termolecular

Termolecular refers to chemical processes that involve three reacting species, typically three molecules colliding or two molecules colliding with a third body. In kinetics, a termolecular process is one that follows an overall third-order rate law, such as rate = k[A][B][C], if all three reactants participate in the same elementary step. In practice, true three-body elementary steps are exceedingly rare, and many processes described as termolecular arise from two-step mechanisms.

Most commonly, a termolecular behavior arises when a fast bimolecular collision forms an energized intermediate that then collides with a third molecule to stabilize the product or to complete the reaction. In such cases the observed rate law appears third-order, even though the underlying mechanism comprises more than one step. The third molecule, often denoted M, acts as a third body that absorbs excess energy and facilitates stabilization rather than being consumed.

In gas-phase kinetics, M is a collider that enables energy transfer. A typical example is A + B + M → AB + M, where the rate depends on [A], [B], and [M]. Because [M] is proportional to pressure, termolecular reactions exhibit strong pressure dependence and become more significant at higher pressures. Notable atmospheric examples include O + O2 + M → O3 + M and H + O2 + M → HO2 + M.

Termolecular processes are central to combustion and atmospheric chemistry, but true elementary third-order steps are uncommon. Consequently, the term often reflects complex mechanisms that collapse to an effective third-order rate law under certain conditions.