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Chapter 9

Chemical Kinetics 11: The Arrhenius

Equation and Graphical Problems

MOLECULARITY
In Chapter 8, the order of a reaction was defined as the sum of the
exponents of the concentration terms in the rate equation. This order
is an experimentally determined quantity not to be confused with a
term called rnolecularity. Most reactions consist of a number of steps,
and each individual step is known as an elementary reaction. Each
elementary reaction can be described by the rnolecularity of the
process:
(a) When a single particle is the only reactant, the reaction is
unirnolecular, i.e. molecularity = 1.
(b) When two particles collide, the reaction is birnolecular, i.e. molecu-
larity = 2.
(c) When three particles collide, the reaction is termolecular, i.e.
molecularity = 3, etc. However, it should be borne in mind that a
full reaction may have substeps, and tennolecular collisions are
infrequent.
This is summarised in Table 9.1 below.

Table 9.1 Molecularity.
1. A + Products Unirnolecular Molecularity = 1
2. A + A -+ Products Birnolecular Molecularity = 2
3. A + B -+ Products Birnolecular Molecularity = 2
4. A + B + C + Products Terrnolecular Molecularity = 3