https://doi.org/10.1351/goldbook.S05962
- In a kinetic analysis of a complex reactioninvolvingunstableintermediates in low concentration, the rate of change of each such intermediate is set equal to zero, so that therate equationcan be expressed as a function of the concentrations ofchemical speciespresent in macroscopic amounts. For example, assume that X is anunstableintermediate in the reactionsequence:Conservation of mass requires that:
which, since is constant, implies: Since is negligibly small, the rate of formation of D is essentially equal to therate of disappearanceof A, and the rate of change of can be set equal to zero. Applying the steady state approximation ( ) allows theeliminationof from the kinetic equations, whereupon therate of reactionis expressed:Note:
The steady-state approximation does not imply that is even approximately constant, only that its absolute rate of change is very much smaller than that of and . Since according to the reaction scheme , the assumption that is constant would lead, for the case in which C is in large excess, to the absurd conclusion that formation of the product D will continue at a constant rate even after the reactant A has been consumed. - In a stirred flow reactora steady state implies a regime so that all concentrations are independent of time.