WAEC Chemistry 2018 Paper

Concept:According to Le Chatelier’s principle, the equilibrium shifts in the direction that opposes any change in conditions.Explanation:The forward reaction has $3$ moles of gas on the reactant side ($2\mathrm{AB}_2 + \mathrm{B}_2$) and $2$ moles of gas on the product side ($2\mathrm{AB}_3$).Thus, a decrease in pressure favors the side with more gas molecules, which is the backward (left) side.Also, the forward reaction is exothermic because $\Delta H = -X\ \mathrm{kJ\,mol}^{-1}$.Decreasing temperature would favor the forward (exothermic) reaction, not the backward reaction.A positive catalyst speeds up both directions equally and does not shift equilibrium.Increasing pressure favors the forward reaction because it has fewer gas molecules.Answer:A. a decrease in pressure