Page 194 - Instant notes

Physical chemistry 180

intrinsically first order in the reactant since the rate of reaction at any time is proportional
only to the concentration of A remaining:
rate=k[A]

The rate law of an elementary bimolecular reaction (i.e. A+A→P, or A+B→ P) is
intrinsically second order since the rate of reaction at any time is proportional to the rate
of collision between the two molecules which in turn is directly proportional to the
concentration of both molecules remaining:
2
rate=k[A]

or
rate=k[A][B]

Complex reaction

The term complex reaction is used for a reaction which consists of more than one
constituent elementary reaction step. A complex reaction proceeds through the
formation and removal of intermediate species not contained in the balanced chemical
equation written for the reaction.
The overall rate law of a complex reaction is derived by combining the rate laws of the
constituent elementary reactions and must be expressed only in terms of concentrations of
reactants or products appearing in the overall balanced chemical equation for the
reaction. It must also agree with the observed rate law under all sets of reaction
conditions. The reverse situation, that of deducing complex behavior from an observed
rate law is often not straightforward. For example, if an observed reaction is presumed to
be genuinely bimolecular then its rate law is, by definition, second order. However, if the
observed rate law is second order, then the reaction may be bimolecular, or it may be
complex, and the latter might only be deduced after detailed investigation across a wide
range of reaction conditions. This is the case for the reaction between H 2 and I 2:
H 2(g)+I 2(g)→2HI(g)

which shows second order kinetics:
rate=k[H 2][I 2]
and was presumed to be a bimolecular reaction between the two reactant diatomics. In
fact there is an underlying chain reaction mechanism involving radical species, as for
the reaction between Br 2 and H 2 (see Topic F6).
Complex reactions are abundant in chemistry. Examples include unimolecular
dissociation (or rearrangement) reactions, enzyme or surface catalysis, and chain and
explosion reactions (see Topics F5 and F6).