MI Prelim 2008 H2 Chemistry Paper 3

Class Adm No Candidate Name: This question paper consists of 11 printed pages. [Turn over Preliminary Examinations 2008 Pre-university 3 H2 CHEMISTRY 9746 / 03 Thursday 11 Sep 2008 2h Additional materials: Foolscap Paper Data Booklet READ THESE INSTRUCTIONS FIRST 1. Do not turn over this question paper until you are told to do so. 2. Write your name, class and index number in the spaces provided at the top of this page and on all the work you hand in. 3. Write in dark blue or black pen on both sides of the paper. 4. You may use a soft pencil for any diagrams or graphs. 5. DO NOT use staples, paper clips, highlighters, glue or correction fluid or tape. 6. Answer FOUR out of five questions. 7. Give non-exact numerical answers correct to 3 significant figures , or 1 decimal place in the case of Mr and Ar, unless a different level of accuracy is specified in the question. 8. The number of marks is given in brackets [ ] at the end of each question or part question. 9. You are reminded of the need for good English and clear presentation in your answers and to show all working in calculations. 10. The use of a calculator is expected, where appropriate.

1. This question explores the physical and c hemical properties of various types of halogen-containing compounds. (a) (i) By considering the structure and bonding in each compound, explain why potassium chloride and silicon (IV) chloride shows a marked difference in their melting points. (ii) State what is observed when separate samples of potassium chloride and silicon (IV) chloride are added to water. Write equation(s) for the reaction(s) which occur and, in each case, state the approximate pH of the solution formed. [4] (a) Potassium chloride and potassium bromide are white solids which dissolve in water to form colourless solutions. To distinguish the above compounds, often silver nitrate solution is used, leading to the formation of white silver chloride and cream silver bromide precipitates. Suggest an alternative chemical test to distinguish the two compounds. State clearly what will be observed for both compounds and write balanced equations for any reactions that occur. [4] (c) A solution containing 1.0 x 10 -4 mol dm-3 of Ag+ ions is slowly stirred into an equal volume of solution which contains KCl and KBr, each with the same concentration of 1.0 x 10-4 mol dm-3. The numerical values of the solubility products are: K sp (AgCl) = 1.8 x 10-10 Ksp (AgBr) = 3.3 x 10-13 Show with clear workings, what will be observed from above. [3] 2

(b) CH3Cl can form (CH3)2NH when reacted with an excess of CH3NH2. (i) Outline a mechanism to show the production of (CH 3)2NH. (ii) Draw the structural formula of two other possible compounds produced in the reaction. [5] (c) CH 3Cl can also react with phenol under various conditions, producing different compounds. State the two different conditions possible for reaction with phenol and draw the products formed respectively. [4] [Total: 20m] 3

2. (a) Explain the underlying chemistry for the following observations. (i) Barium oxide in water produces a solution which reacts with dilute hydrochloric acid to give a solution with a pH of about 7. (ii) Formation of oxides of Group II metals from their respectively carbonates becomes increasingly endothermic down the group. [4] (b) Describe the reaction, if any, of the oxi des of sodium, aluminium and silicon with: (i) NaOH(aq) and (ii) HCl(aq). Write balanced equations for all the reactions involved, if any. [4] (c) Using the following data, draw an energy level diagram to show the stability of benzene relative to the hypothetical cyclohexa-1,3,5-triene. Give a reason for this difference in stability. [2] 4

(d) Consider the following reaction sequence which starts from phenylamine. (i) State and explain the difference in base strength between phenylamine and ammonia. (ii) Suggest a chemical test to dist inguish phenylamine and ammonia, stating clearly the observations for both compounds. (iii) 4–nitrophenylamine can be directly synthesized from phenylamine. Give the reagent(s) and condition required for the conversion. (iv) Name and outline a mechanism for the reaction in (iii). (v) Name the type of linkage which is broken in Step 3 and hence suggest a suitable reagent and condition required for this step. [10] [Total: 20m] 5

3. (a) Industrially, methanol, a useful fuel, can be manufactured from a 2:1 hydrogen:carbon monoxide mixture at high pressure and a temperature of 300 °C in the presence of a finely divided solid catalyst, e.g. Cu-Zn oxide. Under these conditions, the equilibrium yield of methanol is only of the order of 1%, yet these are the conditions chosen for a continuous commercial process in which an overall conversion of 95% can be achieved. CO(g) + 2H 2(g) CH3OH(g) ΔHrxn = – 91 kJ mol–1 (i) Explain why the relatively low temperature of 300 °C is chosen. (ii) Why is a catalyst necessary for this reaction? (iii) What type of catalysis does this illustrate? Explain, in detail, how the catalyst works in the reaction. (iv) Explain how a 95% conversion can be achieved in a reaction which has an equilibrium yield of only 1%. (v) Explain what is meant by the term feasible reaction and determine the temperature conditions at which the synthesis of methanol is no longer f e a s i b l e . [ 1 1 ] (b) There is a need to ensure that there is no leakage in the industrial setup for the production of methanol. This is because carbon monoxide is toxic and can cause brain damage or even death when breathe in. (i) Explain, with reference made to haemoglobin, the toxicity of carbon monoxide. (ii) Suggest a method to reverse the effect of CO poisoning. [3] 6

(c) Using an energy cycle, calculate a value for the standard enthalpy of combustion of methanol with reference to the data provided below. [2] Enthalpy change of combustion of CO –283 kJ mol–1 Enthalpy change of combustion of H2 –286 kJ mol–1 Enthalpy change of vaporization of CH3OH 35.3 kJ mol-1 (d) In a steam generator, the heat released from the combustion of methanol is used for boiling of water to produce steam. The steam produced is then used to generate electricity. About 2.10 x 10 4 kg of methanol is burnt everyday. (i) Given that only 85% of heat released from the combustion process will be effectively used in the steam generator, calculate the initial temperature of water that is used in the steam generator. (Given: Volume of water in steam generator = 1.40 x 10 9 cm3 and density of water = 1 g cm-3) (ii) Sometimes gasoline (C 8H8) can also be used in steam generator. Suggest one environmental problem caused by the burning of gasoline. [4] [Total: 20m] 7

4. (a) A biochemist must prepare a solution for use as a medium in an experiment involving acid-producing bacteria. The acid is produced when the bacteria makes use of the enzymes, e.g. Co–Zy, it synthesises, to break down complex molecules into simpler substances. The pH of the medium must not change with the addition of acid produced by the bacteria. The solution for use as the medium is made by dissolving 43.0 g of a weak base - (HOCH2)3CNH2, also known as TRIS, in 1 dm3 of 0.095 mol dm-3 HCl. (i) Illustrate, using an equation, how the solution can assist in maintaining the pH in an experiment involving acid-producing bacteria. (ii) Given that pK b of TRIS is 5.91, calculate the pH of the solution. (iii) The bacteria produce 0.01 mol of H+ in presence of 1 dm3 of the solution. Calculate the change of pH in the solution. [5] (b)