Question. Hydrogen and iodine react reversibly:H2(g) + I2(g) ⇌ 2HI(g) The table gives Kc at two temperatures.698 K: Kc = 54 764 K: Kc = 46 (a) Deduce the sign of ΔH for the forward reaction. Explain your answer. (2)
(b) State what the value of Kc tells you about the position of equilibrium at 698 K. (1)
(c) State the effect on Kc of (i) doubling the pressure, (ii) adding a catalyst, both at 698 K. (2)Model answer.(a) ΔH is negative (the forward reaction is exothermic). Kc decreases as the temperature increases, so less HI is present at equilibrium at the higher temperature. The equilibrium has shifted to the left, the endothermic direction, so the forward direction must be exothermic.(b) Kc is much greater than 1, so the equilibrium lies to the right: the mixture contains mostly HI.(c)(i) No change. Kc only changes with temperature (and here the number of gas molecules is the same on both sides anyway).
(c)(ii) No change. A catalyst speeds up the forward and backward reactions equally.Examiner's comments. The key idea is that only temperature changes Kc. If Kc falls as the temperature rises, the forward reaction is exothermic; if Kc rises with temperature, the forward reaction is endothermic. Candidates who simply write 'Le Chatelier' without linking the change in Kc to the direction of shift do not get the second mark in (a).
(b) State what the value of Kc tells you about the position of equilibrium at 698 K. (1)
(c) State the effect on Kc of (i) doubling the pressure, (ii) adding a catalyst, both at 698 K. (2)Model answer.(a) ΔH is negative (the forward reaction is exothermic). Kc decreases as the temperature increases, so less HI is present at equilibrium at the higher temperature. The equilibrium has shifted to the left, the endothermic direction, so the forward direction must be exothermic.(b) Kc is much greater than 1, so the equilibrium lies to the right: the mixture contains mostly HI.(c)(i) No change. Kc only changes with temperature (and here the number of gas molecules is the same on both sides anyway).
(c)(ii) No change. A catalyst speeds up the forward and backward reactions equally.Examiner's comments. The key idea is that only temperature changes Kc. If Kc falls as the temperature rises, the forward reaction is exothermic; if Kc rises with temperature, the forward reaction is endothermic. Candidates who simply write 'Le Chatelier' without linking the change in Kc to the direction of shift do not get the second mark in (a).