CHEMISTRY · Single correct · AP · Chemistry · Kinetics · Kinetics
Problem
For the reaction P+Q→S+T, the following two-step mechanism has been proposed.
Step 1 (slow): P+Q→R+S
Step 2 (fast): R→T
A student claims that species R is a catalyst for the reaction. Which statement best explains why this claim is incorrect?
(a) R is involved in the slow, rate–determining step, and catalysts can only participate in fast steps; therefore R cannot be a catalyst.
(b) R is produced in Step1 rather than being initially present, and its net concentration does not remain constant during the reaction.
(c) R does not appear in the overall balanced equation, so it cannot be acting as a catalyst.
(d) R appears and is consumed, so it behaves as a reactant rather than as a catalyst, which must remain entirely unchanged during the mechanism.