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:

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