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.