CHEMISTRY · Single correct · A Level · Chemistry · Kinetics · Kinetics
Problem
Consider two reactions, A and B, occurring in the same gaseous mixture. Reaction A requires surpassing an energy threshold of 40 kJ mol−1, whereas Reaction B requires surpassing 120 kJ mol−1. A student sketches Maxwell–Boltzmann distributions for 290 K and 310 K. Which prediction about the relative rate changes is most consistent with the distributions?
(a) Reaction A’s rate increases far more than Reaction B’s because the lower activation energy makes the exponential factor more temperature-sensitive.
(b) Both reactions increase by roughly 7 % because raising T from 290 K to 310 K only raises the average molecular speed by T2/T1.
(c) Reaction B’s rate increases by a much larger factor than Reaction A’s, since the fraction of molecules with E≥120kJ mol−1 grows proportionally more than the fraction with E≥40kJ mol−1 when T rises.
(d) Reaction B’s rate changes very little; even at 310 K almost no molecules attain 120 kJ, so its effective-collision frequency is nearly unchanged.