 1) Write your name, CT group, centre number and index number clearly in the spaces at the top of this page. 2) Answer all questions in the spaces provided in this Question Paper. INFORMATION FOR CANDIDATES The number of marks is given in brackets [ ] at the end of each question or part question. A Data Booklet is provided. Do not use staples, paper clips, highlighters, glue or correction fluid. You may use a calculator. You are reminded of the need for good English and clear presentation in your answers. For Examiner’s Use 1 / 12 2 / 12 3 / 21 4 / 12 5 / 15 Deductions Total / 72 Calculator Model:

2 © Hwa Chong Institution 2014 9647 / 02 / C2 Prelim 2014 1 Planning (P) For Examiner's use Passing an electrical current through an electrolytic cell can cause a non -spontaneous chemical reaction to occur. The number of moles of products formed is related to the number of moles of electrons that pass through the cell during electrolysis. This technique can be used to determine the Avogadro’s constant. When a current is passed through a solution of acidified aqueous potassium iodide, hydrogen gas will be produced at one electrode, while iodine will be formed at the other electrode. The amount of hydrogen produced can be determined from the volume of hydrogen gas collected by downward displacement of solution. The amount of iodine formed can be found by titration of a portion of the electrolyte with sodium thiosulfate. __ I2 + 2S2O3 2−  2I− + S4O6 2− (a) Give the ion -electron equation, with state symbols, for the reaction which occur at the cathode. …………………………………………………………………………………………................... [1] (b) Calculate the expected volume of hydrogen gas formed at r.t.p. if a current of 0.50 A is passed for 10 minutes. [1] (c) Using your answer to (b), you are required to write a plan to determine the amount of hydrogen and iodine produced in the electrolysis of acidified aqueous potassium iodide, and hence, the Avogadro's constant. You are provided with:  500 cm3 of 0.500 mol dm3 acidified aqueous potassium iodide;  0.0100 mol dm3 aqueous sodium thiosulfate;  common indicators;  milliammeter (range 0 – 1.0 A);  battery  the apparatus and chemicals normally found in a school or college laboratory.

3 © Hwa Chong Institution 2014 9647 / 02 / C2 Prelim 2014 In your plan you should include:  a fully-labelled diagram of the electrolytic setup, showing  how you collect the hydrogen gas,  your choice of electrodes;  an outline of the procedure with essential experimental details, including  quantities of chemicals and appropriate apparatus to be used,  choice of a suitable indicator,  all measurements to be made;  an outline of how you would process the titration results to determine the Avogadro’s constant. For Examiner's use …………………………………………………………………………………………................... …………………………………………………………………………………………................... …………………………………………………………………………………………................... …………………………………………………………………………………………................... …………………………………………………………………………………………................... …………………………………………………………………………………………................... …………………………………………………………………………………………................... …………………………………………………………………………………………................... …………………………………………………………………………………………................... …………………………………………………………………………………………................... …………………………………………………………………………………………................... …………………………………………………………………………………………................... …………………………………………………………………………………………................... …………………………………………………………………………………………................... 1 2 3 4 5 6 7 8 9 10

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5 © Hwa Chong Institution 2014 9647 / 02 / C2 Prelim 2014 BLANK PAGE

6 © Hwa Chong Institution 2014 9647 / 02 / C2 Prelim 2014 2 (a) Sodium fluoride is commonly added to toothpastes or mouth rinses to prevent tooth decay. However, in high concentrations, F – is lethal. A person who has accidentally swallowed some toothpaste may be treated by drinking some milk of magnesia to form magnesium fluoride , MgF 2. T he numerical value of the solubility product of MgF2 at 25 C is 5.16  10–11. For Examiner's use (i) Write an expression for the Ksp of MgF2, stating its units. …………………………………………………………………………………………......... (ii) Calculate the minimum concentration of Mg2+ ions required to form a precipitate in a solution containing 1.0 mg dm3 of F– ions. [3] (b) (i) State the three products formed in the electrolysis of brine ( concentrated sodium chloride solution). …………………………………………………………………………………………......... (ii) Sodium chlorate(V), NaClO3, an ingredient in weed-killer, can be obtained from the products of this electrolysis. Write a balanced equation with state symbols to show how NaClO3 can be formed. …………………………………………………………………………………………......... [2] (c) Explain why hydrogen iodide cannot be prepared with sodium iodide and concentrated sulfuric acid. Support your answer with an appropriate equation. …………………………………………………………………………………………................... …………………………………………………………………………………………................... …………………………………………………………………………………………................... …………………………………………………………………………………………................... [2] 1 2 3 4 5 6 7 8 9 10

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