DHS Prelim H2 CHEM P2 QP

2 This question paper consists of 16 printed pages and 2 blank pages. © DHS 2016 [Turn over Name: Index Number: Class: DUNMAN HIGH SCHOOL Preliminary Examination 2016 Year 6 H2 CHEMISTRY 9647/02 Paper 2 Structured 16 September 2016 2 hours Additional Materials: Data Booklet INSTRUCTIONS TO CANDIDATES 1 Write your name, index number and class on this cover page. 2 Answer all questions. 3 Write your answers in the spaces provided on the question paper. 4 A Data Booklet is provided. 5 The number of marks is given in brackets [ ] at the end of each question or part question. 6 You may use a calculator. Question No. 1 2 3 4 5 Total % Marks 12 15 15 15 15 [72]

2 © DHS 2016 9647/02 [Turn over Answer all questions in the spaces provided. 1 Planning (P) The reaction between peroxodisulfate ions, S 2O82, and ethanedioate ions, C 2O42, is slow and can be catalysed by Cu2+ ions. S2O82 + C2O42  2SO42– + 2CO2 To determine the rate law of this reaction, it is necessary to selectively vary the concentrations of S2O82 and C2O42 ions and determine how the rate of reaction responds to these changes. The concentrations of the reactants are varied in such a way that one is in excess compared to the other in each experiment. A suitable end point (the point at which the final time readin g is made) for the experiments will be when the reaction produced the same volume of CO 2 gas. With the measured reaction time, relative rate of the reaction can be determined. You are required to write a plan to determine the rate law of the reaction between S2O82 and C2O42 ions. (a) Suggest an explanation why this reaction is slow when performed in the absence of a catalyst. [1] (b) You may assume that you are provided with  1.00 mol dm3 peroxodisulfate ions  1.00 mol dm3 ethanedioate ions  10 cm3 aqueous copper (II) ions  the equipment and materials normally found in a school laboratory. It can be assumed that a reactant is in excess if its volume is at least five times the volume of the other reactant used. Your plan should include the following:  quantities of reactants and condition you would use in four different reaction mixtures  the measurements you would take  an outline of how one of the reaction mixtures is prepared  brief, but specific details of how the results would then be used to determine the rate law of reaction

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4 © DHS 2016 9647/02 [Turn over [8] (c) Temperature can also affect the rate of reaction and the activation energy, E a, of this reaction can be found by repeating the experiments at several different temperatures. Using equation (1) and by means of a suitable graph, the activation energy of the reaction can be determined. ln (relative rate) =  )T 1(R Ea + ln (constant)  (1) Briefly describe, with specific experimental details, of how the activation energy, Ea, of the reaction could be determined. [3] [Total: 12]

5 © DHS 2016 9647/02 [Turn over 2 (a) A lead–acid car battery is a reversible cell that consists of a lead anode and a grid of lead coated with lead (IV) oxide as the cathode. The electrolyte is a 30% solution of sulfuric acid. When the cell supplies electric current to power the car, the process is known as discharging. The reversible process is thus known as charging. The overall reaction that takes place in the lead–acid car battery is as follows: (i) Write the half –equations, with state symbols, for the reactions that take place at the anode and the cathode during the discharging process. Hence, state the polarity at the two electrodes. Half–equations Polarity Anode Cathode [3] (ii) When the lead and lead (IV) oxide have been converted to lead (II) sulfate, the cell can no longer give a current and the battery becomes flat. As the car moves, the generator then charges the battery. When the lead– acid battery is fully charged, the sulfuric acid has a relative density of about 1.275. Predict and explain what happens to the relative density of the sulfuric acid during the discharging process. [2] (iii) Overcharging the lead –acid battery with high voltages causes hydrogen and one other gas to form. In addition, the electrolyte level is observed to have dropped. Explain briefly what happens during overcharging and hence, suggest the identity of the other gas that is formed. [2] Pb(s) + PbO2(s) + 2H2SO4(aq) 2PbSO4(s) + 2H2O(l) discharging charging

6 © DHS 2016 9647/02 [Turn over (iv) A simple rechargeable cell similar to that of lead –acid car battery may be constructed by dipping two lead electrodes into aqueous lead (II) nitrate and passing a current for a few minutes. During the charging process, lead (IV) oxide is deposited on one of the electrodes. By reference to the Data Booklet, calculate the value of E when the cell discharges. [2] (v) The voltage of a typical lead–acid battery is 2.0 V. Explain the difference in the voltage and the E calculated in (a)(iv), based on the concentration of ions. [2] (b) (i) Pb2+ ions are often used in qualitative analysis to identify halide ions as they form insoluble precipitates. However, it is not an ideal test to distinguish between C l– and Br – ions as both ions form white precipitate with Pb2+ ions. Suggest another simple chemical test involving precipitation that could be used to distinguish between C l– and Br– ions, stating clearly the steps and reagents involved and the expected observations. [2]

7 © DHS 2016 9647/02 [Turn over (ii) With reference to your answer in (b)(i), explain the chemistry involved and writing equations where appropriate. [2] [Total: 15] 3 Cobalt and vanadium are transition elements with more than one oxidation state and many of its compounds are coloured. (a) When a few drops of aqueous ammonia is added to a test –tube containing Co3+(aq), a brown precipitate, X is observed. The chemical formula of X does not contain water molecules. The precipitate then dissolves in excess aqueous ammonia to give a cation Y which has a co–ordination number of 6. (i) Complete the electronic configuration of cobalt (III) ions. Co3+: 1s22s22p6 [1] (ii) Suggest the identities of X and Y. X: Y: [2]

8 © DHS 2016 9647/02 [Turn over (iii) With the use of appropriate ionic equations, explain the formation of X and Y. [3] (b) When concentrated hydrochloric acid is added to Co 2+(aq), the colour changes from pink to blue. When water is added to the blue solution, it returns to the pink colour. The chemistry of Co2+(aq) with concentrated hydrochloric acid closely resembles that of Cu2+(aq). Write a balanced ionic equation to account for the observations. In your answer, state the change in co–ordination number of cobalt ion in this reaction (if any). [2] (c) Vanadium is also another commercially important transition element. (i) Explain why V2+ is coloured. [2]

9 © DHS 2016 9647/02 [Turn over (ii) V2O5 is used as a catalyst to speed up the conversion of SO 2 into SO3 in the contact process for making sulfuric acid. 2SO2(g) + O2(g) 2SO3(g) ∆H = 197 kJ mol–1 For the reaction above, explain clearly how the catalyst works. [2] (iii) W