Question

# Write balanced redox reactions of the following:$${ H }_{ 2 }{ O }_{ 2 }(aq)$$ reduces $${ Cr }_{ 2 }{ O }_{ 7 }^{ 2- }(aq)$$ to green coloured $${ Cr }^{ 3+ }(aq)$$ in acidic medium.

A
Cr2O27+3H2O2+8H+2Cr3++3O2+7H2O
B
Cr2O29+3H2O2+4H+2Cr3++3O2+7H2O
C
Cr2O27+3H2O2+8H+2Cr3++3O2+9H2O
D
None of these

Solution

## The correct option is B $${ { Cr }_{ 2 }{ O }_{ 7 }^{ 2- }+3H }_{ 2 }{ O }_{ 2 }+8{ H }^{ + }\longrightarrow 2{ Cr }^{ 3+ }+3{ O }_{ 2 }+7{ H }_{ 2 }O$$$${ H }_{ 2 }{ O }_{ 2 }(aq)$$ reduces $${ Cr }_{ 2 }{ O }_{ 7 }^{ 2- }(aq)$$ to green coloured $${ Cr }^{ 3+ }(aq)$$ in acidic medium.The unbalanced redox equation is as follows:$${ { Cr }_{ 2 }{ O }_{ 7 }^{ 2- }+H }_{ 2 }{ O }_{ 2 }\longrightarrow { Cr }^{ 3+ }+{ O }_{ 2 }$$Balance all atoms other than H and O.$${ { Cr }_{ 2 }{ O }_{ 7 }^{ 2- }+H }_{ 2 }{ O }_{ 2 }\longrightarrow 2{ Cr }^{ 3+ }+{ O }_{ 2 }$$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.$${ { Cr }_{ 2 }{ O }_{ 7 }^{ 2- }+3H }_{ 2 }{ O }_{ 2 }\longrightarrow 2{ Cr }^{ 3+ }+3{ O }_{ 2 }$$To balance O atoms, add 7 water molecules on RHS.$${ { Cr }_{ 2 }{ O }_{ 7 }^{ 2- }+3H }_{ 2 }{ O }_{ 2 }\longrightarrow 2{ Cr }^{ 3+ }+3{ O }_{ 2 }$$To balance H atoms, add 8 $$H^+$$ on LHS.$${ { Cr }_{ 2 }{ O }_{ 7 }^{ 2- }+3H }_{ 2 }{ O }_{ 2 }+8{ H }^{ + }\longrightarrow 2{ Cr }^{ 3+ }+3{ O }_{ 2 }+7{ H }_{ 2 }O$$This is the balanced chemical equation.Chemistry

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