AJC H2 Chem 2012 Prelim P3 QP

ANDERSON JUNIOR COLLEGE Preliminary Examinations 2012 CHEMISTRY 9647/03 Higher 2 17 September 2012 Paper 3 Free Response Questions 2 hours Candidates answer on separate paper. Additional Materials: Data Booklet Writing paper Graph paper READ THESE INSTRUCTIONS FIRST Write your name, PDG and register number on all the work you hand in. Write in dark blue or black pen on both sides of the paper. You may use a pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer any four questions. Start each question on a fresh sheet of paper. A Data Booklet is provided. You are reminded of the need for good English and clear 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. This document consists of 12 printed pages. AJC Prelim 2012 9647/03/H2 [Turn over

2 AJC Prelim 2012 9647/03/H2 1 (a) Sodium dichromate(VI), Na 2Cr2O7, is used widely in organ ic synthesis as well as in the process of leather tanning. The principal ore of chromium called chromite, FeCr 2O4, is used in the manufacture of sodium dichromate(VI) via a 2–stage process. In the first stage of the process, the ore is fused with soda ash (sodium carbonate) and air at a temperature of 1000 °C. The following equation represents this stage. 4FeCr2O4 + aNa2CO3 + bO2 cFe2O3 + dNa2CrO4 + eCO2 (i) Complete the balancing of the above equation by deducing the values for a, b, c, d and e. In the second stage, the products are extracted with hot water. Orange crystals of sodium dichromate(VI) separate out during the evaporation stage of the process. (ii) Suggest a reagent that can be added to the hot water extract to produce the dichromate(VI) solution in the second stage of the process. [2] (b) Draw a fully labeled diagram to show how the standard electrode potential of the Cr2O72–(aq)/Cr3+(aq) system could be measured by a standard hydrogen electrode. Show the direction of electron flow in the circuit. [3] (c) When zinc metal is added to an acidifi ed solution of aqueous chromium (III) solution, the solution turns from green to blue. When the excess zinc is filtered off and the blue solution is left to stand for several hours in the absence of air, it slowly turned back to green and a colourless gas is evolved. Explain these observations, quoting appropriate Eo values from the Data Booklet. [3] (d) A chromium–containing ion, CrOxn–, disproportionates in acid solution to produce a mixture of chromium(III) ions and dichromate(VI) ions. A solution containing 7.5 x 10 –3 mol of CrO xn– ions was acidified and the disproportionation occurred. The resulting solution contained 2.5 x 10–3 mol of dichromate(VI) ions. Using the information given, calculate the oxidation number of chromium in the ion CrOxn–. [2]

3 AJC Prelim 2012 9647/03/H2 [Turn over (e) Transition metals such as chromium, iron and zinc and their compounds have found wide applications in electrical energy– storage technology. New technologies involving the inventions of redox flow batteries and metal–air batteries have emerged in recent years. Electrolytes containing chromium ions and iron ions are used in a redox flow battery called iron–chrome battery (ICB). Electricity is generated from the battery by the half– cell reactions of these electrolytes that flow into a cell. The two electrolytes in the cell are separated by an ion selective membrane. Inert electrodes with optimum power acceptance are used in this battery. (i) Using the Data Booklet, select appropriate half–cell reactions of chromium ions and iron ions that would produce the largest standard cell potential in the ICB. Write a balanced equation for the overall reaction that takes place when the battery discharges. (ii) Explain the role of the ion selective membrane in the operation of the ICB. An example of a metal –air battery is the zinc –air battery , commonly used as small button cells in watches and hearing aids. When a gas –permeable, liquid– tight membrane sealing tab in the button cell is removed, oxygen in the air is absorbed into an alkaline electrolyte. The positive electrode is made of porous carbon and the negative electrode consists of zinc. The electrolyte used is a paste containing potassium hydroxide. (iii) A zinc–air button cell has a cell potential of 1.59 V. Using relevant data from the Data Booklet , calculate the electrode potential for the reaction at the anode. Suggest an equation for the reaction that takes place at this electrode. (iv) The zinc electrode of a new zinc –air button cell weighs 1.9 g. The cell can run until 80 % of the zinc is consumed. Calculate the maximum amount of current that can be drawn from the cell if it is expected to last for 30 days. electrolyte tank electrode electrode ion selective membrane electrolyte electrolyte pump pump electrolyte tank electrical load flow of electrolyte flow of electrolyte

4 AJC Prelim 2012 9647/03/H2 (v) Considering the cell reactions in the two batteries described above, suggest the battery that would be expected to discharge electricity at a faster rate. Give a reason for your answer. (vi) Common alkaline batteries contain zinc and manganese (IV) oxide in a paste of potassium hydroxide. The overall cell reaction is shown below. Zn(s) + MnO 2(s) + H2O(l) ZnO(s) + Mn(OH)2(s) Based on this information, suggest an advantage, other than lower cost, that the zinc–air battery has over the common alkaline battery of similar weight. Explain your answer. [10] [Total: 20]

5 AJC Prelim 2012 9647/03/H2 [Turn over 2 (a) Data concerning some Group II sulfates and hydroxides, at 298 K, are given in the table below. Further data may be found in the Data Booklet. solubility / mol dm–3 – lattice energy / kJ mol–1 DHhydration of M2+ / kJ mol–1 DHhydration of SO42– / kJ mol–1 MgSO4 2.2 2959 –1890 –1160 CaSO4 1.5 x 10–2 2704 –1562 –1160 SrSO4 7.1 x 10–3 2572 –1414 –1160 solubility / mol dm–3 – lattice energy / kJ mol–1 DHhydration of M2+ / kJ mol–1 2 x DHhydration of OH– / kJ mol–1 Mg(OH)2 1.6 x 10–4 2993 –1890 –1100 Ca(OH)2 2.5 x 10–2 2644 –1562 –1100 Sr(OH)2 3.4 x 10–2 2467 –1414 –1100 (i) Define the term lattice energy. (ii) Explain the following: I The magnitude of the lattice energy of Group II sulfates decreases from MgSO 4 to SrSO4. II The DHhydration of M2+ becomes less exothermic from Mg2+ to Sr2+. (iii) Hence, suggest qualitatively why the solubility of Group II sulfates decreases from MgSO4 to SrSO4, while the solubility of Group II hydroxides increases fro m Mg(OH)2 to Sr(OH)2. [7] (b) Many metal sulfates have very different industrial uses. For example, the mercury(I) sulfate reference electrode (MSRE) is the second most commonly used reference electrode while calcium sulfate products are used as an economical and FDA– approved source of supplemental calcium. The numerical values of the solubility product, Ksp, for these two sulfates at 298 K are: Ksp of mercury(I) sulfate, Hg2SO4 = 7.4 x 10–7 Ksp of calcium sulfate, CaSO4 = 2.4 x 10–5 During a Chemistry experiment, a student mixed 25.0 cm 3 of 0.1 mol dm–3 Hg+ with 25.0 cm3 of 0.1 mol dm–3 Ca2+ ions and labeled it as solution J. (i) The student was then instructed to add solid potassium sulfate to solution J slowly. Calculate the minimum concentrations of sulfate ions required to precipitate the first trace of mercury(I) sulfate and first trace of calcium sulfate, respectively. (ii) Using your answers to (b)(i), state and