CJC Prelim P3 SOL

[Turn over 9729/03/CJC JC2 Preliminary Examination 2017 CHEMISTRY 9729/03 Paper 3 Free Response 25 August 2017 Candidates answer on separate paper. 2 hours 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. 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. Instruction Catholic Junior College JC2 Preliminary Examinations Higher 2 Suggested Solutions

2 9729/03/CJC JC2 Preliminary Examination 2017 Section A Answer all the questions in this section. 1 Use of the Data Booklet is relevant to this question. (a) Under acidic conditions, ethanol reacts with dichromate( VI) ions quantitatively to give ethanoic acid and chromium(III) ions. 3CH3CH2OH + 2Cr2O72– + 16H+  3CH3CO2H + 4Cr3+ + 11H2O A student carried out the following experiment to determine the concentration of ethanol in a brand of wine. A 10.0 cm3 sample of the wine was diluted to 250 cm3. He then added 25.0 cm3 of 0.156 mol dm –3 K2Cr2O7(aq) and excess dilute H 2SO4 to a 25.0 cm 3 portion of the diluted solution. After allowing the mixture to stand for about an hour at room temperature, the excess K2Cr2O7 in the mixture was then titrated with 0.118 mol dm–3 (NH4)2Fe(SO4)2(aq) using an appropriate indicator. At the end-point of the titration, he used 12.25 cm3 of the Fe2+(aq) solution. (i) Write an ionic equation for the reaction that occurs during titration. [1] Cr2O72– + 6Fe2+ + 14H+  2Cr3+ + 6Fe3+ + 7H2O (ii) Calculate the concentration, in mol dm–3, of ethanol in this brand of wine. [4] mol of excess Cr2O72– = 6 1  mol of Fe2+ used in titration = 6 1  0.118  1000 25.12 = 2.41  10–4 mol mol of Cr2O72– added initially = cV = 0.156  1000 0.25 = 3.90  10–3 mol mol of Cr2O72– that react with CH3CH2OH = (3.90  10–3) – (2.41  10–4) = 3.66  10–3 mol 3CH3CH2OH + 2Cr2O72– + 16H+  3CH3CO2H + 4Cr3+ + 11H2O mol of CH3CH2OH in 25.0 cm3 of diluted soln = 2 3  mol of Cr2O72– reacted = 2 3  3.66  10–3 = 5.49  10–3 mol mol of CH3CH2OH in 250 cm3 of diluted soln = 0.25 250  5.49  10–3 = 0.0549 mol Since 10.0 cm3 of the sample of spirit was diluted to 250 cm3, mol of CH3CH2OH in 10.0 cm3 of spirit = 0.0549 mol  [CH3CH2OH] in the brand of spirit = 0.10 1000  0.0549 = 5.49 mol dm–3

3 [Turn over 9729/03/CJC JC2 Preliminary Examination 2017 (b) The relatively high alcohol content in wine makes it difficult to pair wine with spicy food because the combination can create a painful burning sensation on the lips and tongue that cannot be wiped or rinsed away. Capsaicin, C18H27NO3, is the active ingredient in spicy food that gives the hot taste of chilli peppers. The structure of capsaicin can be deduced from the following reactions. When capsaicin is boiled with dilute sulfuric acid, compound A, C10H18O2, and the salt of compound B, are produced. Compound B, C8H11NO2, has the structure shown, where the CH3O– group can be regarded as inert. compound B When A is heated with concentrated acidified KMnO 4, compound C, C 6H10O4, and compound D, C4H8O2, are produced. C can be prepared from Br(CH 2)4Br in two steps whereas D can be prepared directly from (CH3)2CHCH2OH. Compounds A, C and D all react with aqueous sodium carbonate. (i) What observations and deductions could be made from the reaction of A, C and D with aqueous sodium carbonate? [2] effervescence of CO2 ; A, C and D are acidic/contains –CO2H group. (ii) Draw the structural formula of D and state the reagents and conditions required for its preparation from (CH3)2CHCH2OH. [2] K2Cr2O7 + dil. H2SO4; heat under reflux D is (iii) Show how compound C can be prepared from Br(CH2)4Br, stating clearly the reagents and conditions required for each step, and give the structural formulae of C and the intermediate formed. [4] (iv) Hence suggest the structure for A and two possible structures for capsaicin. [3] A is Br(CH2)4Br ethanolic KCN heat under reflux dilute H2SO4 heat under reflux C

