CJC Prelim P3 QN

[Turn over 9729/03/CJC JC2 Preliminary Examination 2017 CHEMISTRY 9729/03 Paper 3 Free Response 25 August 2017 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. 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. This document consists of 14 printed pages and 0 blank page. Instruction Catholic Junior College JC2 Preliminary Examinations Higher 2

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 th e following experiment to determine the concentration of ethanol in a brand of wine. A 10.0 cm 3 sample of the wine was diluted to 250 cm 3. He then added 25.0 cm 3 of 0.156 mol dm–3 K2Cr2O7(aq) and excess dilute H2SO4 to a 25.0 cm3 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] (ii) Calculate the concentration, in mol dm–3, of ethanol in this brand of wine. [4] (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, C 18H27NO3, is the active ingredient in spicy food that gives the h ot 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, C 4H8O2, 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] (ii) Draw the structural formula of D and state the reagents and conditions required for its preparation from (CH3)2CHCH2OH. [2]

3 [Turn over 9729/03/CJC JC2 Preliminary Examination 2017 (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] (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] (ii) Write a cell representation, indicating clearly on the cell representation the direction of electron flow in the external circuit. [1] (iii) Use appropriate data from the Data Booklet to explain why an acidic electrolyte is often preferred to an alkaline electrolyte. [1] (iv) The design of the above fuel cell could be improved b y replacing the acidic electrolyte solution with a film of solid H+ ion-conducting polymer. Suggest a reason for doing so. [1] (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] [Total: 22]

4 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 II methanol and III H+. [3] (ii) Use your results from (a)(i) to write the rate equation for the reaction. [1] (iii) Calculate the relative rate of reaction for a mixture in which the starting concentrations of all three reactants are 0.20 mol dm–3. [1] (b) The mechanism of acetal formation is proposed to proceed through the following steps. (i) The protonation of the carbonyl group in step I is necessary for step II to take place. Suggest a reason for this. [1] (ii) Using your answer to (a)(ii), state with reasons, which one is the rate-determining step of the reaction mechanism. [2] (iii) The species in the mechanism have various roles. They can be reactants, products, catalysts or intermediates. Suggest, with a reason in each case, the roles of H+ and X. [2]

5 [Turn over 9729/03/CJC JC2 Preliminary Examination 2017 (c) Draw the structure of the acetal B formed from the reaction between phenylethanal and propan-1-ol. [1] (d) Acetal formation is useful in organic synthesis. When an aldehyde or ketone is converted to an acetal, the carbonyl group is protected from attack by reagents such as bases and reducing agents. The acetal can later be hydrolysed by a strong acid to form the original aldehyde or ketone since the formation of acetals is a reversible reaction. Suggest a suitable synthesis route for the following conversion which involves the initial formation of an acetal group using ethane-1,2-diol, . [2] (e) Methanol is currently studied as a potential green fuel that greatly reduces the pollution to the environment. CH3OH(l) + 3 2 O2(g)  CO2(g) + 2H2O(l) (i) The standard molar entropy, So is the entropy content of one mole of substance under standard conditions. Given the following standard molar entropy values, calculate the standard entropy change of combustion of methanol. [1] So /J K−1 mol−1 CH3OH(l) 126.8 O2(g) 205.0 CO2(g) 213.6 H2O(l) 69.9 (ii) Hence, explain the significance of the sign of the standard entropy change of combustion of methanol. [1]

6 9729/03/CJC JC2 Preliminary Examination 2017 (iii) A student carried out an experiment to determine the enthalpy change for the combustion of methanol. A sample of methanol was burnt under laboratory conditions and the following results were obtained by the student