Detailed Summary
Molecularity of a reaction is defined as the total number of reacting species (atoms, ions, or molecules) that collide in a single elementary reaction to produce products. This concept is classified into three types:
- Unimolecular Reactions: Involve a single reacting species. An example includes the decomposition of ammonium nitrite:
$$ \text{NH}_4\text{NO}_2 \rightarrow \text{N}_2 + 2\text{H}_2\text{O} $$
- Bimolecular Reactions: Involve two reacting species that collide simultaneously. A classic example is the dissociation of hydrogen iodide:
$$ 2\text{HI} \rightarrow \text{H} + \text{I} $$
- Termolecular Reactions: Rarely occur as they require three species to collide simultaneously; an example is the reaction of nitrogen monoxide and oxygen:
$$ 2\text{NO} + \text{O} \rightarrow 2\text{NO}_2 $$
Molecularity is an important factor in determining the mechanism of complex reactions, as it helps in identifying whether a reaction proceeds in one step or multiple steps. Importantly, molecularity is an integer (0, 1, 2, ...) and can never be zero or a non-integer. It applies strictly to elementary reactions, while the order of a reaction — which can be zero, fractional, or whole — is determined experimentally and may involve multiple steps of a reaction. Understanding these distinctions allows chemists to predict reaction behavior based on molecular interactions.