2019 CJC Prelim H2 Chem P3 ANS

9729/03/CJC JC2 Preliminary Examination 2019 CANDIDATE NAME CLASS 2T CHEMISTRY 9729/03 Paper 3 Free Response Friday 30 August 2019 2 hours Candidates answer on separate paper. Additional Materials: Answer Paper Data Booklet 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 an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Section A Answer all questions. Section B Answer one question. A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. At the end of 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 8 printed pages. Catholic Junior College JC2 Preliminary Examinations Higher 2 WORKED SOLUTIONS

2 9729/03/CJC JC2 Preliminary Examination 2019 Section A Answer all the questions in this section. 1 (a) Chemical energy can be directly converted to electrical energy in batteries and fuel cells. Fuel cells require a continuous source of fuel and oxygen to sustain the chemical reaction, whereas in batteries, the chemical energy is usually derived from reactions of metals or metal oxides present in the battery. The nickel-cadmium (NiCd) battery is a rechargeable battery used in many devices such as handphones and laptops. A simplified diagram of the NiCd ele ctrochemical cell is as shown: Electrode X is made of a solid nickel compound, NiO(OH), and electrode Y is made of solid cadmium. The electrolyte is aqueous potassium hydroxide. D uring discharge, Ni(OH)2(s) and Cd(OH)2(s) are formed at the respective electrodes. (i) Construct the half-equations at the electrodes of this alkaline NiCd electrochemical cell. Hence, give the balanced overall equation for the reaction that occurs during discharge. [2] Electrode X: NiO(OH) + H2O + e-  Ni(OH)2 + OH- Electrode Y: Cd + 2OH–  Cd(OH)2 + 2e- Overall equation: Cd + 2NiO(OH) + 2H2O Cd(OH)2 + 2Ni(OH)2 (ii) Write a cell representation for the NiCd electrochemical cell, indicating clearly the direction of electron flow in the external circuit. [2] (iii) NiCd batteries can be recharged by applying a current across the two electrodes. How long would it take to recharge a NiCd battery at a current of 2.0 A, if 5.6 g of cadmium was converted to Cd(OH)2? [2] Moles of Cd converted = 5.6 / 112.4 = 0.0498 mol Moles of electrons required = 0.0498 x 2 = 0.0996 mol Cd(s), Cd(OH)2(s) // NiO(OH)(s), Ni(OH)2(s) e– flow

3 9729/03/CJC JC2 Preliminary Examination 2019 [Turn over Total charge required = 0.0996 x 96500 = 9615 C Time required = 9615 / 2.0 = 4807 s (or 80.1 min) (b) Methane gas, CH4, is the simplest alkane used to power a fuel cell. The methane-oxygen fuel cell has recently been shown to be an efficient source of electrical energy. In the simplest version of this fuel cell, an acid is used as the electrolyte and methane undergoes reaction at one of the electrodes while oxygen undergoes reaction at the other electrode. CO2 + 8H+ + 8e– ⇌ CH4 + 2H2O E0 = +0.17 V O2 + 4H+ + 4e– ⇌ 2H2O (i) By using the half –equations above and relevant data from the Data Booklet , calculate the E0cell of this fuel cell. [1] O2 + 4H+ + 4e– ⇌ 2H2O E0 = +1.23 V E0cell = 1.23 – (0.17) = +1.06 V (ii) Draw a fully labelled diagram of the electrochemical cell you could use to measure this E0cell. [3] (c) An alternative fuel used in fuel cells is dimethyl oxalate, (CH 3)2C2O4, which is obtained by the esterification of ethanedioic acid, H 2C2O4, with methanol, CH 3OH. The ethanedioate ion, C2O42-, is commonly found in ionic salts such as XC2O4. When these ethanedioate salts, XC2O4, are oxidised by acidified potassium manganate(VII), KMnO4, both the X2+ and C2O42- ions are oxidised. Carbon dioxide gas is formed as one of the products in the reaction. (i) Write a balanced equation for the reaction between MnO4– and C2O42– under acidic conditions. [1] 5C2O42– + 2MnO4– + 16H+  10CO2 + 2Mn2+ + 8H2O (ii) 25.0 cm3 of 0.1 mol dm-3 XC2O4 solution was titrated with 0.1 mol dm-3 of acidified KMnO4. Determine the volume of KMnO4 needed to oxidise C2O42– only. [2] Moles of C2O42– ions = 0.025 x 0.1 = 0.0025 mol Since C2O42– : MnO4- is 5 : 2, Moles of MnO4– needed to oxidise C2O42– = 0.0025 x 2/5 = 0.001 mol Volume of MnO4– needed = (0.001 / 0.1) x 1000 = 10 cm3 O2 gas at 25oC, 1 bar 1 mol dm–3 HCl (aq) HClH+(aq) Platinum electrode V Platinum electrode 1 mol dm–3 HCl (aq) HClH+(aq) CH4 (g) CO2 (g), 1 bar each at 25oC. salt bridge High resistance voltmeter

