WAEC Chemistry 2013 Paper

Concept:A decrease in temperature shifts an equilibrium toward the exothermic direction to produce heat.Explanation:The given reaction is:$$\mathrm{N_2O_{4(g)} \rightleftharpoons 2NO_{2(g)}}, \quad \Delta H = +x\ \mathrm{kJ\,mol^{-1}}$$The forward reaction is endothermic because $\Delta H$ is positive.Therefore, the reverse reaction, which forms $\mathrm{N_2O_4}$, is exothermic.When the temperature is reduced, the equilibrium shifts in the direction that releases heat.This is the reverse direction, producing more $\mathrm{N_2O_4}$.Hence, the concentration of $\mathrm{N_2O_{4(g)}}$ increases.Answer:B. Concentration of $\mathrm{N_2O_{4(g)}}$ increases.