termolekular

Termolekular, or termolecular, refers to a chemical reaction whose rate depends on the concentrations of three reactant species. In many discussions, a termolecular step is written as A + B + C -> products, but in gas-phase chemistry a more common description is A + B + M -> AB + M, where M is a third body that carries away excess energy and stabilizes an intermediate.

Purely elementary termolecular steps are relatively rare because three molecules must collide simultaneously. More often, the overall process proceeds via a two-stage mechanism: A and B first form an energized adduct AB*, which can dissociate back to A and B or collide with a third molecule M to form AB and M. In such cases the observed rate law is effectively proportional to [A][B][M], though variations can occur and the dependence on pressure can produce non-ideal, falloff-type behavior.

Rate expressions for termolecular steps typically take the form rate = k [A][B][C], with [M] used in gas-phase kinetics. At fixed pressure, [M] is proportional to the total number density, so the rate often exhibits pseudo-first-order or pseudo-second-order behavior depending on which reactants are in excess. In practice, many reactions categorized as termolecular are described using Lindemann-type mechanisms or other multi-step models rather than assuming a single elementary three-body collision.

An important historical and practical example is the formation of ozone in the atmosphere: O + O2 + M -> O3 + M. Termolecular processes also play significant roles in combustion and plasma chemistry, where third-body stabilization influences reaction pathways and energy transfer. The termolekular concept thus covers both truly three-body elementary steps and more complex, multi-step mechanisms that effectively depend on three molecular participants.