Page 61 - Basic physical chemistry for the atmospheric sciences

Chemical kinetics 47

tary processes, which together form what is called the reaction mecha­
nism. For example, the reaction
NO z(g) + CO(g)- NO(g) + CO z(g)
occurs through the following two elementary processes, each of which
is bimolecular

N0 (g) + N02(g)- N03(g) + NO(g)
2
N0 (g) + CO(g - ) N O z(g) + COz(g)
3
In a multistep chemical reaction, the elementary processes must add
u
to give the overall chemical reaction. Th s , adding the above two
elementary processes ,

N02(g) + N0 2 (g) + N03(g) + CO(g)­
N0 3 (g) + NO(g) + NO (g) + CO z(g)
z
and, by canceling species that appear on both sides of the reaction,
we obtain

N02(g) + CO(g)- NO(g) + COz(g)
Although N03(g) appears in the elementary processes, it is neither a
reactant nor a product in the overall chemical reaction ; it is formed in
one elementary process and consumed in another. Such a reagent is
called an intermediate.
Although the order of a chemical reaction cannot be predicted from
the overall reaction, the order of an elementary process is predictable.
For example, for the general unimolecular process

A- products
the rate of decomposition of A at any time will be proportional to
[A] , therefore,
d[A]
- = [A]
k
dt
For the general bimolecular reaction
A + B- products

the rate of collision between the A molecules and the B molecules will
be proportional to [A] and [B] , therefore,