Page 175 - Instant notes

Empirical approaches to kinetics 161

which come together to form a collision complex H-Br-Br that then breaks apart to form
the product species HBr and Br. This is an example of a bimolecular reaction since two
species are involved, H and Br 2.
A unimolecular reaction occurs when a single molecule acquires the necessary energy
to break apart or rearrange its constituent atoms, for example, the thermal dissociation of
azomethane:
CH 3N 2CH 3(g)→2CH 3(g)+N 2(g)
Except in the case of a true elementary reaction the molecularity is independent of the
order. The order of a reaction is based entirely on experimental deduction of a rate law. In
the example of the thermal decomposition of azomethane there are a number of hidden
elementary reactions which determine the proportion of azomethane molecules which
acquire sufficient energy to undergo unimolecular dissociation and the overall order of
the reaction is not well defined (see Topic F5).