At 400 K the equilibrium constant for the reaction is 120

CHEMISTRY · Single correct · A Level · Chemistry · Chemical equilibria (Le Chatelier’s principle and Kc) · Chemical equilibria (Le Chatelier’s principle and Kc)

Problem

At 400 K the equilibrium constant KcK_c for the reaction PCl3(g)+Cl2(g)  ⇌  PCl5(g)\text{PCl}_3(g)+\text{Cl}_2(g)\;\rightleftharpoons\;\text{PCl}_5(g) is 120. The reaction is exothermic in the forward direction. A sealed 1.00 L flask at 400 K contains an equilibrium mixture with [PCl3]=0.400 M[\text{PCl}_3]=0.400\,\text{M}, [Cl2]=0.400 M[\text{Cl}_2]=0.400\,\text{M}, and [PCl5]=19.2 M[\text{PCl}_5]=19.2\,\text{M}. The flask is then cooled rapidly to 350 K, at which temperature Kc=300K_c = 300, and at the same moment 0.0500.050 mol of Cl2\text{Cl}_2 is injected into the flask. Which statement correctly describes the direction of the net reaction immediately after these simultaneous changes? (a) Qc>KcQ_c > K_c; the net reaction will proceed toward additional PCl5\text{PCl}_5 formation (shift right). (b) Qc<KcQ_c < K_c; the net reaction will proceed toward additional PCl5\text{PCl}_5 formation (shift right). (c) Qc<KcQ_c < K_c; the net reaction will proceed toward PCl3\text{PCl}_3 and Cl2\text{Cl}_2 formation (shift left). (d) Qc=KcQ_c = K_c; the mixture remains at equilibrium despite the changes.

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