rate law

Meaning: The rate law tells how fast a reaction goes at a given temperature and how that speed depends on concentration. Each exponent is the order in that reactant, and the sum of the exponents is the overall order. The rate constant k is specific to the reaction and changes with temperature (and with a catalyst), but not with concentration. Orders are found by experiment. For an elementary step, the orders do equal the coefficients (molecularity). For a multistep mechanism, the rate law reflects the slowest step (rate-determining step) and any species that step needs.

Equation: rate = k[A]^m[B]^n, where rate is in M/s (mol L⁻¹ s⁻¹), k has units that depend on overall order (s⁻¹ for first order, M⁻¹s⁻¹ for second order), [A] and [B] are molar concentrations (M), and m and n are the orders (unitless).

On the MCAT: A passage gives a table of initial rates for different starting concentrations. Compare two trials in which only one concentration changes. If doubling [A] doubles the rate, the reaction is first order in A. If it quadruples the rate, it is second order. If the rate does not change, it is zero order. You may also be asked for k and its units, or to pick a mechanism consistent with a given rate law.

Common confusion: Students read the exponents from the balanced equation. That works only for an elementary step. For an overall reaction, the orders must come from data or from the rate-determining step.