CJC Prelim H2 CHEM P3 ans

CATHOLIC JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATIONS Higher 2 CHEMISTRY 9647/03 Paper 3 Free Response Friday 26 August 2016 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. Answer any four questions. A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. You are reminded of the need for good English and clear presentation in your answers. 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 documents consists of 17printed pages and 1 blank pages. 9647/03/CJC JC2 Preliminary Examination 2016 [Turn over ANSWER SCHEME & EXAMINERS’ COMMENTS

2 9647/03/CJC JC2 Preliminary Examination 2016 Answer any four questions. 1 Use of the Data Booklet is relevant to this question. (a) When potassium manganate(VII), KMnO4, is heated with concentrated sodium hydroxide, NaOH, bubbles of oxygen are seen and a green solution of MnO42- is obtained. The addition of barium chloride, BaC l2, to this solution precipitated out a solid X with composition by mass of 53.5% barium, 21.5% manganese and 25.0% oxygen. (i) Calculate the empirical formula of X. [1] Element Ba Mn O % mass 53.5 21.5 25.0 Ar 137 54.9 16.0 No. of moles 0.391 0.392 1.56 Mole ratio 1 1 4 Empirical formula of X is BaMnO4 (ii) When the green solution above is acidified, a brown precipitate in a purple solution is formed. Suggest the identities of the species formed upon acidification and hence state the type of reaction that has occured. [3] Brown precipitate is MnO2 Purple solution is MnO4– Disproportionation (b) A solution of acidified potassium manganate(VII), KMnO 4, can be standardi sed through titration against sodium ethanedioate, Na 2C2O4. The ethanedioate ion is oxidised to carbon dioxide in this reaction. (i) State the oxidation numbers of C in the ethanedioate ion and in carbon dioxide. [1] O.N. of C in C2O42– = +3 O.N. of C in CO2 = +4 (ii) Write the half-equation for the oxidation of the ethanedioate ion. [1] C2O42–  2CO2 + 2e– (iii) Hence, w rite the overall balanced equation for the reaction between acidified manganate(VII) and ethanedioate ions. [1] 2MnO4– + 16H+ + 5C2O42–  2Mn2+ + 10CO2 + 8H2O

3 9647/03/CJC JC2 Preliminary Examination 2016 [Turn over (iv) In one such standardi sation procedure, 25.0 cm 3 of 0.0150 mol dm ‒3 Na2C2O4 solution required 28.85 cm3 of acidified KMnO4 to reach the end-point. Determine the concentration of KMnO 4 in the solution. You may assume that 2MnO4– ≡ 5C2O42–. [3] No. of moles of C2O42– = 25.0 1000 ×0.0150 = 0.000375 mol Since 2MnO4– ≡ 5C2O42–, No. of moles of MnO4– = 2 5 ×0.000375 = 0.000150 mol Concentration of KMnO4 = 0.000150 28.85 ×1000 = 0.00520 mol dm‒3 (c) Myrcene, A and ocimene, B, are isomers with the molecular formula C 10H16. When subjected to hydr ogen with platinum catalyst, both isomers give 2,6 -dimethyloctane, C10H22. When treated with hot concentrated acidified KMnO4, A gives CO 2 and compound C, C7H10O4; B gives CO2 and compounds D, C5H8O3, and E, C3H4O3. Compounds C, D and E give one mole of CH I3 with alkaline aqueous iodine. On addition of aqueous NaHCO3, compounds C, D and E produce effervescence. Suggest structures for A-E, and explain the observations described above. [10]

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5 9647/03/CJC JC2 Preliminary Examination 2016 [Turn over 2 The table below lists the standard enthalpy change of formation of four compounds. Compound ∆Hf o /kJ mol–1 H2O(l) – 286 HCl(g) – 92.0 SiO2(s) – 910 SiCl4(l) – 640 (a) What is meant by the term, standard enthalpy change of formation of a compound? [1] The enthalpy change when 1 mol of a compound is formed from its elements (in its most stable form), under standard conditions (25oC, 1 atm pressure). (b) SiCl4(l) undergoes hydrolysis to give SiO2(s). (i) Write the balanced equation with state symbols for the hydrolysis reaction. [1] SiCl4(l) + 2H2O(l) → SiO2(s) + 4HCl(g) (ii) Using the above data, calculate the standard enthalpy cha nge for the hydrolysis reaction. State one assumption made in your calculation. [1] ∆Hro = ∑∆Hfo(products) – ∑∆Hfo(reactants) = [– 910 + 4(-92.0)] – [– 640 + 2(– 286)] = – 66.0 kJ mol-1 Assumption: - SiCl4(l) is in excess. The hydrolysis gives HCl(g) instead of HCl(aq). - Hess’ Law is followed/obeyed. (iii) Does the hydrolysis have a positive, negative or zero entropy change? Explain your answer. [1] Positive entropy change. 4 moles of gas (HC l) are produced. There is an increase in disorder of the system as there are more ways of arranging the partciples in the system. (iv) Hence, by the use of the Gibbs free energy, ∆Go, explain why the hydrolysis of SiCl4(l) is always a spontaneous process. [1] ∆Go = ∆Ho - T∆So Since ∆Ho is negative and ∆So is positive, ∆Go is always negative. So the process is spontaneous. (c) Consider reaction 1 and reaction 2 shown below.

6 9647/03/CJC JC2 Preliminary Examination 2016 (i) Both reaction 1 and reaction 2 proceed via a similar mechanism . Name the type of reaction undergone. [1] Electrophilic substitution. (ii) Suggest why reaction 1 gives a mixture of organic products while reaction 2 gives only one mono-substituted product. [2] Reaction 1 The CH3 is an electron donating or activating group. It activates the benzene ring and so makes the delocalised 𝝅 electrons more susceptible/available to electrophilic attack/electrophiles . This favours further substitution to give multi-substituted products. Reaction 2 The C=O is an electron withdrawing or deactivating group. It deactivates the benzene ring and so makes the delocalised 𝝅 electrons less susceptible/available to electrophilic attack/electrophiles. Further substitution is less favourable. (d) The organic product in reaction 2, phenylethanone, , reacts with HCN to produce the corresponding cyanohydrin under certain experimental conditions. (i) State the experimental conditions. [1] Trace amount of NaOH/NaCN and 10 – 20oC (ii) Describe the mechanism of reaction, of phenylethanone with HCN, showing clearly the curly arrows to indicate the movement of electrons and all charges. [3] Nucleophilic Addition Mechanism. HCN H+ + :CN- (iii) Hence, suggest the reagents and conditions used in the following conversion, stating clearly the intermediate products in the process of synthesis. [2]

7 9647/03/CJC JC2 Preliminary Examination 2016 [Turn over (e) The rate of reaction between phenylethanone and iodine in acid medium to give 2-iodo-1-phenylethanone, , is found to be independent of [I2], but directly proportional to [H+] and directly proportional to [phenylethanone]. (i) Write the rate equation for this reaction and state the overall order and the units of the rate constant. [2] Rate = k [ ][H+] Overall order = 2, units: mol-1 dm3 s-1 (ii) The reaction between phenylethanone and bromine proceeds by a similar mechanism. How would you expect the rate of this reaction to compare with that of the above reaction? Explain your answer. [1] The reaction rate is unchanged as it is also independent of [Br2] like I2. (iii) When a basic medium is used instead for the reaction betwee