4 - Simplified Kinetics of Sequences at the Steady State

Abstract

This chapter discusses simplified kinetics of sequences at the steady state. Even with a small number of steps, the rate expression of a sequence is rather complicated despite the steady-state approximation. A simplification is in order whenever possible. Frequently, for a restricted range of experimental conditions, it can be assumed with considerable success that a given step in the sequence is rate-determining. If this occurs, all the other steps in the sequence will be in quasi-equilibrium and the kinetic problem is reduced to a consideration of the kinetics of a single step and the thermodynamic equilibrium of all the other steps. The chapter discusses shift in rate-determining step and rate-determining active centers. While the steady-state approximation is rarely in error and constitutes an almost universal tool for the kinetic treatment of sequences involving active centers, the assumption of a rate-determining step must be made with great caution. If such a step exists at all, there is no guarantee that it will not shift as process conditions are changed. The variables affecting rates are temperature, pressure, and composition and hence, a shift in rate-determining step may occur as these variables are changed. According to the concept of a rate-determining step, if there is one, all other steps in a sequence are in quasi-equilibrium. Thus, these other steps must be reversible.