JAMB Chemistry 2015 Paper

Concept:A decrease in pressure shifts a gaseous equilibrium toward the side with more gas molecules.Explanation:The reaction is: $\mathrm{PCl_5(g) \rightleftharpoons PCl_3(g) + Cl_2(g)}$.On the reactant side, there is $1$ mole of gas.On the product side, there are $2$ moles of gas.When pressure decreases, the equilibrium moves in the direction that produces more gas molecules.Therefore, the forward reaction is favored.More $\mathrm{PCl_3}$ and $\mathrm{Cl_2}$ will be formed.The yield of $\mathrm{PCl_3}$ increases.The yield of $\mathrm{PCl_5}$ would decrease, not increase.Pressure change mainly shifts the equilibrium position rather than simply speeding up or slowing down the reaction.Answer:B. Increase yield of $\mathrm{PCl_3}$.