| Deduction of Rate Laws from Mechanisms |
The mechanism of a reaction is the series of elementary steps by which
the reaction takes place. We have already seen that in a series of steps
there will usually be one (the slowest or rate-determining step) which is
slower than the others and which therefore controls the rate of reaction.
|
If the rate-determining step is the first or the only in a sequence,
then the rate law can be written down directly from the stoichiometry of the
first step; e.g. the overall reaction
|
N2O5(g) + NO(g) -----> 3 NO2(g)
|
There are only two reactants and it is a one-step reaction. Therefore
the rate law can be determined to be
|
rate = k [N2O5][NO]
|
In an reaction that is not a one-step reaction then you must first
determine what the slowest step in the mechanism will be. Then once you've
decided on the slowest step, the rate law can be written directly from the
slowest step.
|