First-order kinetics

The rate law or rate equation for a chemical reaction is an equation that links the reaction rate with the concentrations or pressures of the reactants and constant parameters (normally rate coefficients and partial reaction orders). For many reactions the rate is given by a power law such as

where [A] and [B] express the concentration of the species A and B, respectively (usually in moles per liter (molarity, M)). The exponents x and y are the partial reaction orders and must be determined experimentally; they are often not equal to the stoichiometric coefficients. The constant k is the reaction rate constant or rate coefficient of the reaction. The value of this coefficient k may depend on conditions such as temperature, ionic strength, surface area of an adsorbent, or light irradiation.

For elementary reactions, which consist of a single step, the order equals the molecularity as predicted by collision theory. For example, a bimolecular elementary reaction A + B → products will be second order overall and first order in each reactant, with rate equation ${\displaystyle r\;=\;k[\mathrm {A} ][\mathrm {B} ]}$. For multistep reactions, the order of each step equals the molecularity, but this is not generally true for the overall rate.

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