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
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
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
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
mechanisms, and see how they account for the observed rate