rate-determining step

Meaning: A mechanism breaks a reaction into elementary steps. The slowest step acts as a bottleneck, like the narrowest point in a pipe, so speeding up other steps changes the overall rate very little. On an energy diagram it is the step with the largest activation energy, meaning the biggest climb from the preceding low point to the next transition state. A catalyst that lowers this barrier speeds up the overall reaction.

Equation: rate = k[A]^m[B]^n, where rate is in mol/L·s, k is the rate constant of the slow step (units depend on overall order), [A] and [B] are the concentrations of the reactants in that step in mol/L, and m and n are their coefficients in that elementary step.

On the MCAT: A passage gives a proposed mechanism and an experimental rate law, and you decide which step is slow. Or you read an energy diagram and pick the step with the tallest barrier. If the slow step contains an intermediate formed in a fast equilibrium before it, you use Keq of that fast step to rewrite the intermediate in terms of reactants. A reactant that enters only after the slow step does not appear in the rate law.

Common confusion: Students read orders from the coefficients of the overall balanced equation. Only the elementary step that is rate-determining gives the rate law. The slow step is also not automatically the first step, and it is set by the highest barrier, not by which step is most exergonic.