# Hydrogen peroxide acts both as an oxidising and as a reducing agent depending upon the nature of the reacting species. In which of the following cases ${\mathrm{H}}_{2}{\mathrm{O}}_{2}$ acts as a reducing agent in acid medium ?

1. A

${\mathrm{MnO}}_{4}^{-}$

2. B

${\mathrm{Cr}}_{2}{\mathrm{O}}_{7}^{2-}$

3. C

${\mathrm{SO}}_{3}^{2-}$

4. D

$\mathrm{KI}$

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### Solution:

With ${\mathrm{MnO}}_{4}^{-}$ in acidic medium, ${\mathrm{H}}_{2}{\mathrm{O}}_{2}$ acts as a reducing agent.

$2{\mathrm{MnO}}_{4}^{-}+6{\mathrm{H}}^{+}+5{\mathrm{H}}_{2}{\mathrm{O}}_{2}\to 2{\mathrm{Mn}}^{2+}+8{\mathrm{H}}_{2}\mathrm{O}+5{\mathrm{O}}_{2}$

${\mathrm{H}}_{2}{\mathrm{O}}_{2}$ oxidizes ${\mathrm{SO}}_{3}^{2-}$ to ${\mathrm{I}}^{-}$ to ${\mathrm{I}}_{2}$ and  ${\mathrm{Cr}}_{2}{\mathrm{O}}_{7}^{2-}$ to ${\mathrm{CrO}}_{3}$ in acidic medium.

${\mathrm{SO}}_{3}^{2-}+{\mathrm{H}}_{2}{\mathrm{O}}_{2}\to {\mathrm{SO}}_{4}^{2-}+{\mathrm{H}}_{2}\mathrm{O}$

$2{\mathrm{I}}^{-}+2{\mathrm{H}}^{+}+{\mathrm{H}}_{2}{\mathrm{O}}_{2}\to 2{\mathrm{H}}_{2}\mathrm{O}+{\mathrm{I}}_{2}$

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