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.
          Go to the Molecular Rate Law Worksheet
          Go to the Molecular Rate Law Extra Worksheet