An overall balanced chemical equation records the net transformation of reactants into products. It usually does not show the individual molecular events by which that transformation occurs.

A reaction mechanism is a sequence of elementary reactions whose equations add to the overall reaction. An elementary reaction represents one molecular-scale step.

The molecularity of an elementary reaction is the number of reacting entities involved in that step:

• one reacting entity: unimolecular;
• two reacting entities: bimolecular;
• three reacting entities: termolecular.

Termolecular elementary events are uncommon because three suitably arranged particles must encounter one another essentially simultaneously.

For an elementary step, the rate law follows from the reactant molecularity. For example,

$$\mathrm{A\rightarrow products}$$

has an elementary rate law

$$r=k[A],$$

while

$$\mathrm{A+B\rightarrow products}$$

has

$$r=k[A][B].$$

This direct connection is valid for an elementary step, not for an arbitrary overall reaction equation.

Intermediates

A species formed in one elementary step and consumed in a later step is a reaction intermediate. It does not appear in the net balanced equation because it cancels when the mechanism steps are added.

For example,

$$\mathrm{NO_2+NO_2\rightarrow NO_3+NO}$$

followed by

$$\mathrm{NO_3+CO\rightarrow NO_2+CO_2}$$

gives, after cancelling one $\mathrm{NO_2}$ and the intermediate $\mathrm{NO_3}$,

$$\boxed{\mathrm{NO_2+CO\rightarrow NO+CO_2}}.$$

The mechanism therefore contains chemistry that the overall equation cannot reveal.

A catalyst differs from an intermediate: a catalyst is consumed in an early step and regenerated later, while an intermediate is produced during the mechanism and then consumed. Neither appears in the net equation when fully cancelled.

Mechanisms are models constrained by experimental evidence. Their elementary steps must sum to the overall stoichiometry, but stoichiometric agreement alone is not enough; the mechanism must also be consistent with the observed kinetics.