The mechanism given above seems to demand a three-body collision between one H2 molecule and two I atoms, but this, too, can be avoided by assuming a rapid equilibrium between H2 and another intermediate molecular species, H2I:


This is the mechanism outlined in fig 6 opposite. The rate then is given by

Once again, this is the same expression that would be expected from a simple bimolecular reaction. It emphasizes the truth of the statement that neither the overall chemical equation nor even the correct rate equation is enough by itself to tell you the detailed mechanism of a reaction.