# Hydrogen peroxide: catalyst changes colour during peroxide decomposition

When adding iron(III) chloride to hydrogen peroxide, the hydrogen peroxide decomposes.

Iron(III) chloride is a catalyst.

But when it is first added, it changes its colour to dark brown, then near the end it turns to orange.

Why does the colour change?

I heard that it has to do something with reaction intermediate. Correct?

It is seen on: iron(III) chloride

Also, the colour changes when different catalyst is used like iron(III) nitrate and potassium iodide potassium iodide, iron(III) nitrate

At first, I thought that the brown colour is due to ferrous chloride that formed during the course of reaction but since the reaction is occuring in a aqueous solution, it should be green in colour. Ferrous chloride is brown in anhydrous form. So I thought to be either due to formation of a complex, iron(II, III) oxide($\ce{Fe2O3}$), or iron(III) oxide-hydroxide($\ce{FeOOH.nH2O}$) or a mixed salt of oxide-chloride i.e. iron oxychloride($\ce{FeOCl}$).

The following information is from the paper 2 of that answer:

According to the mechanism proposed by KREMER and STEIN, an intermediate oxygen complex of iron with oxidation number +V is primarily formed by the reaction of $\ce{Fe^3+}$ with $\ce{H2O2}$. This complex reacts with another $\ce{H2O2}$ molecule to water and oxygen thereby reforming $\ce{Fe^3+}$.

$$\ce{Fe^3+ + H2O2 <=> [Fe^{III}OOH]^2+ + 2H+ <=> [Fe^{V}O]^3+ + H2O ->[H2O2] Fe^3+ + 2H2O + O2}$$

According to the mechanism proposed by HABER and WEISS the $\ce{Fe^3+}$ ions initiate a radical reaction, after which the chain reaction consumes the hydrogen peroxide. This mechanism can explain the high reaction rate very well.

Chain initiation: $\ce{Fe^3+ + H2O2 <=> [Fe^{III}OOH]^2+ + 2H+ <=> Fe^2+ + HOO. + H+}$

Chain propagation: $\ce{Fe^2+ + H2O2 -> Fe^3+ + 2OH.}$ $\ce{Fe^3+ + H2O2 + OH. -> Fe^3+ + HOO. + H2O -> Fe^2+ + H+ + O2 + H2O }$

That intermediate oxygen complex of iron with oxidation number +V maybe responsible for the brown colour of solution.

As for the potassium iodide, the color you are seeing is basically food coloring. Various food coloring is used to make the foam produced colorful. If food-color is not used, still color is seen. That color is basically color of pure soap foam. This elephant toothpaste experiment not uses food-color but still it has color that is color of soap-foam.

I have never seen this reaction before, but if I recall correctly, chloride ions ($\ce{Cl-}$) are known to give a solution an orange appearance.

I am assuming that the $\ce{H2O2}$ oxidizes the iron, leaving free chloride ions in solution.

• Does NaCl dissolved in water appear orange? – airhuff Jan 28 '17 at 23:15
• No, I do not believe it does. It's possible I was thinking of chlorine - in which case chloride ions would be reacting with each-other near the end of the reaction. – Bob Jan 28 '17 at 23:23
• Why would hydrogen peroxude oxidize the iron? The catalyst is not being used up – didgocks Jan 29 '17 at 0:10
• That is indeed the definition of a catalyst. But, I think the oxidation would actually be from O2. H2O2 -> H2O + O2 O2 + Fe(III) -> Fe2O3 I'm mostly just speculating... Sorry – Bob Jan 29 '17 at 0:18
• If the solution only contains water, hydrogen peroxide, and iron chloride.. And the solution is a different color at the end of the reaction. There is no possible way that there are not any new-formed compounds. As water and H2O2 are both clear, and O2 would either evaporate, or remain dissolved. And dissolved O2 would still be clear. – Bob Jan 29 '17 at 0:29