Besides their usefulness as solvents, acids and bases are important
as catalysts. Because of their small size, mobility, and charge,
H+ and OH- ions from acids and bases can attack
compounds in such a way as to make reactions occur more easily and
faster. This is the key to their catalytic effectiveness.
If a substance provides a faster pathway for reaction but is regenerated
again at the end of the process, it is a true catalyst. If catalysts
are ions or molecules dissolved in the same solution as the reactants
and products, they are known as homogeneous catalysts. This is the
type of catalysis discussed in the postscript to this chapter.
In Chapter 15 we saw examples of heterogeneous catalysts, in which
the catalyst is a separate phase - a surface to which the gaseous
or dissolved reactants diffuse and from which the products separate.
In either type of catalysis the principle is the same: A catalyst
is a substance that accelerates a thermodynamically spontaneous
reaction by providing an alternate mechanism, without itself being
consumed by the overall reaction. It can participate in several
steps of the process, as long as it is regenerated at the end.
Acids and bases are widely used homogeneous catalysts.