CJC Prelim H2 CHEM P3 qns

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 10 printed pages. 9647/03/CJC JC2 Preliminary Examination 2016 [Turn over

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] (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] (b) A solution of acidified potassium manganate(VII), KMnO 4, can be standardised 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] (ii) Write the half-equation for the oxidation of the ethanedioate ion. [1] (iii) Hence, w rite the overall balanced equation for the reaction between acidified manganate(VII) and ethanedioate ions. [1] (iv) In one such standardis ation 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] (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] [Total: 20]

3 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] (b) SiCl4(l) undergoes hydrolysis to give SiO2(s). (i) Write the balanced equation with state symbols for the hydrolysis reaction. [1] (ii) Using the above data, calculate the standard enthalpy cha nge for the hydrolysis reaction. State one assumption made in your calculation. [1] (iii) Does the hydrolysis have a positive, negative or zero entropy change? Explain your answer. [1] (iv) Hence, by the use of the Gibbs free energy, ∆Go, explain why the hydrolysis of SiCl4(l) is always a spontaneous process. [1] (c) Consider reaction 1 and reaction 2 shown below. (i) Both reaction 1 and reaction 2 proceed via a similar mechanism . Name the type of reaction undergone. [1] (ii) Suggest why reaction 1 gives a mixture of organic products while reaction 2 gives only one mono-substituted product. [2] (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] (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]

4 9647/03/CJC JC2 Preliminary Examination 2016 (iii) Hence, suggest the reagents and conditions used in the following conversion, stating clearly the intermediate products in the process of synthesis. [2] (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] (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] (iii) When a basic medium is used instead for the reaction between phenylethanone and iodine, a different mechanism of reaction is followed and different organic products are produced. (I) State the type of reaction undergone. [1] (II) Give the structures of all the organic products formed in this reaction. [2] [Total: 20]

5 9647/03/CJC JC2 Preliminary Examination 2016 [Turn over 3 Fireworks are a traditional part of many celebrations. Their colours can come from heating of Group II metals and their salts. These salts often contain anions such as chlorates, perchlorates, and nitrates which are oxidising agents. (a) One of the Group II metals, M, when heated with oxygen under pressure, forms a metal peroxide which can be use d as an oxidising agent in fireworks to produce a vivid green colour. The equation for the reaction is shown below: 2MO(s) + O2(g) ⇌ 2MO2(s) When equilibrium is attained, the mixture of oxides was found to have a mass of 1.60 g, where MO2 has a percentage mass of 18.7%. When the mixtu re was treated with dilute sulfuric acid, a white solid was precipitated and a solution of hydrogen peroxide was produced. An excess of acidified potassium iodide was added to the hydrogen peroxide solution. The reaction between hydrogen peroxide and acidified potassium iodide is as follows. H2O2 + 2I– + 2H+ → I2 + 2H2O The iodine liberated needed 11.80 cm3 of 0.300 mol dm–3 sodium thiosulfate, Na2S2O3, for complete reaction. I2 + 2S2O32– → S4O62– + 2 I– (i) Write a balanced equation to represent the reaction between MO2 and sulfuric acid. [1] (ii) By determining the amount of MO2 that has reacted, calculate the relative atomic mass of M and suggest its identity. [5] (b) Strontium nitrate, Sr(NO3)2 is an oxidising agent and is commonly used as a red colouring agent in fireworks. (i) Write a balanced equation for the reaction that occurs when Sr(NO 3)2 undergoes thermal decomposition. [1] (ii) Predict whether the decomposition temperature of Sr(NO 3)2 will be higher or lower than the decomposition temperature of Mg(NO3)2. Explain your answer. [3] (c) Another oxidising agent used in fireworks is potassium chlorate(V), KClO3. When KClO3 undergoes thermal decomposition, two products are obtained, one of which is KCl. (i) Write a balanced equation to show how KC lO3 is produced from C l2 gas and hot, aqueous potassium hydroxide. [1] (ii) Write a balanced equation for the thermal decompositio