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