HCI H2 Chem Prelim P2

1 Candidate Name: _____________________________________ CT Group: 07S______ HWA CHONG INSTITUTION C2 PRELIMINARY EXAMINATION 9746 H2 CHEMISTRY Paper 2 Structured 16 September 2008 1 hour 30 minutes Do not open this booklet until you are told to do so. READ THESE INSTRUCTIONS FIRST Write your name and class clearly in the spaces at the top of this page. Write in dark blue or black pen. You may use a soft pencil for any diagrams, graphs or rough working. Answer all questions in the spaces provided in this question booklet. A Data Booklet is provided. You may use a calculator. The number of marks is given in brackets [ ] at the end of each question or part question. You are reminded of the need for good English and clear presentation in your answers. For Examiner’s Use 1 /11 2 /5 3 /9 4 /15 5 /8 6 /12 Total /60 This question booklet consists of 14 printed pages.

2 Answer all the questions. 1 (a) The table below gives the melting points of some elements of Period 3. Element phorus phos sulphur chlorine Melting point / K 317 392 172 In terms of structure and bonding, explain why sulphur has the highest melting point among the three elements. ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... [2] (b) Sulphur dissolves in aqueous sodium hydroxide to form sodium thiosulphate, Na 2S2O3, and sodium sulphide, Na2S. (i) State the type of reaction taking place. ............................................................................................................................................. (ii) Write ionic half-equations for the reaction and hence construct a balanced equation, including state symbols, for the reaction. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. [3] 9746/02/HCI/C2 Prelim/2008

3 (c) The following test-tube experiments are carried out starting from solid aluminium chloride. (i) Explain with the help of a balanced equation, why A lCl 3(s) dissolves to form an acidic solution. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. (ii) Identify W. ............................................................................................................................................. (iii) Write a balanced equation, including state symbols, for the reaction of W with excess NaOH(aq). ............................................................................................................................................. (iv) The chlorides of the elements, sodium to phosphorus, dissolve in or react with water. On the axes below, sketch the variation in pH of the solution obtained when each of these chlorides is dissolved in water. [6] [TOTAL: 11] Na2CO3(aq) water acidic solution AlCl3(s) white precipitate W + effervescence excess NaOH(aq) precipitate dissolves pH 7 14 0 NaCl MgCl2 AlCl3 SiCl4 PCl5 9746/02/HCI/C2 Prelim/2008

4 2 (a) Give an example of a reaction that uses a transition metal or its compound as a heterogeneous catalyst. Equation for reaction: ..................................................................................................................... Heterogeneous catalyst: ................................................................................................................ [2] (b) With reference to your example in (a), explain the mode of action of a heterogeneous catalyst. ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... ...................................................................................................................................................... [2] (c) In heterogeneous catalysis, the reactants must be carefully purified to prevent poisoning of the catalyst. Suggest what is meant by the terms in italics. ...................................................................................................................................................... ...................................................................................................................................................... [1] [TOTAL: 5] 9746/02/HCI/C2 Prelim/2008

5 3 (a) Meals-ready-to-eat (MRE) are military meal s that can be heated on a flameless heater. The heat is produced by the following reaction : Mg(s) + 2H2O(l) → Mg(OH)2(s) + H2(g) (i) Use the data below to calculate the standard enthalpy change for this reaction. Compound ∆H, f / kJ mol−1 H2O(l) −286 Mg(OH)2(s) −925 (ii) A MRE pack contains 2.4 g of magnesium which comes in contact with 100 g of water upon breaking a valve. Using your answer in (a)(i), determine whether this pack can be used to raise the temperature of the water from 25 oC to its boiling point. Specific heat capacity of water = 4.2 J g −1 K−1 [3] 9746/02/HCI/C2 Prelim/2008

6 (b) The following data relate to the energy changes which occur when sodium hydroxide, NaOH, and magnesium hydroxide, Mg(OH)2, dissolve in water. s modium agnesium H − −ydration energy of metal ion / kJ mol−1 390 1890 L − −attice energy of metal hydroxide / kJ mol−1 896 2995 E − x nthalpy change of solution of metal hydroxide / kJ mol−1 44 E (A 5 15 xtent of hydration of metal ion verage number of attached water molecules per metal ion) (i) Explain why the hydration energy of Mg2+ is more exothermic than that of Na+. ................................................