26.1.5—Mechanisms and rate-determining step

For a multi-step mechanism, the slow rate-determining step controls the observed rate. A proposed mechanism must reproduce the overall equation and the experimentally observed rate equation.

Intermediates are formed then consumed; catalysts are used then regenerated. If a fast pre-equilibrium precedes the slow step, substitute its relationship to eliminate intermediates from the rate law.

If the slow step is A + I → products and a fast step makes I proportional to [B], the observed rate may be k[A][B], even when I is absent from the overall equation.

Do not copy overall stoichiometric coefficients into the rate equation automatically, and do not leave an intermediate in the final observable law.