1800f_PChem Rev 2 Ans

1800f_PChem Rev 2 Ans - Suggested Answers to Physical...

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1 Suggested Answers to Physical Chemistry Revision Exercise Part 2 Chemical Equilbria 1(a) SO 2 (g) + Cl 2 (g) SO 2 Cl 2 (g) [1] K c = [SO 2 Cl 2 ]/ [SO 2 ] [Cl 2 ] [1] 1(b) At 7 minutes, K c = 0.03/ (0.07 x 0.09) = 4.76 mol –1 dm 3 [1] At 9 minutes, K c = 0.04/ (0.08 x 0.06) = 8.33 mol –1 dm 3 [1] 1(c) As temperature decreases, K c increases => forward reaction is exothermic. [1] At lower temperature, equilibrium shifts to the right to favour the exothermic reaction. [1] 1(d) 4th minute: SO 2 has been added or [SO 2 ] is increased. [1] When [SO 2 ] is increased, by Le Chatelier’s Principle, equilibrium shifts to the right to remove some of the added SO 2 / reduce SO 2 b [SO 2 ] increases and then decreases. [1] [Cl 2 ] is decreased as some Cl 2 reacted with the added SO 2 to form more product, SO 2 Cl 2 . As more SO 2 Cl 2 is produced, [SO 2 Cl 2 ] increases. [1] 1(e) A decrease in volume results in an increase in pressure for the system. Hence, equilibrium shifts right to form a smaller number of gaseous molecules so as to reduce pressure [1] but K c remains constant. [1] [JJC 2005] 2(a) COCl 2 (g) CO(g) + Cl 2 (g) Initial no. of mol 3 0 0 Eqb no of mol 0.9 2.1 2.1 Eqb conc/ mol dm –3 0.09 0.21 0.21 [2] (b) K c = 0.21 2 / 0.09 = 0.49 mol dm –3 [1] (c) When temperature increases, equilibrium shifts to the right. [1] The forward reaction is endothermic since endothermic reactions are favoured by increase in temperature. By Le Chatelier’s Principle, when temperature of the system is increased, the system favours the endothermic reaction in order to remove the excess heat. [1] (d) K c remains unchanged as K c is affected by changes in temperature only. [1] [HCJC 2005] 3(a)(i) Colour intensity is proportional to the concentration of NO 2 in the mixture, since NO 2 is the only coloured component in the mixture. [1] (ii) The initial slope of the graph is proportional to the initial rate of reaction. [1] The graph at 330 o C has greater initial slope compared to that at 300 o C, hence Its initial rate is greater. [1] By collision theory – the greater rate at higher temperature is due to more reacting molecules having energy greater than or equal to activation energy. This leads to an increase in the frequency of effective collisions. Thus, the rate of reaction increases. [1] (iii) The system has reached a state of dynamic equilibrium, i.e., the rates of the forward and backward reactions are the same. [1] (iv) At the higher temperature of 330 o C, colour intensity is lower=> less NO 2 is produced. This suggests the backward reaction is favoured at higher temperatures. [1] By Le Chatelier’s Principle, when temperature of the system is increased, the system favours the endothermic reaction in order to remove the excess heat. [1] => the backward reaction is endothermic=> the forward reaction must be exothermic.
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2 (b) (i) NO 2 (g) + CO(g) NO(g) + CO 2 (g) Initial n/ mol x 2x 0 0 Change in n/ mol -1/2x -1/2x + 1/2x +1/2x Eqb n/ mol +1/2x 1.5x
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This note was uploaded on 10/12/2010 for the course C 11 taught by Professor Na during the Spring '10 term at Rappahannock Community College.

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1800f_PChem Rev 2 Ans - Suggested Answers to Physical...

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