Two-Step Reactions with an Element of Reversibility

Suppose that an irreversible reaction between A and B leading to 95% product or products, designated D, is examined in the usual way. One of the reactants, B, is held in excess and the loss of A monitored. It is likely that the loss will be a first-order process (rate constant k). At low concentrations of B (but still [A]), the value of k may be proportional to the concentration of B. At higher concentrations of B however this direct proportionality may disappear and eventually k will become independent of [B]. Obviously, a second-order reaction at low reactant concentrations has lost its simplicity at higher reactant concentrations and eventually turned over to first-order in A alone. Such a situation is accommodated by a rate law of the form 

This behavior is sometimes referred to as saturation kinetics. When ft [B] 1, the observed second-order is easily understood (rate constant = a). When 6 [B] 1 there is a mixed-order 

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