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.