4 9729/03/CJC JC2 Preliminary Examination 2019 (iii) Given that 15.00 cm3 of acidified KMnO4 was required to completely oxidise 25.0 cm3 of 0.1 mol dm -3 XC2O4 solution, and using your answer in (c)(ii), find the oxidation state of X in the product. [3] Volume of MnO4– ions needed to oxidise X2+ ions = 15 – 10 = 5 cm3 Moles of MnO4- ions = (5 / 1000) x 0.1 = 0.0005 mol From Data Booklet, MnO4- + 8H+ + 5e- Mn2+ + 4H2O Moles of electrons gained by MnO4- = 0.0005 x 5 = 0.0025 mol Moles of X2+ions = 0.025 x 0.1 = 0.0025 mol Therefore, mole ratio of X2+ : e = 1 : 1 Since 1 mole of X2+ loses 1 mole of electrons, Oxidation state of X in the product = +3 (d) Potassium manganate(VII), KMnO4, can be used to oxidise alkenes in different ways. Alkene P, C8H16, reacts with hot acidified KMnO4 to form one organic product only, which gives a yellow precipitate with hot aqueous alkaline iodine. P also reacts with cold alkaline KMnO4 to form an optically inactive diol , Q. When P is treated with chlorine in ultraviolet light, it produces a mixture of chlorinated compounds, including R, C8H14Cl2, which does not have any chiral carbon . When R is reacted with ethanolic NaOH, compound S, C8H12, is the only product formed. S produces two compounds, CO2 and CH3COCO2H in equimolar amounts when it is oxidised by hot acidified KMnO4. (i) Suggest the structural formulae for compounds P, Q, R and S. [4] P : C C C C C H C CH3CH3 HH H H H H H H H Q : R : C C C C C H C CH3CH3 HH H H H Cl H H Cl

5 9729/03/CJC JC2 Preliminary Examination 2019 [Turn over S : C C C C C H C CH3CH3 HH H H H (ii) Draw the structural formula of an isomer of R, C8H14Cl2, that could give S on dehydrochlorination. [1] C C C C C H C CH3CH3 HH H H H H Cl H Cl or C C C C C H C CH3CH3 HH H H H H Cl Cl H (iii) Suggest, with a reason, the number of stereoisomers for compound S. [1] There is 1 alkene double bond that is capable of exhibiting cis -trans isomerism as there are 2 different groups attached to each C atom across C=C double bond; no of stereoisomers= 2. [Total:22]

6 9729/03/CJC JC2 Preliminary Examination 2019 2 With the prospect that fossil fuels will become increasingly scarce in the future, many compounds are being considered for use in internal combustion engines. One of these is DME or dimethyl ether, CH3OCH3. DME is a gas which can be synthesised from methanol. Methanol can be obtained from biomass, such as plant waste from agriculture. (a) Write a balanced equation to define the term standard enthalpy change of combustion, Hc o , of DME. [1] CH3OCH3(g) + 3O2(g)  2CO2(g) + 3H2O(l) (b) A 0.31 g sample of DME was completely burnt in air. The heat produced raised the temperature of 200 g of water by 11.7 °C. Assume no heat losses occurred during this experiment. (i) Use relevant data from the Data Booklet to calculate the amount of heat released in this experiment. [