1
1. Chemical kinetics
Kinetics is the study of the rates of chemical processes in an effort to
understand what it is that influences these rates and to develop theories
which can be used to predict them.
A knowledge of reaction rates has many
practical applications, for example in designing an industrial process, in
understanding the complex dynamics of the atmosphere and in
understanding the intricate interplay of the chemical reactions that are the
basis of life.
At a more fundamental level we want to understand what happens to the
molecules in a chemical reaction – that is what happens in a single reactive
encounter between two reagent molecules.
By understanding this we may
be able to develop theories that can be used to predict the outcome and rate
of reactions.
1.1 Books
Any general physical chemistry text (such as P W Atkins
Physical
Chemistry
, OUP, any edition) will have several chapters on chemical
kinetics.
There is a nice small book about kinetics in the Oxford Chemistry
Primers series: B G Cox
Modern Liquid Phase Kinetics
, OUP, 1994.
For
more detail (beyond the scope of this course),
Reaction Kinetics
by
M J Pilling and P W Seakins (OUP 1995) is a good source.
2. Rates, rate laws and rate constants
In this Section we will introduce the language and terms used to describe
the rates of chemical reactions.
At this stage we will not be concerned with
the theory of reactions or mechanisms, but just stand back and describe the
overall rates.
2.1
Rate of reaction
The
rate
is defined as
t
t
c
∆
∆
∆
in time
,
ion,
concentrat
in
change
We can talk about the rate of formation or loss of any species – reactant,
intermediate or product.
It is, however, important to specify which species
we are talking about.
The rate can be positive or negative: a positive rate
means that the concentration is increasing with time
e.g.
a product; a
negative rate means that the concentration is falling with time
e.g.
a
reactant.
The rate may vary with time (and concentration), so it is usual to define
the rate over a very small time,
∆
t.
We think of the rate as the
derivative
of
concentration
with respect to time
[]
t
d
ion
concentrat
d
rate
=
This derivative is the slope of a graph of concentration against time, taken
at a particular time.
concentration,
c
time,
t
∆
t
∆
c
concentration
time
The rate is the instantaneous
slope, and this varies with time.
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From its definition it is clear that the units of a rate are concentration per
unit time, for example mol dm
–3
s
–1
.
There are other measures of
concentration, for example in the gas phase pressure is proportional to
concentration, so a rate can be expressed in torr min
–1
(1 torr
= 1 mm Hg, a
measure of pressure).
It is also common to express concentration not in
moles
per unit volume but in
molecules
per unit volume, so the rate would
be expressed in molecules dm
–3
s
–1
or molecules cm
–3
s
–1
.
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 Winter '07
 EStyne
 Kinetics, Chemical reaction, Rate equation, ea

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