Molecularity in chemistry is the number of colliding molecular entities that are involved in a single reaction step. While the order of a reaction is derived experimentally, the molecularity is a theoretical concept and can only be applied to elementary reactions. In elementary reactions, the reaction order, the molecularity and the stoichiometric coefficient are the same, although only numerically, because they are different concepts.
- A reaction involving one molecular entity is called unimolecular.
- A reaction involving two molecular entities is called bimolecular.
- A reaction involving three molecular entities is called termolecular or trimolecular. Termolecular reactions in solutions or gas mixtures are very rare, because of the improbability of three molecular entities simultaneously colliding. However the term termolecular is also used to refer to three body association reactions of the type:
Where the M over the arrow denotes that to conserve energy and momentum a second reaction with a third body is required. After the initial bimolecular collision of A and B an energetically excited reaction intermediate is formed, then, it collides with a M body, in a second bimolecular reaction, transferring the excess energy to it.
The reaction can be explained as two consecutive reactions:
These reactions frequently have a pressure and temperature dependence region of transition between second and third order kinetics.
Catalytic reactions are often three-component, but in practice a complex of the starting materials is first formed and the rate-determining step is the reaction of this complex into products, not an adventitious collision between the two species and the catalyst. For example, in hydrogenation with a metal catalyst, molecular dihydrogen first dissociates onto the metal surface into hydrogen atoms bound to the surface, and it is these monatomic hydrogens that react with the starting material, also previously adsorbed onto the surface.
Notes and references
- IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (1996) "molecularity".
- Discussion on the improbability of termolecular reactions
- Text discussing rate constants for termolecular reactions 
- IUPAC definition of Troe expression, a semiempirical expression for the rate constant of termolecular reactions 
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