Reaction Mechanism and Rate Law

A mechanism breaks a reaction into elementary steps, each a single molecular event. Think of a factory line: if one station takes ten minutes per item and the rest take seconds, the whole line only outputs one item every ten minutes. That slow station is the rate-determining step, and chemically it behaves the same way. The overall rate can never outrun its slowest step, so the rate law for the whole reaction matches the rate law of that one elementary step.

Writing the rate law from an elementary step is direct: the exponents equal the coefficients of the reactants in that step, because an elementary step's rate law comes from its molecularity, not from experimental data. If A + B → C is the slow step, rate = k[A][B]. If the slow step is not first, things get more complex, since earlier fast steps can produce or consume species that feed into the slow step, sometimes requiring substitution of an intermediate's concentration using a fast pre-equilibrium. That complication sits just past this topic, so for now focus on mechanisms where the first step is rate-determining.

This connects directly to experimental rate laws from earlier in the unit: a mechanism is only acceptable if the rate law it predicts matches the one measured in the lab. Getting the correct overall balanced equation is easy to do many different ways, but only mechanisms whose slow step reproduces the observed orders and rate constant survive. That distinction, between a mechanism that merely balances and one that is actually consistent with data, is the real test chemists use.