Thus far we have established that the
rates of chemical
reactions depend on the concentrations of reactant species,
and that this dependence often has a complicated form that cannot
be predicted directly from the coefficients of the balanced chemical
equation (as the equilibrium-constant
The reason for these complications is
that the actual mechanism
of reaction may involve a series of small, one or two-molecule
reactions, with intermediate complexes of atoms that are used up
in subsequent steps. The rate
equation only summarizes the overall process and does not tell
us what is happening in the individual steps.
Nevertheless, if we can come up with a
series of hypothetical reaction steps that faithfully reproduce
the observed rate expression, then we feel confident that our hypothetical
mechanism must be close to the actual mechanism.
Molecules react when they collide, provided
that the collision generates enough energy to tear the atoms apart
from one another and rearrange them into new molecules. Bimolecular
collisions (between two molecules) are common in gases, but simultaneous
trimolecular collisions are a thousand times rarer, and four-molecule
simultaneous impacts are so infrequent as to be eliminated from consideration.
Then how does the following multimolecular smog reaction take place?