Mechanisms describe in a stepwise manner the exact collisions and
events that are required for
the conversion of reactants into products. Mechanisms achieve that goal by
breaking up the overall
balanced chemical equation into a series of elementary steps. An
elementary step is written to
mean a single collision or molecular vibration that results in a chemical
reaction. The following
picture of an elementary step shows a single collision between
water and boron
trifluoride:
Figure %: Schematic representation of an elementary step
The molecularity of an elementary step describes the number of reactive
partners in the
elementary step. For example, the above elementary step is called
bimolecular because two
molecules collide. Commonly, elementary steps are mono-, bi-, or
termolecular. The probability of
four molecules colliding at exactly the same place and time is so small
that we can safely assume that
no reaction will ever be tetramolecular. Because
take up a large
amount of space, we will represent elementary steps in this SparkNote as
normal reactions with molecular formula line equations. You will
know from the context (i.e. talking about the steps of a mechanism)
whether
the reaction is an
elementary step or an overall reaction.
To better understand mechanisms, let's consider the following mechanism for
the decomposition of
ozone, O3:
The above mechanism exhibits a property of all mechanisms: it is a series of
elementary steps whose sum is the overall balanced reaction. Note the
presence of the oxygen
atom, O, intermediate in the above equation. It is an intermediate
because it is both created and
destroyed in the mechanism and does not appear in the net equation.
Another property of mechanisms is that they must predict the experimentally
determined rate
law. To calculate the rate law from a mechanism you need to first know
the rate limiting step.
The rate limiting step determines the rate of the reaction because it is
the slowest step. You can
rationalize that a reaction can only go so fast as its slowest step by
thinking about what happens when
you encounter an accident on the highway that closes all but one lane. You
may have been able to
race along at 65 m.p.h. (depending on your state's laws) before you reached
the lane closure but the
slow passage of cars past the accident limits your rate. You can only go
as fast through that one lane
as the slowest car in front of you.
In the above , the first reaction is labeled as
"slow". This reaction is the rate determining step because it is the slowest
step. As we
have stated, that means that
the rate of the overall reaction is equal to the rate of the rate
determining step. The rate of an
elementary step is the rate constant for that step multiplied by the
concentrations of the reactants
raised to their stoichiometric powers. Note that this rule only applies for
elementary steps. The rate
of an overall reaction is NOT the product of the concentrations of
the reactants raised to
their stoichiometric powers. The rate law for the first elementary step in
the is rate = k [O3]. Because this step is the
rate determining step,
the rate law is also the rate law for the overall reaction. Using similar
techniques we can calculate the
rate law predicted by any mechanism. We then check the predicted rate
law against the
experimentally determined rate law to test the validity of the proposed
mechanism.
