3 - The Steady-State Approximation: Catalysis

Abstract

This chapter discusses the steady-state approximation. One-step reactions between stable molecules are very rare. This is only natural as a stable molecule is, by definition, quite unreactive, and complicated rearrangements of bonds are usually required to go from reactants to products. This means that most directly observed reactions do not proceed in a single elementary step. Instead, a sequence of elementary steps is necessary and reactants or products of these are not the stable reactants or products but are highly reactive intermediates that shall be called active centers. Active centers may be of several different chemical types such as free radicals, free ions, solvated ions, complexes at surfaces, complexes in a homogeneous phase, and complexes with enzymes. Many active centers may be involved in a given reaction. Yet it is found that the advancement of the reaction can still be described by means of a single parameter — the extent of reaction. If this is the case, the reaction is said to be single.