NYJC H2 Chem 2013 Prelim P3 QP

NANYANG JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 CHEMISTRY 9647/03 Paper 3 Free Response 19 Sep 2013 2 hours Candidates answer Section A on the Question 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 on both sides of the paper. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer any four questions. A Data Booklet is provided. 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. Section B Answer all questions on separate answer paper. For Examiner’s Use Section A 1 2 3 4 5 Total This document consists of 11 printed pages 40

2 Answer any four questions. 1 The elements of Group VII of the Periodic Table are called halogens, which mean "salt formers". They lack only one electron to fo rm a complete shell, and are extremely active chemically. This question examines the chemistry of two reactions of halogens. (a) In the first reaction, a 1:2 mixture of H 2 and I 2 gases was mixed at a constant temperature of 430 oC and a constant pressure of 2 atm. The following equilibrium was set up. H2 (g) + I2 (g) 2HI (g) ∆H = −11 kJ mol1 (i) Given that degree of dissociation of H 2 is 86 %, calculate the equilibrium constant, Kp for the above reaction under these conditions. (ii) Suggest, with reasoning, a way to increase degree of dissociation of H 2 for the above reaction. (iii) Deduce how K p for the following system would be compared to your answer in (i). H2 (g) + Br2 (g) 2HBr (g) [7] In another reaction, halogens can react with unreactive alkanes to form mono- substituted product. An example is the reaction of chlorine with 2-methylpentane shown below. 2-methylpentane (b) (i) Explain why alkanes are generally unreactive. (ii) This reaction is seldom used for synthesis as there are many associated problems. Firstly, several isomeric pr oducts are formed. The relative ratio of the isomeric products may be more a ccurately determined if relative rates of abstraction of H atoms are taken into account. The relative rates of abstraction of H atoms are Type of H atoms Relative rate of abstraction Primary 1 Secondary 4 Tertiary 6 By examining the difference in stab ility of the intermediates formed when different types of H atom are abstracted, explain the trend in relative rate of abstraction. 41

3 H2 Chemistry 9647/03 NYJC J2/2013 Prelim [Turn