Reactions of thermal cracking

These reactions are known in industrial processes and occur simply by decomposition and in gas phase. However, it is a nonelementary chain reaction and occurs in different elementary steps. For reactions such as thermal cracking in gas phase, there are several kinetic models, but the most interesting one is the model of Rice-Herzfeld. The main idea is that in the first step of the reaction, the abstraction of the H from the reactant molecule occurs and subsequently there is decomposition with formation of a new radical. The process takes place throngh the steps of initiation, propagation, and termination. 

Since the initiation rate is low, we can neglect the first term. (-r ) = 2[Ri][A] 

Substituting Equation 8.3 into Equation 8.2, simplifying and considering that the initial rate is low compared to the rates of propagation and termination, we obtain the following expression  

The rate of conversion of A is of first order in relation to the concentration of [A], but the reaction rate constant encompasses the four specific rates of the mechanism. The apparent constant is determined experimentally, i.e.  

Note that the specific rates depend on the temperature and have their own activation energies t, satisfying the Arrhenius equation, i.e.  

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