Non-Elementary Reactions and Stationary State Approximations

Non-elementary reactions are reactions for which there are no one-to-one correspondences between the observed rate expression and the stoichiometric equation (Section 2.1.4). Rate equations for such non-elementary reactions are derived by proposing a reaction mechanism that defines the way in which the non-elementary reaction is split into a number of elementary reaction steps. For example, Lindemann (1922) proposed a mechanism to explain the spontaneous decomposition of azomethane 

This reaction is of the type A B + C and is split into three elementary steps  

One molecule of A collides with another molecule of A producing an extra-energetic molecule A, which is unstable. 

Energised molecule A returns to a stable state by collision with a molecule of A 

It is assumed that the slowest of the three steps is step 3, which is the formation of products B and C by decomposition of A. In any multistep reaction, the slowest step is the rate-controlling step and the rate of the slowest step is taken as the overall rate of reaction. Thus, the rate of reaction r is 

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