NYJC H2 Chem 2013 Prelim P3 Soln

1 H2 Chemistry 9647/03 NYJC J2/2013 Prelim [Turn over Answer for Paper 3 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. H 2 (g) + I 2 (g) 2 HI (g) Initial amt / mol X 2x 0 Change / mol 0.86x 0.86x +1.72x Equilbrium / mol 0.14x 1.14x 1.72x . n(total) = 1.14x + 0.14x + 1.72x = 3x K p = 22 2 2 1.72(2 )() 3 0.14 0.14() ( ) (2 ) (2 )33 HI HI x P x x xPP xx  = 18.5 (ii) Suggest, with reasoning, a way to increase degree of dissociation of H 2 for the above reaction. Decrease temperature. By Le Chatelier’s Principle, when temperature is decreased, The forward exothermic reaction is favoured. Hence POE shift to the right, and degree of dissociation of H 2 increases. 66

(iii) Deduce how Kp for the following system would be compared to your answer in (i). H2 (g) + Br2 (g) 2HBr (g) It will be larger since a stronger HBr bond is fo rmed. Extent of equilibrium lies more to the right. [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. The C-H bonds are non-polar and very strong (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 stabilit y of the intermediates formed when different types of H atom are abstracted, explain the trend in relative rate of abstraction. The tertiary radical is the most stable as it contains the most electron- donating alkyl groups which help to stab ilise the electron-deficient radical centre. Hence, it is abstracted more easily/faster. (iii) Predict the ratio of the following two products A and B, taking into account the relative rates of abstraction given in (ii). Explain your reasoning. Cl and A B Cl 67

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