Box 44 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.:

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 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 t

System energy

Decreasing

Increasing

Decreasing

Reaction: A + B ^ C + D

AB

Activation

/ \

energy

A + B

C + D

Reaction proceeding

Reaction proceeding

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

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