The correct option is A 4Zn+NO−3+7OH−⟶NH3+4ZnO2−2+2H2O
The unbalanced redox equation is as follows:
Zn+NO−3+OH−⟶NH3+ZnO2−2
All atoms other than H and O are balanced.
The oxidation number of Zn changes from 0 to 2. The change in the oxidation number of Zn is 2.
The oxidation number of N changes from 5 to -3. The change in the oxidation number is 8.
The increase in the oxidation number is balanced with decrease in the oxidation number by multiplying Zn and ZnO2−2 with 4.
4Zn+NO−3+OH−⟶NH3+4ZnO2−2
O atoms are balanced by adding 4 water molecules on LHS.
4Zn+NO−3+OH−+4H2O⟶NH3+4ZnO2−2
To balance H atoms, add 6 H+ on RHS.
4Zn+NO−3+OH−+4H2O⟶NH3+4ZnO2−2+6H+
Since the reaction occurs in basic medium, add 6 hydroxide ions on both sides of the equation.
4Zn+NO−3+4H2O+7OH−⟶NH3+4ZnO2−2+6H++6OH−
On RHS, 6 H+ ions combine with 6 OH− ions to form 6 water molecules out of which 4 cancel out with 4 water molecules on LHS.
4Zn+NO−3+7OH−⟶NH3+4ZnO2−2+2H2O
This is the balanced chemical equation.