Reaction between Molecular Hydrogen and Chlorine

The thermal and photochemical reactions between molecular hydrogen and chlorine show some resemblance to the hydrogen-bromine reactions, but the mechanisms are more complex. Since it is easier to understand its main features, we will consider first the photochemical reaction. The mechanism has to account for the profound effect of oxygen 

The concentration of chlorine atoms [Cl] in this mechanism is much lower than that of [Br] in the H2/Br2 reaction. This is a direct consequence of the lower activation energy of C1+H2 HC1+H when compared with of Br+H2 HBr+H, as will be discussed in Section 12.4. 

The rate of Cl formation in step (1) is 21, where I is the intensity of the absorbed light in appropriate units, because two radicals are formed for each absorbed photon. This rate has the following expression  

Introduction of this expression into eq. (12.14), neglecting the small quadratic term in [O2], leads to 

Subtraction of eq. (12.15) from eq. (12.18) leads to an expression for the rate law for the H2/CI2 photochemical reaction that can be inserted into eq. (12.17) to give 

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Frequently, the basic kinetic laws of chain reactions are described in the one-centered approximation [1]. Certainly such an ideal situation is not of general nature. It is easy to note that even a relatively simple example of a chain chemical reaction between molecular hydrogen and chlorine is realized by means of two active reaction centers, the chain carriers, the H and Cl atoms. 

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