An Introduction to Environmental Chemistry

eqn. 2.8

The formation of these ion pairs (see Box 6.4) further reduces the effective
concentration, and the frequency of collisions increases as the total amounts of
chemical species in the solution increase. The effective concentration of an
ion therefore becomes an important consideration in concentrated and complex
solutions like seawater.
In order to predict accurately chemical reactions in a concentrated solution,
we need to account for this reduction in effective concentration. This is done
using a concentration term known as ‘activity’ that is independent of electrosta-
tic interactions. Activity is the formal thermodynamic representation of concen-
tration and it describes the component of concentration that is free to take part
in chemical reactions. Activity is related to concentration by an activity coeffi-
cient (g).

eqn. 2.9

Equation 2.9 shows that units of activity and concentration are proportional; in
other words gcan be regarded as a proportionality constant. These constants,
which vary between 0 and 1, can be calculated experimentally or theoretically and
are quite well known for some natural solutions. Having said this, measuring
gin complex solutions like seawater has proved very difficult. Most importantly
for our purposes, as solution strength approaches zero, gapproaches 1. In other
words, in very dilute solutions (e.g. rainwater), activity and concentration are
effectively the same.
In this book activity is expressed in units of mol l-^1 in the same way as con-
centration, but activity is denoted by the prefix ‘a’ in equations.
We should also note that all thermodynamic terms (e.g. equilibrium constants,
see Box 3.2) are expressed as activity. Thus, measured concentrations of any
chemical species should usually be converted to activities before comparison with
thermodynamic data.

2.7 Organic molecules—structure and chemistry

Organic matter and organic compounds are integral components of all environ-
mental reservoirs; it is therefore important to understand some of the basic facts
about their structure and chemistry. Organic molecules contain carbon, hydro-
gen and often some other non-metallic elements such as oxygen, nitrogen,
sulphur or halogens such as chlorine. Organic molecules are often complex struc-
tures, typically a skeleton of carbon atoms arranged in chains, branched chains
or rings. It is more convenient to draw these complex structures as a simple
picture (Fig. 2.4) rather than write the formula. The atoms in organic molecules
are usually held together by covalent bonds, depicted as lines joining atoms
(Section 2.3.1 & Fig. 2.4).
The simplest organic compounds contain only hydrogen atoms bonded to the
carbon skeleton and are known as hydrocarbons, for example methane, the main

activity=concentration¥g

Na+()aq +SO4aq^2 ()- ªNaSO4aq-()

Environmental Chemist’s Toolbox 23