As with the NO reaction just discussed, one can write an equilibrium constant for the dissociation and reassociation of I2:

The rate of reaction is determined by the slowest step, and hence is

Using the equilibrium expression to eliminate the concentration of the short-lived I atom intermediate, produces a rate expression that is identical with that predicted from simple collision theory:
[I]2 = Keq[I2]


How then can one decide which mechanism is right, bimolecular collision, or dissociation of I2 and subsequent reaction of I atoms?

In 1967, J. H. Sullivan found an ingenious way to decide. At equilibrium at any given temperature, the iodine molecule and atom concentrations always will be linked by the equilibrium expression