Rate law

A rate law is an equation relating a chemical reaction’s rate to reactant concentrations, usually through experimentally determined exponents. It describes how concentration changes affect rate under specified conditions.

Reaction rate: The change in concentration of a reactant or product per unit time. A rate law expresses reaction rate as a function of reactant concentrations.
Molar concentration: The amount of a substance per unit volume of solution, commonly expressed in moles per liter. Rate laws commonly use reactant molar concentrations as their variables.
Arrhenius equation: An equation relating a reaction rate constant to temperature through an activation energy and a pre-exponential factor. It adds temperature dependence to the rate constant in a rate law.
Reaction stoichiometry: The quantitative relationships among reactants and products in a balanced chemical equation. Overall stoichiometric coefficients generally do not determine rate-law exponents.
Reaction order: The exponent of a concentration term in a rate law; the overall order is the sum of these exponents. Its exponents specify how strongly each reactant concentration affects rate.
Chemical kinetics: The study of reaction rates and the molecular processes that produce them. Rate laws are central mathematical descriptions in this broader study.
Pseudo-first-order reaction: A reaction that behaves as first order when one reactant’s concentration remains effectively constant. Excess reactant can simplify a rate law for kinetic measurement.
Molecularity: The number of reacting particles involved in a single elementary reaction step. Unlike reaction order, molecularity applies only to an elementary step and is not experimentally fitted.
Elementary reaction: A single molecular event in a reaction mechanism, with no smaller reaction steps. For an elementary step, its molecularity determines the rate-law exponents.
Initial rates method: A method for determining reaction orders by comparing reaction rates measured shortly after reactions begin at different initial concentrations. It estimates concentration exponents without requiring a full reaction-time curve.

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