Reaction between Molecular Hydrogen and Iodine

The reaction between H2 and I2 was extensively studied by Bodenstein [9-11] at the end of the nineteenth century. For many years it was considered to be a true bimolecular reaction. However, in 1959, Sullivan showed the presence of free radicals in the system above 600 K, leading to the proposed mechanism [12-14] 

Inserting the expressions for [I] and [H2I], obtained from the equilibrium processes (1) and (2), one gets 

k = k K2lK. This mechanism, together with mechanism (12.III) and a few others, all appear to make contributions to the overall reaction. Another process that is almost certainly involved proceeds via the elementary step Ij + 2HI, which proceeds via a 

This illnstrates that a single reaction can often occur by more than one mechanism proceeding simnltaneously. In general there is no nnique mechanism corresponding to a given empirical law, although under certain experimental conditions one may be dominant. 

---