An Introduction to Environmental Chemistry

9.75 years. This represents the average lifetime of a CH 4 molecule in the atmos-

phere (at least, it would if the atmosphere was very well mixed).

Residence time is the fundamental quantity that describes systems in steady

state. It is a very powerful concept that plays a central role in much of environ-

mental chemistry. Compounds with long residence times can accumulate to rel-

atively high concentrations compared with those with shorter ones. However,

even though gases with short residence times are removed quickly, their high

reactivity can yield reaction products that cause problems.

The famous atmospheric chemist C.E. Junge made an important observation

about residence times and the variability of gases in the atmosphere. If a gas has

The Atmosphere 37

Box 3.2 Chemical equilibrium

Many chemical reactions occur in both
directions such that the products are able to
re-form the reactants. For instance, in rainfall
chemistry, we account for the hydrolysis
(i.e. reaction with water) of aqueous
formaldehyde (HCHO) to methylene glycol
(H 2 C(OH) 2 ) according to the equation:

eqn. 1

but the reverse reaction also occurs:

eqn. 2

such that the system is maintained in
dynamic equilibrium, symbolized by:

eqn. 3

The relationship between the species at
equilibrium is described in terms of the
equation:

eqn. 4

where adenotes the activities of the entities
involved in the reaction. Remember from
Section 2.6 that activities are the formal
thermodynamic representations of
concentration. However, in dilute solutions
activity and concentration are almost
identical. Dilute solutions, such as rainwater,
are almost pure water. The activity of pure
substances is defined as unity, so in the
case of rainwater the equation can be
simplified:

K

a

=aa

( )

◊

HCOH

HCHO H O

(^22)
2
HCHO()aq+H O 22 ()lªH C OH( ) 2 ()aq
H C OH 2 ( ) 2 ()aqÆ+HCHO()aq H O 2 ()l
HCHO()aq+ÆH O 22 ()l H C OH( ) 2 ()aq
eqn. 5
Kis known as the equilibrium constant and
in this case it has the value 2000. An
equilibrium constant greater than unity
suggests that equilibrium lies to the right-
hand side and the forward reaction is
favoured. Equilibrium constants vary with
temperature, but not with concentration if
the concentrations have been correctly
expressed in terms of activities.
The equilibrium relationship is often
called the law of mass action and may be
remembered by the fact that an equilibrium
constant is the numerical product of the
activity of the products of a reaction divided
by the numerical product of the reactants,
such that in general terms:
eqn. 6
eqn.7
It may be easier to grasp the notion of shifts
in equilibrium in a qualitative way using the
Le Chatelier Principle. This states that, if a
system at equilibrium is perturbed, the
system will react in such a way as to minimize
this imposed change. Thus, looking at the
formaldehyde equilibrium (eqn. 3), any
increase in HCHO in solution would be
lessened by the tendency of the reaction to
shift to the right, producing more methylene
glycol.
K aaaa
mn
= kl
◊
◊
CD
AB
klmnAB C D++ª
K=aHCOHaHCHO^2 ( )^2