15. The Rates of Chemical Reactions   Previous PageNext Page
       Reaction Mechanisms
It would be difficult or impossible to measure the concentration of N202 because the molecule is a short-lived reaction intermediate, but fortunately this is unnecessary. The equilibrium-constant expression relates the N202 concentration with that of the reactant N0 concentration:
 

Substituting in the rate equation, we get
 

in which k = k3Keq.

This is the same rate expression that would have resulted if the reaction had occurred by the simultaneous three-molecule collision of N0, N0, and 02, but the mechanism just proposed assumes a series of two-molecule collisions instead. One cannot decide the actual mechanism of a reaction from the rate equation alone.

For this example, it would be necessary to carry out chemical experiments to determine the presence or absence of reaction intermediates such as N202. Finding them would support the proposed mechanism; but not finding them might only mean that the chemical detection methods were not sensitive enough.

This is why the number of proposed reaction mechanisms in the chemical literature is much greater than the number of well-established mechanisms.

We will look at three other examples of reaction mechanisms, and see how they account for the observed rate expressions.

 

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