All reactions other than spontaneous decompositions of single particles involve collisions between molecules. As we have just seen, some reactions that look simple from their overall stoichiometric equations actually involve elaborate chain mechanisms, and it is not possible to predict the true mechanism just by looking at the equation for the chemical reaction.

Even these more complex processes, however, involve separate steps that usually are simple two-body collisions, and what we derive from bimolecular reactions below can be applied to each of the individual steps. The principles are the same for both simple and complex mechanisms.

A reaction that does appear to proceed by a simple two-body collision is the following:

The rate of the forward reaction, by which NO2 and CO molecules collide and react, can be written as
  rate = k[NO2] [CO]

We know that we can increase the rate of the forward reaction by increasing the concentrations of NO2 and CO. Can we also find some way to increase the rate constant, k?