What is the mechanism of the reaction between iodide ion and hydrogen peroxide?

There are two different reactions between hydrogen peroxide and iodide:

1. The iodine clock reaction. In acidic solution, the overall reaction is

   3 I^-(aq) + H₂O₂(aq) + 2 H⁺(aq) I₃^-(aq) + 2 H₂O(l)

   which is first order in both peroxide and iodide. That's consistent with a rate-limiting step like

   H₂O₂(aq) + I^-(aq) IO^-(aq) + H₂O(l)

   The triiodide ion is rapidly produced by reaction of the hypoiodous acid with iodide:

   IO^-(aq) + H⁺(aq) HOI(aq)
   HOI(aq) + 2 I^-(aq) + H⁺(aq) I₃^-(aq) + H₂O(l)

   Other reactions can occur here, and each of these reactions can be composed of a series of other steps. But these equations are the basis for the first order behavior.
2. The iodide-catalyzed decomposition of hydrogen peroxide. This reaction produces oxygen gas, but no triiodide ion.

   2 H₂O₂(aq) O₂(g) + 2 H₂O(l)

   The experimental rate law is first order in both peroxide and iodide- suggesting a mechanism like

   H₂O₂(aq) + I^-(aq) IO^-(aq) + H₂O(l)
   H₂O₂(aq) + IO^-(aq) I^-(aq) + H₂O(l) + O₂(g)

   where the first reaction is the rate-determining step- the same one seen in the iodine clock reaction.

Author: Fred Senese senese@antoine.frostburg.edu