# Hydrogen peroxide and potassium iodide reactions

What is the explanation for the two possible reactions of $$\ce{H2O2}$$ with $$\ce{KI}$$ in acid media (iodide-catalyzed decomposition of hydrogen peroxide or iodide oxidation by hydrogen peroxide) and what determines the prevalence of one over the other?

It is known that if the hydrogen peroxide is concentrated, the pathway which results in the decomposition of hydrogen peroxide is favored. Otherwise, the iodide oxidation is predominant.

Two well-known chemistry demonstrations (elephant's toothpaste and the iodine clock) rely on these reactions.

The first one involves the decomposition of hydrogen peroxide

$$\ce{2H2O2 ->[\ce{KI}] 2H2O + O2}$$ which is believed to occur through the following mechanism $$\ce{H2O2 +I- + H+ -> HOI + H2O}$$ $$\ce{HOI + H2O2 -> H2O + O2 +I- + H+} \text{(rate-determining step)}$$

On the other hand, the reaction that happens in the iodine clock is $$\ce{H2O2 + 3 I- + 2 H+ -> I3- + 2H2O}$$

I would like to understand this phenomenon based on thermodynamic or kinetic data, since I could not find an explanation for it.

• Mar 14 at 1:15
• What is the mechanism of the 'clock' reaction? Does it also form $HOI$ initially? What is its (likely) rate-determining step? Assuming that both types of reactions can happen in parallel, what decides which one is prevalent must be kinetics, as far as I can tell. So if you can write the differential expression of the rds of each, you should be able to answer your question. Mar 14 at 13:07
• Apr 8 at 8:04