Consider the elementary gas–phase reaction

CHEMISTRY · Single correct · A Level · Chemistry · Kinetics · Kinetics

Problem

\text{X}_2(g) \rightleftharpoons 2\text{X}(g)

, which is endothermic with

\Delta H^\circ = +150\,\text{kJ\,mol}^{-1}

. The uncatalyzed activation energy for dissociation (forward direction) is 200 kJ mol

^{-1}

, and that for recombination (reverse direction) is 50 kJ mol

^{-1}

. A homogeneous catalyst lowers each activation energy by 40 kJ mol

^{-1}

. Which statement is most consistent with these data? (a) The catalyst decreases

\Delta H^{\circ}

40\,\text{kJ mol}^{-1}

, making the reaction exothermic and shifting the equilibrium toward

\mathrm{X_2}

. (Distractor) (b) By lowering activation energies, the catalyst increases the equilibrium constant

K

because product formation is favored. (Distractor) (c) The catalyst decreases the time required to reach equilibrium but leaves both

K

and

\Delta H^{\circ}

unchanged. (Correct) (d) Lowering each activation energy by the same amount sets

\Delta H^{\circ}

to zero, so the reaction is neither endothermic nor exothermic at equilibrium. (Distractor)