DHS Y5 Revision Lecture_Chem Equilibria (With suggested answer)

2024 Y5 CHEMISTRY REVISION LECTURE Term 4 Week 1 Discussion Questions 2 (a) Nitrogen monoxide in the air can be converted to nitric acid, which results in acid rain. Both nitrogen monoxide and nitrogen dioxide also cause ozone layer depletion. One way of forming nitrogen monoxide is through the dissociation of nitrogen dioxide. 2NO2(g) ⇌ 2NO(g) + O2(g) ∆H = +114.2 kJ mol-1 At 494 °C, the value of Kp for the above reaction is 36.9 kPa. When nitrogen dioxide is placed in a sealed container at 494 °C, equilibrium is reached when 40% of the original nitrogen dioxide has dissociated. (i) Given that the standard enthalpy change of formation of NO(g) is +90.3 kJ mol-1, calculate the standard enthalpy change of formation of NO2(g). [1] (ii) Write an expression for the equilibrium constant, Kp, for the reaction. [1] (iii) Calculate • the initial partial pressure of nitrogen dioxide and • the total pressure at equilibrium. [3] (iv) State and explain the impact on the equilibrium yield of NO(g) and value of Kp if the process was now conducted at 400 oC. [2] 1 (a) A sample of chlorodifluoromethane, CHClF2, undergoes pyrolysis in a closed vessel of volume 18.5 dm3 according to the following equilibrium. 2CHClF2(g) ⇌ C2F4(g) + 2HCl(g) ∆H = +128 kJ mol−1 At the reaction temperature, the degree of dissociation was found to be 75%. (i) Write the expression for the equilibrium constant, Kc, for the reaction. [1] (ii) Calculate the initial amount of CHClF2 in the sample given that the magnitude of Kc at the reaction temperature was 0.780. [2] (iii) Explain how the Kc of the reaction will change when the reaction is carried out at a lower temperature. [2]

(v) The volume of the sealed container was instantaneously doubled at constant temperature, and the system allowed to reach equilibrium. Explain the effect this will have on the partial pressures of the individual gases. [3] (vi) Hence, predict how the value of Kp will change. [1] N2018/P2/2 MCQ 1 A gas P decomposes to two other gases, Q and R, according to the equation: 2P(g) ⇌ 3Q(g) + R(g) The graph below represents the decomposition of 1.0 mole of P in the presence of a catalyst at variou