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?

  • 1
    $\begingroup$ Yes, H2O2 is both easily reduced and oxidised. $\endgroup$ – Mithoron Jun 20 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 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 at 18:42
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    $\begingroup$ @sochemistry, Your textbook is right. It is just that question is wrong. $\endgroup$ – M. Farooq Jun 20 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. $\endgroup$ – Bloke Down The Pub Jun 22 at 22:37

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).

  • $\begingroup$ Thanks for the explanation so it means that all the options are incorrect for the given question isn't it? $\endgroup$ – ecneics Jun 20 at 18:40
  • 1
    $\begingroup$ Yes, the whole question is wrong. $\endgroup$ – M. Farooq Jun 20 at 18:41
  • 2
    $\begingroup$ @sochemistry See Fenton reagent $\endgroup$ – Poutnik Jun 21 at 5:59

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