Page 60 - Basic physical chemistry for the atmospheric sciences

46 Basic physical chemistry

which is a (termolecular) third-order reaction 1 that is of importance in
the atmosphere. The rate law for this reaction is

In the atmosphere [M) and [S0 (g)] are generally essentially constant;
2
therefore, their concentrations can be combined with the value of �"
to give

d[OH(g)]
- kpseudo[OH( g) )
dt
where kpseudo i s a pseudo first-order rate coefficient.

3.2 Reaction mechanisms
How are reactants converted into products in a chemical reaction?
The simplest process is single step and unimolecular, in which one
molecule gains sufficient energy to break a chemical bond. For ex­
ample,

o;(g)� 02(g) + O(g)
where the superscript asterisk indicates that the ozone molecule i s
sufficiently energized ( o r excited) t o dissociate. Single-step ( o r ele­
mentary) processes can also involve two reactant molecules (a bimo­
lecular process). For example,

Or, three reactant molecules (a termolecular process) can be involved

In this example, as in most termolecular processes, two molecules
combine, and the third molecule removes the excess energy produced
by the combination. (If this excess energy is not removed, the 0 3
.
molecule is likely to dissociate again into 0 and 02 ) Elementary
processes involving more than three molecules are not known, since
the chance that more than three molecules will collide simultaneously
is so small that such a process would produce a negligibly small
reaction rate.
Chemical reactions generally involve several single steps or elemen-