1799f_PChem Rev 2 Qn

1799f_PChem Rev 2 Qn - Raffles Junior College H2 Chemistry...

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1 Raffles Junior College H2 Chemistry 2008/9 Physical Chemistry Revision Part 2 (Chemical Equilibria, Ionic Equilibria, Electrochemistry) Prepared by Mr Eric Gwee Dr Wong Poh Lee Chemical Equilibria 1. SO 2 and Cl 2 were mixed in a closed vessel and the whole system was allowed to reach equilibrium. The product of the reaction is SO 2 Cl 2 . The concentration of all gases were measured at one-minute interval for 4 minutes, and then one of the operating conditions was altered. The effects are shown graphically below. 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 0 1 2 3 4 5 6 7 8 9 10 Time/ min Concentration/ mol dm-3 (a) Write a balanced equation for the reversible reaction and an expression for the equilibrium constant, K c . [2] (b) Calculate the value of the equilibrium constant for the reaction at 7 minutes and at 9 minutes. [2] (c) Given that the temperature was decreased at 7 minutes and in the light of your answer to (b) above, is the forward reaction exothermic or endothermic? Explain your answer. [2] (d) What change to the system must have occurred at the 4 th minute? Account for the observed changes in concentration of the gases as a result of this change. [2] (e) Explain the effect on the equilibrium position and K c if the volume of the container is decreased keeping the temperature constant. [2] [JJC 2005] 2. Phosgene, COCl 2 , can decompose to gaseous chlorine and carbon monoxide under suitable conditions as shown in the equation below:
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2 COCl 2 (g) CO(g) + Cl 2 (g) The reaction was carried out at 300 o C in a 10.0 dm 3 vessel. At the start of the reaction, 3 moles of phosgene were introduced into the vessel. When equilibrium was established, it was found that 30% of the phosgene was left. (a) Calculate the equilibrium concentrations of the three gases. [2] (b) Determine the equilibrium constant, K c , for the reaction at 300 o C. [1] (c) When the temperature was increased to 400 o C, it was found that only 20% of the phosgene was left. Deduce whether the reaction is endothermic or exothermic. [2] (d) Explain how the equilibrium constant, K c will be affected when the volume of the vessel is halved. [1] [HCJC 2005] 3(a) NO 2 can be obtained by the reversible reaction between NO and O 2 . The equation for the reaction is 2NO(g) + O 2 (g) 2NO 2 (g) Equimolar mixture of NO and O 2 are made at a constant pressure P but at two different temperature of 300 o C and 330 o C, and the variation in colour intensity was monitored over a period of time. The graphs below show the variation of colour intensity with time. (i) What is the significance of the colour intensity in the reaction? (ii) Explain, in terms of collision theory, the difference in the initial slopes of the two graphs. (iii)
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1799f_PChem Rev 2 Qn - Raffles Junior College H2 Chemistry...

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