Hydrogen Combustion as Model Reaction

As stated before (p. 190), the hydrogen combustion is one of the reactions providing the experimental data underlying the theory of branched chain reactions. However, the basic features of this reaction are also inherent in the combustion of other gases. Consequently, the hydrogen combustion can be taken as a model reaction, to some or other extent representing combustion in general. 

The most precise mechanism of hydrogen combustion accepted at present involves the following elementary steps, though not all are important for the qualitative understanding  

In terms of this mechanism hydrogen and oxygen atoms, as well as OH and HO2 radicals, are the active centers of the hydrogen combustion. All these have directly been found in the hydrogen combustion zone, the concentrations of atomic oxygen, atomic hydrogen, and hydroxyl radicals exceeding the equilibrium values by factors of hundreds and thousands [230]. 

The HO2 radical is formed in the termolecular process (14). The resulting homogeneous chain termination was introduced as a hypothetical process in order to account for the second ignition limit. Later on, its validity was supported by many experiments. 

It has to be expected that in the low-temperature range the relative HO2 concentrations are high. 

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