In chemistry, molecularity is the number of molecules that come together to react in an elementary (single-step) reaction and is equal to the sum of stoichiometric coefficients of reactants in the elementary reaction with effective collision (sufficient energy) and correct orientation.
Depending on how many molecules come together, a reaction can be unimolecular, bimolecular or even trimolecular.
The kinetic order of any elementary reaction or reaction step is equal to its molecularity, and the rate equation of an elementary reaction can therefore be determined by inspection, from the molecularity.
<chem display="block">CH3Br + OH^- -> CH3OH + Br^-</chem>
Termolecular reactions
A termolecular (or trimolecular) reaction in solutions or gas mixtures involves three reactants simultaneously colliding, with appropriate orientation and sufficient energy. However the term trimolecular is also used to refer to three body association reactions of the type:
<chem display="block">A + B ->[\ce{M}] C</chem>
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:
<math chem display="block">\ce{A + B -> AB}^*</math>
<math chem display="block">\ce{AB}^*\ce{ + M -> C + M}</math>
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.
Reactions of higher molecularity are not observed due to very small probability of simultaneous interaction between 4 or more molecules. Molecularity, on the other hand, is deduced from the mechanism of an elementary reaction, and is used only in context of an elementary reaction. It is the number of molecules taking part in this reaction.
This difference can be illustrated on the reaction between nitric oxide and hydrogen:
<chem display="block">2NO + 2H2 -> N2 + 2H2O,</chem>
where the observed rate law is <math chem>v = k\ce{[NO]^2[H2]}</math>, so that the reaction is third order. Since the order does not equal the sum of reactant stoichiometric coefficients, the reaction must involve more than one step. The proposed two-step mechanism