Two different mechanisms (I and II) accomplish the overall reaction A → D
CHEMISTRY · Single correct · A Level · Chemistry · Kinetics · Kinetics
Problem
Two different mechanisms (I and II) accomplish the overall reaction A → D.
Mechanism I: highest transition-state energy = 75 kJ relative to A; ΔH = −40 kJ.
Mechanism II: highest transition-state energy = 55 kJ relative to A; ΔH = +20 kJ.
Under identical conditions, which statement is most consistent with these energy profiles?
(a) Mechanism I should proceed faster because its overall reaction is more exothermic, releasing 40kJ.
(b) Mechanism II should proceed faster because its rate-determining activation energy (55kJ) is lower than that for mechanism I.
(c) Mechanism II should proceed slower because converting A to an endothermic product requires extra energy beyond the transition state.
(d) Both mechanisms should proceed at similar rates because each contains the same number of elementary steps.