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](https://youtu.be/qT3mJrmhJHU?t=2m54s)


Also, the colour changes when different catalyst is used like iron(III) nitrate and potassium iodide [potassium iodide](https://youtu.be/mNRUlQ1TfV4?t=3m16s), [iron(III) nitrate](https://youtu.be/lWkfJsugkwE)




[Recently, I answered your question and relevant information can be found from that answer.](https://chemistry.stackexchange.com/questions/66949/demonstrating-decomposition-of-hydrogen-peroxide-using-ironiii-nitrate-catalys/67059#67059)


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($\ce{Fe2O3}$),](https://en.wikipedia.org/wiki/Iron(III)_oxide) or [iron(III) oxide-hydroxide($\ce{FeOOH.nH2O}$)](https://en.wikipedia.org/wiki/Iron(III)_oxide-hydroxide) or a mixed salt of oxide-chloride i.e. [iron oxychloride($\ce{FeOCl}$).](https://en.wikipedia.org/wiki/Iron_oxychloride)


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.




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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.](https://www.youtube.com/watch?v=UTfMrx7275w)


 3

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.


 0

The orange-yellow colour is caused by the chloride ions taking the place of water in the coordination complex, anhydrous iron (III) chloride is a dark black or purple crystal, it's easier to see with a bigger anion that won't interfere with the hexaaquairon (III) complex. The red iron (IV) solution is formed on addition of hydrogen peroxide to the orange solution of [pentaaquachloridoiron (III)]2+ chloride solution turning some of those metal ion complexes to [tetraaquaoxoiron(IV)]+ chloride ions.


 -1