Page 193 - Instant notes

F4
FORMULATION OF RATE LAWS

Key Notes
An elementary reaction is a single reaction step. When only a
single molecule is involved (A→P) the elementary reaction is
unimolecular with a first order rate law (rate=k[A]). If two
reactant molecules are involved (A+B→P) the elementary
reaction is bimolecular with a second order rate law
(rate=k[A][B]).
A complex reaction is one which proceeds through more than one
constituent elementary reaction step. Unimolecular reactions,
chain reactions, catalytic and enzyme reactions are all examples
of complex reactions.
This is the assumption that the concentrations of all intermediate
species in a reaction mechanism remain constant during the
reaction. Hence the net change in concentration [I] of any
intermediate with time can be set to zero, d[I]/dt≈0 which means
that the rates of formation and removal for each intermediate
must balance.
The overall rate law of a complex reaction mechanism is
formulated by combining the first and second order rate laws of
the constituent elementary reactions, usually by applying the
steady state assumption or, analogously, by assuming that some
equilibrium is attained. The formulated rate law must be
consistent with the observed rate law.
The rate determining step is the slowest step in a reaction
mechanism. The rate of this reaction determines the maximum
overall rate of formation of products.

Related topics Empirical approaches to kinetics Rate laws in action (F5)
(F1)
Rate law determination (F2) The kinetics of real systems
(F6)

Rate laws of elementary reactions

An elementary reaction is a single, discrete reaction step. The molecularity of the
reaction is the number of reactant molecules involved in this discrete reaction step (see
Topic F1). The rate law of an elementary unimolecular reaction (i.e. A→P) is