Chemical Mechanisms
By describing how atoms and molecules interact to generate products,
mechanisms help us to
understand how the world around us functions at a fundamental level. A
mechanism is a series of elementary steps whose sum is the overall
reaction. An elementary step
is a reaction that is meant to represent a single collision or vibration
that leads to a chemical change.
For a mechanism to be considered valid, its sum must equal the overall
balanced equation, its
predicted rate law must agree with experimental data, and its predictions of
intermediates must not be
contrary to experimental
observations. A mechanism may never be proven because we cannot
ever see a chemical
reaction--both the time scale of an elementary step and the size of
atoms are too small.
Furthermore, we must guess at the identity of many intermediates
because they are usually so
reactive that they can not be isolated. Instead, a chemist proposes reaction
mechanisms and tests their validity against experimental data, ruling out
mechanisms that are inconsistent with results. These experiments may be
strategically designed to trap an intermediate product to prove its existence as
a stepping-point in the total reaction.
To aid in our understanding of mechanisms, we will draw reaction
coordinate diagrams that
trace the free energy path of a reaction from reactants to products. The
activation energy of a
reaction represents the difference in energy between the reactants and the
highest point on a reaction
coordinate diagram. We will derive the Arrhenius Equation, which relates the
rate constant for a
reaction to its activation energy. Local minima on the reaction coordinate
diagram are positions occupied by intermediates.
By comparing the reaction coordinate diagram for a catalyzed and a
uncatalyzed process, we can
see that catalysts function by altering the route the reaction takes from
reactants to products without
the catalyst being altered.
