Write balanced redox reactions of the following:

$H_{2} O_{2} (a q)$ reduces ${C r}_{2} O_{7}^{2 -} (a q)$ to green coloured ${C r}^{3 +} (a q)$ in acidic medium.

{{C r}_{2} O_{7}^{2 -} + 3 H}_{2} O_{2} + 8 H^{+} ⟶ 2 {C r}^{3 +} + 3 O_{2} + 7 H_{2} O

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{{C r}_{2} O_{9}^{2 -} + 3 H}_{2} O_{2} + 4 H^{+} ⟶ 2 {C r}^{3 +} + 3 O_{2} + 7 H_{2} O

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{{C r}_{2} O_{7}^{2 -} + 3 H}_{2} O_{2} + 8 H^{+} ⟶ 2 {C r}^{3 +} + 3 O_{2} + 9 H_{2} O

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None of these

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Solution

The correct option is B ${{C r}_{2} O_{7}^{2 -} + 3 H}_{2} O_{2} + 8 H^{+} ⟶ 2 {C r}^{3 +} + 3 O_{2} + 7 H_{2} O$
$H_{2} O_{2} (a q)$ reduces ${C r}_{2} O_{7}^{2 -} (a q)$ to green coloured ${C r}^{3 +} (a q)$ in acidic medium.

The unbalanced redox equation is as follows:
${{C r}_{2} O_{7}^{2 -} + H}_{2} O_{2} ⟶ {C r}^{3 +} + O_{2}$

Balance all atoms other than H and O.
${{C r}_{2} O_{7}^{2 -} + H}_{2} O_{2} ⟶ 2 {C r}^{3 +} + O_{2}$

The oxidation number of Cr changes from 6 to 3. The change in the oxidation number per Cr atom is 3. For 2 Cr atoms, the change in the oxidation number is 6.

The oxidation number of O changes from -1 to 0. The change in the oxidation number per O atom is 1. For 2 O atoms, the change in the oxidation number is 2.

To balance the increase in the oxidation number with a decrease in the oxidation number multiply $H_{2} O_{2}$ and $O_{2}$ with 3.

${{C r}_{2} O_{7}^{2 -} + 3 H}_{2} O_{2} ⟶ 2 {C r}^{3 +} + 3 O_{2}$

To balance O atoms, add 7 water molecules on RHS.

${{C r}_{2} O_{7}^{2 -} + 3 H}_{2} O_{2} ⟶ 2 {C r}^{3 +} + 3 O_{2}$

To balance H atoms, add 8 $H^{+}$ on LHS.

${{C r}_{2} O_{7}^{2 -} + 3 H}_{2} O_{2} + 8 H^{+} ⟶ 2 {C r}^{3 +} + 3 O_{2} + 7 H_{2} O$

This is the balanced chemical equation.

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Similar questions

{C r}_{2} O_{7}^{2 -} (a q) + 3 {S O}_{3}^{2 -} (a q) + 8 H^{+} ⟶ 2 {C r}^{3 +} (a q) + 3 {S O}_{4}^{2 -} (a q) + 4 H_{2} O

{C r}_{2} O_{7}^{2 -} + {F e}^{2 +} + H^{\oplus} ⟶ {C r}^{3 +} + {F e}^{3 +} + H_{2} O

6 {F e}^{2 +} + 14 H^{\oplus} + {C r}_{2} O_{7}^{2 -} ⟶ 6 {F e}^{3 +} + 2 {C r}^{3 +} + 7 H_{2} O

H_{2} O_{2} (a q) + {C r}_{2} O_{7}^{2 -} ⟶ O_{2} (g) + {C r}^{3 +} (a q)

{C r}_{2} O_{7}^{2 -} (a q) + H {N O}_{2} (a q) ⟶ {C r}^{3 +} (a q) + {N O}_{3}^{-} (a q)

{C r}_{2} O_{7}^{2 +} (a q) + 14 H^{+} (a q) + 6 {F e}^{2 +} (a q) \to 6 {F e}^{3 +} (a q) + 2 {C r}^{3 +} (a q) + 7 H_{2} O (l)

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