Molecularity of a Reaction

While order is found purely from experimental rate data, molecularity is a theoretical

concept that comes from the reaction mechanism -- the sequence of individual molecular-level steps,

called elementary reactions, believed to actually take place as reactants are converted to

products. The molecularity of an elementary step is simply the number of reacting species (atoms,

ions, or molecules) that must collide simultaneously in that one step for it to occur: a step

involving one species reacting alone (e.g. a spontaneous decomposition) is unimolecular; one

requiring two species to collide is bimolecular; one requiring three species to collide at once is

termolecular. Because a simultaneous collision of four or more particles at exactly the same

instant is statistically extremely improbable, elementary steps with a molecularity greater than three

are essentially never observed.

Molecularity, unlike order, must always be a whole positive integer -- it can never be zero, negative,

or fractional, because it is simply a count of colliding particles in one specific, physically real

step; there is no such thing as "half a molecule colliding."

Elementary versus complex reactions. Many reactions occurring in a single elementary step (e.g. the

gas-phase reaction $2\text{HI} \rightarrow \text{H}_2 + \text{I}_2$, believed to proceed via a single

bimolecular collision) have their rate law follow directly from that step's molecularity, so that order

and molecularity coincide for such a reaction. However, most reactions -- especially those with more

complicated balanced equations -- are complex (multi-step) reactions, proceeding through a sequence

of two or more elementary steps whose sum gives the overall balanced equation. For a multi-step

reaction, the overall rate is governed by whichever step is the slowest, called the

rate-determining step, and the experimentally observed overall order need bear no simple

relationship to the coefficients of the overall balanced equation at all -- it reflects only the

molecularity of that one slow, rate-determining elementary step (and, often, the concentrations of

species from faster steps that precede it).

Because molecularity is defined only for a single elementary step, it is meaningless to speak of the "molecularity" of a complex, multi-step overall reaction -- one can only assign a molecularity to

each individual elementary step of its mechanism. Order, by contrast, is always a well-defined, …