A mechanism predicts that a reaction is first order in reactant C under the…
CHEMISTRY · Single correct · AP · Chemistry · Kinetics · Kinetics
Problem
A mechanism predicts that a reaction is first order in reactant C under the conditions where the pre-equilibrium approximation is valid. The following slopes were obtained from linear plots of log(rate) versus log[C] at two different concentration ranges:
• Low [C] range: slope =0.98
• High [C] range: slope =0.35
Which statement best explains the observed change in slope?
(a) At high [C], detector saturation under-reports the initial rates, giving a smaller slope.
(b) Because taking the logarithm compresses large numbers more than small ones, the log[C] axis is effectively “squashed” at high concentrations, making the slope appear lower.
(c) At high [C], the initial fast step no longer remains in equilibrium, so the pre-equilibrium approximation breaks down.
(d) At high [C], the rate-determining step switches to an elementary step that is zero order in C, lowering the observed slope.