reaction. However, we can collect experimental evidence to supporta proposed
mechanism, or to show that some proposed mechanism is incorrect. (Thus the
scientific axiom that any multitude of experiments can suggest that a hypoth-
esis is correct, but only one experiment is needed to show that a hypothesis is
incorrect.) Experimental techniques used to try to support a proposed mech-
anism include stopped-flow experiments for solution-phase chemistry and
ultrafast (on a femtosecond timescale; a femtosecond is 10^15 second) laser
spectroscopy for gas-phase reactions. We will not dwell on such techniques
here; rather, we will focus on the elementary processes that such experiments
might study.
For example, in the reaction of hydrogen and oxygen gases, the first ele-
mentary process in the overall reaction might be
H 2 O 2 →2OH (20.58)That is, the two diatomic molecules collide in space and rearrange to form two
new molecules, OH. Notice that this is notthe hydroxide ion! It is a combina-
tion of one oxygen atom and one hydrogen atom, and as an uncharged di-
atomic molecule it has an odd number of electrons. Such odd-electron species
are rare in main-group compounds. Typically, odd-electron molecules are re-
active and short-lived; they are called free radicals,or more simply,radicals.
This diatomic product, OH, also violates our “normal” rules of valence. But
we don’t mind in this case, because this is simply the first step in an overall
mechanism and not the balanced chemical reaction. We presume that this prod-
uct will react further with other species to ultimately give the final product of
the reaction. But what you can see is that we are allowed to step outside the
regular rules for making compounds when dealing with mechanisms, because
typically the intermediate chemical species aren’t our final products anyway.
There are some basic guidelines, however. Since chemical species are inter-
acting in three-dimensional space, we presume that individual elementary
processes involve a single species, two species coming together (colliding), and
rarelythree species coming together. (Again, we emphasize “rarely”: What are
the odds that three different atoms or molecules will come to the exact point
in three-dimensional space at the exact time in the necessary orientation so
that a reaction will occur? It would be very rare. Elementary processes involv-
ing more than three species in the gas phase are not even seriously considered
as possibilities.) Occasionally, collision with an inert reactant or the wall of the
container can be invoked as part of an elementary process; such mechanisms
are sometimes necessary to remove excess energy from two colliding reactants.
But for the most part, elementary processes will involve one or two (maybe,
but rarely, three) reactant species.
Also, the overall sum of all elementary processes must yield the balanced
chemical reaction. This may seem obvious, but might be easily forgotten when
proposing an overall mechanism.
Finally, the proposed mechanism must be consistent with the overall rate
law of the reaction, which is determined experimentally. This point is impor-
tant and useful. Earlier, we made the point that the exponents on the concen-
trations in the rate law, the orders with respect to each concentration, were not
necessarily equal to the coefficient in the balanced chemical reaction. However,
for elementary processes, the rate law isdetermined directly from the stoi-
chiometry of the process. Instead of using the term “the order with respect to
the reactants,” we refer to “the molecularitywith respect to the individual
species” in the elementary process.
20.7 Mechanisms and Elementary Processes 707