Download a Printable Version Here (Adobe
Acrobat Format)
Reaction Kinetics & Chemical
Reaction Models
General
Features of Reaction Mechanisms
There are fundamental features in most reaction
mechanisms occurring in the gas phase that can be used to assemble such mechanisms. The
most important one is the role played by free radicals in the decomposition of any
reactant. A very simple example found in freshman chemistry books is the formation of
hydrogen iodide starting from hydrogen and iodine (Chang, 1994). The proposed mechanism is
a two-step mechanism,
 |
(3) |
 |
(4) |
The first step involves the formation of iodine radicals in reaction (3), and it is
followed by the formation of hydrogen iodide. Another example is the oxidation of
hydrocarbons at low temperatures initiated by the following reactions (Dryer, 1991)
 |
(5) |
 |
(6) |
in which the hydrocarbon radical R plays a role in reaction (6) forming a species that
can lead to the formation of oxygenated species such as alcohols and aldehydes. These
types of reaction seem to take place in the urban atmosphere (Seinfield, 1989). If the
temperature is high enough, the decomposition of the reactant is initiated by a thermal
decomposition such as reaction (7),
 |
(7) |
Reaction (7) is endothermic and its extent would depend on temperature, and results in the
formation of methyl radical. This type of reaction would occur in shock tubes, for
example, where the decomposition of the fuel is initiated by a traveling shock wave
causing temperature and pressure to rise considerably in a very short time.
The above examples then illustrate a very important point to remember when examining
reaction mechanisms: the decomposition of the reactant is initiated by formation of
radical species that can later participate in the reactions leading to the product. The
concentration of the radicals formed increase as the reaction time or temperature
increase.
Another important consideration is the formation of chain reactions. The basic premise of
chain reaction mechanisms is also that free radicals play a leading role in the
destruction of reactant molecules. The chain reaction mechanism itself consists of several
steps: initiation, propagation, branching (not always present), and termination. This can
be illustrated, for certain range of temperature and pressure, by some of the reactions in
the following Hydrogen oxidation mechanism:
General Features of Reaction Mechanisms |