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A rate law specifies how a reaction rate depends on concentrations under stated conditions. A common form iswhere $k$ is a rate constant and the exponents are partial orders of reaction. Determine them experimentally, or derive them from a justified reaction mechanism. Do not infer $\alpha =a$ and $\beta =b$ from the net chemical equation: its coefficients describe consumption ratios, whereas the exponents describe sensitivity to concentration. See IUPAC Gold Book: rate law and Flowers et al., Chemistry 2e, §12.3: rate laws.

A rate law becomes a prediction of time evolution when combined with stoichiometry in a rate equation, for example $d[\mathrm{A}]/dt=-av$. Agreement with a measured rate law is useful evidence for a reaction mechanism, but several routes can give the same dependence.

A reaction intermediate is formed in one step of a reaction mechanism and consumed in a later step, cancelling from the net chemical equation. A reaction intermediate can connect the transformations in energy coupling; it is distinct from a transition state. See the IUPAC Gold Book: intermediate.

A reaction mechanism proposes the sequence of elementary reactions that produces an overall chemical reaction. Its predicted rate law must agree with observations; matching one rate law need not uniquely identify a reaction mechanism. See Flowers et al., Chemistry 2e, §12.6: reaction mechanisms.