Phosphorus pentachloride decomposes according to PCl _5(g) PCl _3(g) + Cl _2(g) H^ = +92

CHEMISTRY · Multi-part · A Level · Chemistry · Chemical equilibria (Le Chatelier’s principle and Kc) · Chemical equilibria (Le Chatelier’s principle and Kc)

Problem

Phosphorus pentachloride decomposes according to PCl5(g)⇌PCl3(g)+Cl2(g)ΔH∘=+92 kJ.\mathrm{PCl}_5(g) \rightleftharpoons \mathrm{PCl}_3(g) + \mathrm{Cl}_2(g) \quad \Delta H^\circ = +92\,\text{kJ}. A mixture at 600 K600\,\text{K} is at equilibrium with the following partial pressures: PCl5=0.60 atm\mathrm{PCl}_5 = 0.60\,\text{atm}, PCl3=0.25 atm\mathrm{PCl}_3 = 0.25\,\text{atm}, Cl2=0.25 atm\mathrm{Cl}_2 = 0.25\,\text{atm}. The system is then simultaneously subjected to two stresses: • The temperature is lowered by 50 K50\,\text{K}. • Some Cl2\mathrm{Cl}_2 is quickly removed, lowering its partial pressure to 0.10 atm0.10\,\text{atm} before any shift can occur. (a) Considering only the temperature change, predict the direction of the equilibrium shift. Justify your answer. (b) Considering only the removal of Cl2\mathrm{Cl}_2, predict the direction of the equilibrium shift. Justify your answer. (c) Taking both stresses together, determine the net direction in which the system will shift. Explain your reasoning in terms of QQ relative to the new KK.

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