Question

Write balanced redox reactions of the following.

$H_2{O}_2\left(aq\right)$ oxidises ${Mn}^{2+}\left(aq\right)$ to $Mn{O}_2$ in basic medium.

• A. ${Mn}^{2+}+H_2{O}_2+5{OH}^{-}⟶Mn{O}_2+2H_{2}O$
• B. ${Mn}^{2+}+H_2{O}_2+2{OH}^{-}⟶Mn{O}_2+2H_{2}O$
• C. ${Mn}^{2+}+{2H}_2{O}_2+2{OH}^{-}⟶Mn{O}_2+3H_{2}O$
• D. None of these

Answer 1

Answer: (B)

${Mn}^{2+}+H_2{O}_2+2{OH}^{-}⟶Mn{O}_2+2H_{2}O$

$H_2{O}_2\left(aq\right)$ oxidizes ${Mn}^{2+}\left(aq\right)$ to $Mn{O}_2$ in basic medium.
The unbalanced redox equation is as follows:
${Mn}^{2+}+H_2{O}_2⟶Mn{O}_2$
All atoms other than $H$ and $O$ are balanced..
The oxidation number of $Mn$ changes from 2 to 4. The change in the oxidation number of $Mn$ is 2.
The oxidation number of $O$ changes from -1 to -2. The change in the oxidation number per $O$ atom is 1. For $2O$ atoms, the change in the oxidation number is 2.
The increase in the oxidation number is balanced with a decrease in the oxidation number.
O atoms are balanced. To balance H atoms, add 2 $H^{+}$ on RHS.
${Mn}^{2+}+H_2{O}_2⟶Mn{O}_2+2H^{+}$
Since the reaction occurs in basic medium, add 2 hydroxide ions on both sides of the equation.
${Mn}^{2+}+H_2{O}_2+2O{H}^{-}⟶Mn{O}_2+2H^{+}+2O{H}^{-}$
On RHS, 2 $H^{+}$ ions combine with 2 $O{H}^{-}$ ions to form 2 water molecules.
${Mn}^{2+}+H_2{O}_2+2{OH}^{-}⟶Mn{O}_2+2H_{2}O$
This is the balanced chemical equation.