1.9 Kinetic Models for Non-Elementary Reaction

It is usually assumed that a set of elementary reactions occur to constitute a non-elementary reaction. We can not measure the intermediates formed. Also we assume that these intermediates are present in very small amounts. For example, if \([X]\) represents the concentration of intermediate \(X\), then \[\frac{d[X]}{dt} \approx 0\] This is called steady state hypothesis (SSH) or steady state approximation (SSA).

In addition to SSA, equilibrium assumption is also used in order to explain the mechanism of a non-elementary reaction.

Usually a mechanism is postulated for a non-elementary reaction and the same will be tested for consistency with kinetic data. Once tested successfully, we now have an equation for \(-r_A\) (rate law).