4 9729/03/CJC JC2 Preliminary Examination 2017 capsaicin is and (c) Methanol, CH3OH, is the simplest alcohol that could be used to power a fuel cell in an electric car. Figure 1.1 shows a preliminary design of a methanol fuel cell. The fuel and oxygen are fed continuously to the two electrodes which are made of platinum. Methanol is oxidised when the fuel cell is operated. Fig. 1.1 (i) Write balanced equations for the half reactions at each electrode and for the overall cell reaction. [2] Cathode: O2(g) + 4H+(aq) + 4e–  2H2O(l) Anode: CH3OH(l) + H2O  CO2(g) + 6H+ + 6e– Overall reaction: 2CH3OH(l) + 3O2(g)  2CO2(g) + 4H2O(l) (ii) Write a cell representation, indicating clearly on the cell representation the direction of electron flow in the external circuit. [1] Pt(s)CH3OH(l)/CO2(g) // O2(g)/ H2O(l), H+(aq)Pt(s) e– flow

5 [Turn over 9729/03/CJC JC2 Preliminary Examination 2017 (iii) Use appropriate data from the Data Booklet to explain why an acidic electrolyte is often preferred to an alkaline electrolyte. [1] In an acidic medium, E oO2/H2O = +1.23 V In an alkaline medium, E oO2/OH = +0.40 V Acidic electrolyte is preferred because o xygen is a stronger oxidising agent in an acidic medium. (iv) The design of the above fuel cell could be improved by replacing the acidic electrolyte solution with a film of solid H+ ion-conducting polymer. Suggest a reason for doing so. [1] Using a film of solid H+ ion-conducting polymer as electrolyte would eliminate the possibility of acid leakage from the fuel cell. (v) Methanol could also be used as fuel in an internal combustion engine. Suggest one advantage of the methanol fuel cell over such an internal combustion engine that uses methanol. [1] In the methanol fuel cell, there is complete combustion of CH3OH and so, no C nor CO (atmospheric pollutants) is produced. [Total: 22]

6 9729/03/CJC JC2 Preliminary Examination 2017 2 Acetals are molecules that contain 2 –OR groups bonded to the same carbon and are formed when aldehydes are reacted with an alcohol in the presence of an acid. The reaction between ethanal and methanol in the presence of acid, to produce acetal A was studied. (a) When the initial rate of this reaction was measured at various starting concentrations of ethanal, methanol and H+, the following results were obtained: Experiment number [CH3CHO] / mol dm-3 [CH3OH] / mol dm-3 [H+] / mol dm-3 relative rate 1 0.20 0.10 0.05 1.00 2 0.25 0.10 0.05 1.25 3 0.25 0.32 0.05 4.00 4 0.10 0.16 0.10 1.60 (i) Use the data in the table to determine the order with respect to I ethanal By inspection Using experiments 1 and 2, when [CH3CHO] increased by 0.25/0.20 = 1.25 times and the concentration of other reactants remain constant, the relative rate increases by 1.25 times. Order of reaction wrt ethanal = 1 II methanol and By inspection Using experiments 2 and 3, when [CH3OH] increased by 0.32/0.1 = 3.2 times and the concentration of other reactants remain constant, the relative rate increases by 4.00/1.25 = 3.2 times. Order of reaction wrt ethanal = 1 III H+. [3] By inspection Using experiments 1 and 4, when [CH3CHO] is halved and [CH3OH] is i