Bimolecular reactions are elementary reactions involving two distinct entities that combine to form an activated complex. For reactions in solution, the solvent contributes to the reaction's molecularity only when it is a re-actant of the system. Bimolecular reactions are usually second order, but it is important to stress that some second order reactions need not be bimolecular.

Bimolecular rate constants typically have units of M"1 s"1. Strict compliance to SI units for bimolecular rate

Binding Change Mechanism constants requires expressing concentrations in the SI system as mol-m-3, but concentrations are more frequently given as mol-L-1 (or, M-1 or mol-dm-3). When gas-phase reactions are studied (and even in the case of gases in solution), "concentrations" are often reported in terms of partial pressures rather than molarity. Nevertheless, it is usually preferable to convert such units to true concentration units when able.

Bimolecular processes are very common in biological systems. The binding of a hormone to a receptor is a bimolecular reaction, as is substrate and inhibitor binding to an enzyme. The term "bimolecular mechanism'' applies to those reactions having a rate-limiting step that is bimolecular. See Chemical Kinetics; Molecularity; Reaction Order; Elementary Reaction; Transition-State Theory

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