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I have studied that hydrogen peroxide always oxidizes ferrous ion to ferric ion (source of study : NCERT Chemistry Part II, Textbook for Class XI), but a question came in IIT JEE 2015 which states:

Question 36 IITJEE 2015 Paper 1

The answer given is A and B but I doubt that the answer is correct as I have studied the following reactions in my book :

$$\ce{2Fe^{2+}(aq) + 2H^{+}(aq) + H_2O_2(aq) -> 2Fe^{3+}(aq) + 2H_2O(l)}$$

$$\ce{2Fe^{2+} + H_2O_2(aq) -> 2Fe^{3+} + OH^{-}}$$

Where am I possibly going wrong?

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    $\begingroup$ Yes, H2O2 is both easily reduced and oxidised. $\endgroup$
    – Mithoron
    Jun 20, 2021 at 18:08
  • $\begingroup$ You are right, and your book is wrong. It is easy to show in a simple test tube. In presence of $\ce{H2O2}$, the $\ce{Fe^{2+}}$ ion is always oxidized, whatever the presence of an acid or a base in solution. $\endgroup$
    – Maurice
    Jun 20, 2021 at 18:33
  • $\begingroup$ @Maurice I think you mean to say that the question is wrong as my book states that in presence of $H_2O_2$, $Fe^{2+}$ is always oxidized to $Fe^{3+}$. $\endgroup$
    – ecneics
    Jun 20, 2021 at 18:42
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    $\begingroup$ @sochemistry, Your textbook is right. It is just that question is wrong. $\endgroup$
    – AChem
    Jun 20, 2021 at 18:46
  • $\begingroup$ The book example looks like the peroxide is oxidising ferrous to ferric, which is kind of what I'd expect an oxidising agent to do. Think someone made a mistake (or two) writing the question. $\endgroup$ Jun 22, 2021 at 22:37

2 Answers 2

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You are right. This question is "practically" incorrect, although on paper it might appear so if someone is unaware of real chemistry (the question setter). Iron (II) will readily precipitate in alkaline medium and then immediately oxidize to iron (III) in the presence of hydrogen peroxide. It does not proceed backwards, i.e., hydrogen peroxide will not reduce Fe(III). It will start catalytic decomposition rather.

In fact iron (II)+ hydrogen peroxide mixture is a very interesting system and used in environmetal cleaning. It is quite complex chemistry of free radicals. It is called Fenton's reagent (always used in mildly acidic conditions).

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  • $\begingroup$ Thanks for the explanation so it means that all the options are incorrect for the given question isn't it? $\endgroup$
    – ecneics
    Jun 20, 2021 at 18:40
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    $\begingroup$ Yes, the whole question is wrong. $\endgroup$
    – AChem
    Jun 20, 2021 at 18:41
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    $\begingroup$ @sochemistry See Fenton reagent $\endgroup$
    – Poutnik
    Jun 21, 2021 at 5:59
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Test questions usually require regurgitating something already taught (whether that teaching is absolutely correct or not is not really an issue).

However, while it may be accurate, as the OP stated, that hydrogen peroxide always oxidizes ferrous ion to ferric, it is not incorrect to state that hydrogen peroxide also reduces ferric ion to ferrous.

How does this occur? Consider a beaker of ferric chloride to which is added some hydrogen peroxide:

Fe$^{3+}$ + H2O2 --> Fe$^{2+}$ + $^.$OOH + H$^+$

Fe$^{3+}$ + $^.$OOH --> Fe$^{2+}$ + H$^+$ + O2

These equations are the second and third from the question asked over a year ago (Ref 1). They amount to a hydrogen peroxide molecule going to O2, and 2 protons and two electrons reducing two ferric ions to ferrous. In the Fenton scheme, as the H2O2 runs out, the hydroperoxyl moiety will be the last oxidant in solution, since it is not as active as the hydroxyl radical, which will have been consumed too. So any ferric ions will be reduced to ferrous, but there will be no H2O2 to reoxidize the ferrous to ferric.

So H2O2 can reduce ferric ion to ferrous. Ferrous ions can be reoxidized to ferric only as long as some H2O2 is present:

Fe$^{2+}$ + H2O2 --> Fe$^{3+}$ + $^.$OH + OH$^-$

(the first equation in the question of Ref 1).

While the question may be questionable, with the above analysis, answers A and B could be correct. The pH is an important factor because at low pH, complexation of Fe$^{2+}$ occurs, limiting its ability to react with H2O2 (Ref 2).

Ref 1. Is the effect of hydroperoxyl radicals significant in the oxidation of organic compounds in the Fenton reaction? Why or why not?

Ref 2. https://en.wikipedia.org/wiki/Fenton%27s_reagent

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