15. The Rates of Chemical Reactions   Previous PageNext Page
       The Hydrogen-Iodine Reaction
Sullivan found a way to change the relative concentrations of I2 and I by using ultraviolet light from a mercury vapor lamp to cause more iodine molecules to dissociate:

In effect, the photons from the lamp gave this reaction a vigorous kick to the right. Sullivan then could alter the relative amounts of I2 and I at will, by controlling the amount of ultraviolet light from the lamp.

The question became: Is the rate of reaction dependent on the concentration of I2 molecules, or of I atoms? The answer from the experiments was clear; the rate depends on the concentration of I atoms. The actual rate equation under all conditions is


The simpler form, which makes it look as if the rate depends on the first power of the concentration of iodine molecules, is valid only because in the absence of UV disturbance, I2 and I always are in rapid equilibrium.

The Sullivan mechanism for HI was startling because it destroyed what was long believed to be a classical example of true bimolecular collision.


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