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


Jim Fisher (jrf9g@virginia.edu)

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



General Chemistry Online! What is the mechanism of the reaction between iodide ion and hydrogen peroxide?

Copyright © 1997-2010 by Fred Senese
Comments & questions to fsenese@frostburg.edu
Last Revised 02/15/10.URL: http://antoine.frostburg.edu/chem/senese/101/kinetics/faq/mechanism-h2o2-iodide.shtml