reaction mechanism

Meaning: A mechanism breaks an overall reaction into elementary steps, each a single molecular event, so each step's rate law follows its coefficients (A + B → C is first order in A and first order in B). An intermediate is made in one step and consumed in a later one; a catalyst is consumed in one step and regenerated in a later one. Both cancel from the overall equation. The slowest step is rate-determining and controls the observed rate law.

Equation: rate = k[A][B] for the step A + B → C, where k is the rate constant (m³ mol⁻¹ s⁻¹ for this second-order step) and [A] and [B] are concentrations (mol/m³).

On the MCAT: Passages give a multistep mechanism and an experimental rate law. You identify the intermediate, the catalyst, the slow step, or whether the mechanism is consistent (the steps must sum to the overall equation). On an energy diagram, the highest peak marks the rate-determining step, the peaks are transition states, and the valleys between peaks are intermediates.

Common confusion: Reaction orders come from experiment, not from the overall equation's coefficients; only for an elementary step do the coefficients give the order. Also, agreement with rate data can rule out a mechanism but cannot prove it.