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Elementary Reaction

What is an Elementary Reaction [1-2]

An elementary reaction is a chemical reaction where reactants directly form products in a single step with a single transition state. The reaction may combine to form complex or non-elementary reactions.

Molecularity and Rate Law of Elementary Reaction [1-4]

The molecularity of an elementary reaction can be defined as the number of reactant particles involved in the reaction mechanism. It is a determining factor for the classification of elementary reactions.

Rate law gives a relationship between the rate of reaction and the concentration of the reactants. It is derived from the molecularity of an elementary reaction.

The order of these reactions can be determined by the number of molecules participating in a collision. It is equal to the stoichiometric coefficients for each reactant. The overall reaction order, i.e., the sum of stoichiometric coefficients of reactants, is always equal to the molecularity of the elementary reaction.

1. Unimolecular Reaction

It is an elementary reaction that involves just one reactant species. Here a molecule gets decomposed or rearranged.

General Reaction: A → products

Rate Law: Rate = k[A]

Where [A] is the concentration of A in moles per liter.

It is a first-order reaction.

Example: The gas-phase decomposition of cyclobutane (C4H8) to ethylene (C2H4) occurs via a unimolecular, single-step mechanism.

C4H8 → 2 C2H4

∴ Rate = k [C4H8]

2. Bimolecular Reaction

A bimolecular reaction is an elementary reaction involving two reactant species.

General Reaction:

A + A → Products          Rate = k[A]2

A + B → Products          Rate = k[A][B]

These are second-order reactions.

Example: The reaction between nitrogen dioxide (NO2) and carbon monoxide (CO) gives nitric oxide (NO) and carbon dioxide (CO2).

NO2(g) + CO(g) → NO(g) + CO2(g)

∴ Rate = k [NO2][CO]

3. Termolecular Reaction

An elementary termolecular reaction involves the simultaneous collision of three atoms, molecules, or ions. Some termolecular reactions exist but are rare because it is difficult for three atoms or molecules to collide at once with proper orientation.

General Reaction:

2A + B → products          Rate = k[A]2[B]

A + B + C → products          Rate = k[A][B][C]

These are third-order reactions.

Example: The reaction of nitric oxide (NO) with chlorine (Cl) gives nitrosyl chloride (NOCl).

2NO + Cl2 -> 2NOCl

∴ Rate = k [NO]2[Cl2]

Elementary and Non-elementary Reaction [1]

In elementary reaction, reactants directly form the products. In contrast, in a non-elementary or complex reaction, reaction intermediates go on to form the final products.

FeaturesElementary ReactionNon-elementary or Complex Reaction
DefinitionA type of chemical reaction involving a single stepA type of chemical reaction involving multiple steps
No. of transition statesOneMore than one
IntermediatesCannot be detectedCan be detected


  1. Elementary Reaction Definition –
  2. Reaction Mechanisms –
  3. Reaction Mechanisms –
  4. Elementary Reactions –

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