If a reaction mechanism contains a slow step and a fast step, which expression best describes the dependence of the rate on reactant concentrations for an elementary slow step A + B → products?

• A. Rate = k1[A][B]
• B. Rate = k2[A]
• C. Rate = k1[A]^2
• D. Rate = k3[B]^2

Answer: (A)

In a mechanism, the overall rate is governed by the slowest step—the rate-determining step. For an elementary step, the rate law mirrors its molecularity. Here the slow step is A + B → products, a bimolecular collision, so the rate is proportional to the chance that an A molecule meets a B molecule. That gives a rate proportional to [A][B], with its own rate constant k1. Therefore Rate = k1[A][B]. The other forms would imply the step involves only one species or a collision between the same species (A with A or B with B), which isn’t the case for this slow step.