The steady state in chemical kinetics: Characterization in terms of the first and second steady state rate laws

Abstract

Chemical reaction rate laws facilitate the design and control of chemical processes. In turn rate laws are often arrived at after assumption  of the steady state approximation, whereby the steady state is characterized by the First State Rate Law, defined for  reactions involving very reactive intermediates, by assuming that the concentrations of the chemical species, involved as very reactive  intermediates can be eliminated by equaling their rate of formation to their rate of disappearance, in which case the concentration of the  intermediate can be assumed to be constant. In this communication theoretical evidence, as well as experimental evidence from the  literature, is presented showing that the steady state can also be characterized by the Second Steady Rate Law based on pseudo-zero  order kinetics. In addition, equations are derived showing that the concentration of the steady state intermediate can be expressed in  terms of the initial concentration of the reactant, and that the steady state approximation is valid within the kinetic limits bounded by truly first (or second) order rate laws and the truly equilibrium rate law. The article has the following highlights: 1. The steady state in  chemical kinetics is defined by the First Steady State Rate Law in terms of the constancy of the concentration of the reaction intermediate,  and by the Second Steady State Rate Law in terms of the constancy of the rate of reaction. 2. Both First and Second Steady  State Rate Laws can be used as bases for assuming the steady state approximation. 3. The limits of the validity of the steady state  approximation are defined.