My chemistry books details out both, the oxidizing and reducing properties of hydrogen peroxide, $\ce{H2O2}$.

  • Its reducing properties:

    • It reduces $\ce{Cl}$ to $\ce{HCl}$
    • It reduces $\ce{Ag2O}$ to $\ce{Ag}$
    • It reduces $\ce{K3[Fe(CN)6]}$ (potassium ferricyanide) to $\ce{K4[Fe(CN)6]}$ (potassium ferrocyanide)
    • Etc.
  • Half equations for reduction:

    • In acidic medium: $\ce{H2O2 -> 2H+ + O2 + 2e-}$
    • In basic medium: $\ce{H2O2 + 2OH- -> 2H2O + O2 + 2e-}$
  • Its oxidizing properties:
    • It oxidizes sulphites to sulphates
    • It oxidizes arsenites to arsenates
    • It oxidizes nitrites to nitrates
    • It oxidizes potassium ferricyanide to potassium ferrocyanide
    • Etc.
  • Half equations for oxidation:
    • In acidic medium: $\ce{H2O2 + 2H+ + 2e- -> 2H2O}$
    • In basic medium: $\ce{H2O2 + 2e- -> 2OH-}$

I was unable to find any pattern in all of the reactions I studied above (except for potassium ferrocyanide, for which the medium triggers the electron addition/removal) to recognize when hydrogen peroxide behaves as an oxidising agent and when it behaves as a reducing agent.

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    $\begingroup$ Are you familiar with reduction potentials? $\endgroup$ – gsurfer04 Jan 16 '17 at 16:49
  • $\begingroup$ @gsurfer04 So you are trying to say it depends on the relative reduction potential ?Is this the only reason(I find so many exceptions in chemisty that I suspect there might be some exception here too) $\endgroup$ – Xasel Jan 17 '17 at 5:52
  • $\begingroup$ The standard electrode potential ($\pu{1M}$ solution at STP) of $\ce{H2O2}$ to $\ce{H2O}$ is $\pu{+1.78V}$. The $\ce{O2}/\ce{H2O2}$ potential is $\pu{+0.70V}$. As always, changing the reaction conditions will change the thermodynamics. So if you get water, $\ce{H2O2}$ is reduced and if you get oxygen it has been oxidised. I've just noticed that you've put the ferri/ferrocyanide reaction as both reduction and oxidation. $\endgroup$ – gsurfer04 Jan 17 '17 at 11:00
  • $\begingroup$ @gsurfer04 I fixed your comment (almost two years later :D). However, you generally have two options here: delete and repost, custom flag for moderator. $\endgroup$ – Martin - マーチン Nov 9 '18 at 14:15
  • $\begingroup$ @gsurfer04 What about the reactions in which H2O2 is converting to both water and oxygen? As in the reaction of hydrogen peroxide with ozone or silver oxide. $\endgroup$ – Apurvium Dec 14 '19 at 9:53

Let us say that $\ce{H2O2}$ reacts with some ion $\ce{Y+}$. If $+1$ is yttrium's highest oxidation state then it will have no other option but to to get reduced to one of its lower oxidation states, and hydrogen peroxide acts as reducing agent. If this is not the case it will get oxidized to some higher oxidation state, and hydrogen peroxide will act as oxidizing agent. This is because $\ce{H2O2}$ is both an oxidizing and reducing agent.

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In a Fenton system, $\ce{H2O2}$ can act as an oxidizing and reducing agent.

\begin{align} \ce{Fe(II) + H2O2 &-> Fe(III) + OH^. + OH-}\\ \ce{Fe(III) + H2O2 &-> Fe(II) + HO2^. + H+} \end{align}

See reactions (14) and (15) in the following publication:
Muruganandham, M.; Suri, R. P. S.; Jafari, S.; Sillanpää, M.; Lee, G.; Wu, J. J.; Swaminathan, M. Recent Developments in Homogeneous Advanced Oxidation Processes for Water and Wastewater Treatment. Int. J. Photoenergy 2014, 2014, 1–21 DOI: 10.1155/2014/821674.

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Hydrogen peroxide is a redox substance. Its behaviour as oxidising agent and reducing agent depends upon nature of substance that reacts with hydrogen peroxide. If a substance is oxidisable then H2O2 will acts as oxidising agent.

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    $\begingroup$ Then what decided whether subject is oxidisable or not,and if we go along the lines of modern definition of Oxidation,pretty much every substance can be oxidized? $\endgroup$ – Xasel Jan 16 '17 at 14:45

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