Reaction order and molecularity are both numbers used in chemical kinetics, but they describe different ideas. Reaction order comes from the experimentally determined rate law. Molecularity describes the number of reacting species involved in a single elementary step.
Confusing them is common because, for an elementary reaction, the rate-law exponents may correspond to stoichiometric participation. That relationship does not generally hold for an overall multi-step reaction.
At a glance
| Point | Reaction order | Molecularity |
|---|---|---|
| How determined | Experimentally from kinetics | From the elementary-step mechanism |
| Applies to | Overall or elementary reactions | Only an elementary step |
| Possible values | Can be 0, fractional, integer and sometimes effective/non-simple values | Positive whole-number count, normally 1, 2 or rarely 3 |
| Relation to stoichiometry | Not generally equal to overall stoichiometric coefficients | Directly tied to the participants in that elementary step |
| Can be zero? | Yes | No |
| Example | rate = k[A]^2[B] gives overall order 3 | A + B → products as one elementary collision is bimolecular |
Reaction order
The sum of the exponents of reactant concentrations in an experimentally determined rate law.
Molecularity
The number of reacting molecules, atoms or ions participating in a single elementary reaction event.
Why order is experimental
A balanced chemical equation tells you how much material is consumed and produced, but it does not necessarily reveal the rate-determining microscopic steps. Therefore you cannot normally read the reaction order from the coefficients of an overall equation.
Chemists determine the rate law by measuring how the rate changes when reactant concentrations change.
Why high molecularity is rare
An elementary event requiring three particles to collide simultaneously with suitable energy and orientation is much less probable than a one- or two-particle event. That is why unimolecular and bimolecular elementary steps dominate most mechanisms.
An apparent overall reaction involving many molecules is usually built from a sequence of simpler elementary steps.
When order and molecularity coincide
For a genuine elementary step, the kinetic rate law often follows directly from that step’s reactant stoichiometry. In that special case, the numerical order and molecularity can coincide.
For an overall reaction, however, order must be measured or derived from a validated mechanism.
Frequently asked questions
Can reaction order be fractional?
Yes. Fractional orders can arise from multi-step mechanisms, equilibria, adsorption processes or effective kinetic models.
Can molecularity be fractional?
No. It counts discrete reacting species in one elementary event.
Can an overall reaction have molecularity?
Strictly, molecularity is defined for elementary steps, not a multi-step overall equation.
Is a second-order reaction always bimolecular?
No. A second-order rate law does not prove that the overall reaction occurs as one bimolecular elementary collision.
Sources and further reading
KnowDifferences Editorial Team
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