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