An Introduction to Environmental Chemistry

80 Chapter Four

Box 4.4 Metastability, reaction kinetics, activation energy and catalysts

Some reduced compounds appear to be

stable at Earth surface temperatures despite

the presence of atmospheric oxygen.

Graphite, for example, is a reduced form of

carbon which we might expect to react with

oxygen, i.e.:

eqn. 1

Although the reaction of oxygen with

graphite is energetically favoured, graphite

exists because the reaction is kinetically very

slow. Many natural materials are out of

equilibrium with their ambient environment

and are reacting imperceptibly slowly. These

materials are metastable. Metastability can

be demonstrated using a graph of energy in

a chemical system in which substances A and

B react to give C and D (Fig. 1). In order for

reaction to take place, A and B must come

into close association and this usually requires

an input of energy (activation energy). Under

cold (low-energy) conditions a small number

of A and B will occasionally have the energy

to overcome the activation energy, but this

COCOgraphite+Æ 22 ()g ()g

will be rare and the reaction will proceed

slowly. If the energy of the reactants is

increased (e.g. by heating), then the reaction

will be able to proceed more quickly because

more A and B will have the required

activation energy.

An alternative way to increase the rate of

a reaction is to lower the activation energy.

This is done by a catalyst—a substance that

alters the rate of a chemical reaction without

itself undergoing any overall chemical

change. In our hypothetical reaction, the

catalyst acts as an intermediate compound

that allows A and B to come together more

readily. In the environment biological enzyme

systems—especially those of microorganisms—

catalyse reactions that would not otherwise

proceed spontaneously because of kinetic

inhibition. Water, strictly speaking, is not

a chemical catalyst; however its solvent

properties provide a kind of catalytic role as

an intermediary, allowing closer interaction

of ions and substances than is possible under

dry conditions.

Increasing

Reaction: A + B C + D

System

energy

Decreasing

A + B

Reaction

proceeding

Activation

energy

C + D

AB

Fig. 1Schematic representation of energy in a chemical system.