Up: Back to the Chemistry 2710 assignment
index
Chemistry 2710 Review Problem for the March 8 Lecture:
Solution
Apply the equilibrium approximation to the first step. The result is
.
The rate of the reaction is
The overall reaction is .
If this were an elementary reaction, the rate would be proportional to
the product of the hydrogen and iodine concentrations. This is exactly
what we get with the approximate rate law, so we can't tell that this
reaction isn't elementary from the rate law. There are at least two
ways to show experimentally that this reaction isn't elementary, one of
which is based on kinetics:
- When we say that the first step is fast, we mean that
it is fast compared to the rate of the second step
under normal experimental conditions. However, since these
rates depend on the concentrations, we can imagine reducing the
concentration and increasing the
concentration so that this is no longer the case. If we can
make the second step reasonably fast, then the equilibrium
approximation is no longer appropriate. Rather, we should use
the steady-state approximation to deal with the highly reactive
radical intermediate. The result of this approximation is
(Try to derive this equation for yourself.)
This is one of the rare cases where, in a relatively simple
mechanism, the functional form of the
SSA is different from that of the EA.
The dependence of the rate on the concentration of
is quite different from what we would find in an
elementary reaction. It only remains to know whether the
experimental conditions required to bring out this relationship
are feasible.
- Free radicals can be observed directly by electron spin resonance
(sometimes called electron paramagnetic resonance). The
observation of iodine radicals would of course strongly suggest
a non-elementary reaction.
Marc Roussel
Wed Mar 8 11:05:00 MST 2000