For example, one cannot light a match to burn glucose in the human body; this reaction must be carried out at approximately 98.6⁰F. It is for such reactions that catalysis becomes useful.

In general, catalysts lower the activation barrier for a reaction (Ea) thereby making the rate constants larger and the reactions faster. This is represented schematically by the drawing opposite.

Lowering E_a means finding an alternative pathway or mechanism for the reaction, in which the intermediate states (activated complexes) at all times are at a lower energy.

Both the forward and the reverse reactions are speeded up by a catalyst, since lowering the forward E_a necessitates lowering the reverse E_a by the same amount. A catalyst has no effect on K_eq or on the ultimate equilibrium conditions for a reaction; it only provides a way in which a spontaneous but slow reaction can arrive at equilibrium faster.

If the reaction is not already thermodynamically spontaneous, a cataIyst will be of no use. Thermodynamics does not tell a chemist how to find a catalyst for a given reaction, but it does tell him when it is, or is not, worth his time to look for one.