© Ultrafast Systems LLC /Vernier Software & Technology
15
5
The Basic Principles of
Chemical Kinetics
Most chemical reactions that occur, either in nature or in the laboratory, are complex. That is,
they occur via a series of simple, or elementary, processes, which can proceed at different rates. An
elementary process is a single-step reaction in which there are no chemically-identifiable intermediates.
For a given complex reaction, its rate will be determined by the rate of the slowest of the participating
elementary reactions. The major advantage of using photons to initiate reactions is that in this way it is
relatively straightforward to initiate elementary processes and it is these that are best used to understand
the fundamental principles of chemical kinetics.
Now some necessary definitions:
5.1 Stoichiometry
Every chemical change can be represented by a stoichiometric, or balanced, equation in symbolic
form. For example, the acid-base reaction between sulfuric acid and a metal hydroxide is represented by
2
4
2
4
2
2
2
H SO
NaOH
Na SO
H O
This equation tells us nothing about the mechanism of the process; it simply expresses the
overall chemical equivalency.
5.2 Reaction
Rate
The rate of a chemical reaction is expressed as the variation with time of the concentration of
either reactants or products. In solution phase reactions, as studied here, the units of reaction rate are
concentration units per second, i.e., mole per liter per second, Ms
-1
.
In the (elementary) reaction
A B
C
Equation
1
The rate (change in concentration as a function of time) is expressed in the form of a derivative
Rate =
[ ]
[ ]
[ ]
d A
d B
d C
dt
dt
dt
Ms
-1
Equation 2
