In chemistry, an elementary reaction is a chemical reaction that proceeds in a single step with only one transition state (reactants → products). An elementary reaction cannot be broken down into simpler reactions and generally has no intermediates. In contrast, a complex reaction or nonelementary reaction or composite reaction consists of multiple elementary reactions, with intermediates and multiple transition states (reactant → intermediates → products).
Examples of Elementary Reactions
Elementary reactions are common in chemistry. Examples include:
- Cis-trans isomerization
- Thermal decomposition reactions:
CuCO₃(s) → CuO(s) + CO₂(g)
2HI → H2 + I2
C4H8 → 2 C2H4
- Ring opening reactions
- Many reactions between gases:
NO2(g) + CO(g) → NO(g) + CO2(g)
2NO(g) + Cl2(g) → 2NOCl(g)
- Radioactive decay
- Nucleophilic substitution
Types of Elementary Reactions
One method of classifying elementary reactions is according to their molecularity. Molecularity refers to the number of reactant particles involved in a chemical reaction. Because we’re talking about whole atoms or molecules, molecularity has an integer value: unimolecular (1), bimolecular (2), or termolecular (3). Termolecular reactions are rare. There aren’t any known elementary reactions involving four or more molecules.
Here is a table summarizing the types of elementary reactions, their molecularity, and their rate laws:
|Molecularity||Elementary Step||Rate Law||Example|
|Unimolecular||A → Products||rate = k[A]||N2O4(g) → 2NO2(g)|
|Bimolecular||A + A → Products||rate = k[A]2||2NOCl → 2NO(g) + Cl2(g)|
|Biomolecular||A + B → Products||rate = k[A][B]||CO(g) + NO3(g) → NO2(g) + CO2(g)|
|Termolecular||A + A + A → Products||rate = k[A]3|
|Termolecular||A + A + B → Products||rate = k[A]2[B]||2NO(g) + O2(g) → 2NO2(g)|
|Termolecular||A + B + C → Products||rate = k[A][B][C]||O(g) + O2(g) + M → O3(g) + M|
Order of Reactions
Note that the reaction order differs, depending on the nature of the elementary reaction:
- Unimolecular elementary reactions are first order reactions.
- Bimolecular reactions are second order reactions.
- Termolecular reactions are third order reactions.
Direct vs Indirect Reaction
Sometimes the definition of an elementary reaction states that it has no intermediates (reactive complexes). In practice, this isn’t strictly true. An elementary reaction may have no intermediates or else they exist only very briefly or their existence is not needed in order to describe how the reaction occurs. The nature of the intermediate, if it exists, leads to classifying an elementary reaction as a direct reaction or as an indirect reaction.
A direct reaction has a reactive complex with a lifetime shorter than its period of rotation. An indirect reaction or complex-mode reaction has a reactive complex with a lifetime longer than its period of rotation. But, in either case, the intermediate doesn’t stick around long enough to be observable under ordinary conditions.
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