The **rate law** or **rate equation** for a chemical reaction is an equation that links the reaction rate with concentrations or pressures of reactants and constant parameters (normally rate coefficients and partial reaction orders). To determine the rate equation for a particular system one combines the reaction rate with a mass balance for the system. For a generic reaction *a*A + *b*B → C with no intermediate steps in its reaction mechanism (that is, an elementary reaction), the rate is given by

where and express the concentration of the species A and B, respectively (usually in moles per liter (molarity, M)); *x* and *y* are the respective stoichiometric coefficients of the balanced equation; they must be determined experimentally. *k* is the *rate coefficient* or *rate constant* of the reaction. The value of this coefficient *k* depends on conditions such as temperature, ionic strength, surface area of the adsorbent or light irradiation. For elementary reactions, the rate equation can be derived from first principles using collision theory. Again, x and y are not always derived from the balanced equation.

The rate equation of a reaction with a multi-step mechanism cannot, in general, be deduced from the stoichiometric coefficients of the overall reaction; it must be determined experimentally. The equation may involve fractional exponential coefficients, or it may depend on the concentration of an intermediate species.

The rate equation is a differential equation, and it can be integrated to obtain an **integrated rate equation** that links concentrations of reactants or products with time.

If the concentration of one of the reactants remains constant (because it is a catalyst or it is in great excess with respect to the other reactants), its concentration can be grouped with the rate constant, obtaining a **pseudo constant**: If B is the reactant whose concentration is constant, then . The second-order rate equation has been reduced to a **pseudo-first-order** rate equation. This makes the treatment to obtain an integrated rate equation much easier.

Read more about Rate Equation: Stoichiometric Reaction Networks, Zero-order Reactions, First-order Reactions, Second-order Reactions, Summary For Reaction Orders 0, 1, 2, and *n*, Equilibrium Reactions or Opposed Reactions, Consecutive Reactions, Parallel or Competitive Reactions, General Dynamics of Unimolecular Conversion

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