Chemistry Reference
In-Depth Information
5.3 Reaction kinetics and mechanism
reagents. Such a situation might occur when the sol-
vent was also one of the reagents.
One of the ways in which we can obtain informa-
tion about a mechanistic sequence is to study the
rate of reaction. The dependence of the reaction rate
on the concentration of reagents and other variables
indicates the number and nature of the molecules
involved in the
rate-determining step
of the reac-
tion. The rate-determining step is defined as the slow-
est transformation in the sequence, all other transfor-
mations proceeding much faster than this. Consider
a turnstile at a football match. This limits the rate at
which spectators enter the ground. How rapidly peo-
ple walk towards the turnstile or away from it once
they are in the ground cannot influence the rate at
which they get through the turnstile.
The
rate of reaction
is given by the equation
Rate =
k
[A][B] =
k
′
[A]
A + B
C + D
B is also the solvent and
thus present in large excess
[B]
appears constant,
with no significant change
during reaction
Despite occasional apparent anomalies such as this,
the rate expression gives us valuable information
about the likely reaction mechanism. If the reaction
is unimolecular, the rate-determining step involves
just one species, whereas the rate-determining step
involves two species if it is bimolecular. As indicated
in Table 5.1, we can then deduce the probable reac-
tion, and our proposed mechanism must reflect this
information. The kinetic rate expressions will be con-
sidered further as we meet specific types of reaction.
concentrations of A, B, ...
Rate =
k
[A][B]...
5.4 Intermediates and transition
states
rate constant
in which
k
is the rate constant, and A, B, etc. are the
variables on which the rate depends. Square brackets
are used to indicate concentrations. It is rare for more
than two variables to be involved, and often it is only
one. The most common types of rate expression are
given in Table 5.1.
In first-order reactions, the rate expression depends
upon the concentration of only one species, whereas
second-order reactions show dependence upon two
species, which may be the same or different.
The
molecularity
, or number of reactant molecules
involved in the rate-determining step, is usually
equivalent to the kinetic reaction order, though there
can be exceptions. For instance, a bimolecular reac-
tion can appear to be first order if there is no appar-
ent dependence on the concentration of one of the
Any realistic mechanism will include a number of
postulated structures, perhaps charged structures or
radicals, which lie on the pathway leading from reac-
tants to products. Some of these intervening structures
are termed intermediates, and others transition states.
These are differentiated by their stability, and whether
they can be detected by appropriate analytical meth-
ods. A diagram that follows the energy change during
the reaction can illustrate their involvement. The
x
-
coordinate is usually termed 'reaction coordinate',
and in many cases equates to time, though the possi-
bility that the reaction is reversible prevents us from
showing this as a simple time coordinate.
Consider the energy profile in Figure 5.1, in which
reactants are converted into products. The difference
between the energy of the reactants and products is
called the
standard free energy change
G
◦
for the
reaction. As shown, the change in energy is negative,
so that the reaction liberates energy and is potentially
favourable. It does not occur spontaneously, however,
since the reactants need to acquire sufficient energy
to collide and react. This energy is termed the
acti-
vation energy
- even gunpowder needs a match to
set off the explosion! The high-energy peak in the
curve is termed the
transition state
or sometimes,
Table 5.1
Rate
expressions,
reaction
order,
and
molecularity
Rate
expression
Reaction
order
Probable
reaction
Molecularity
k
[A]
first
A
→
unimolecular
k
[A][B]
second
A
+
B
→
bimolecular
k
[A]
2
second
A
+
A
→
bimolecular
