Question
Describe the steps involved in balancing redox reactions by the oxidation number method.
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Answer
$i.$ Step $1:$ Write the unbalanced equation for redox reaction. Balance the equation for all atoms in the reactions, except $H$ and $O.$ Identify the atoms which undergo change in oxidation number and by how much. Draw the bracket to connect atoms of the elements that changes the oxidation number.
$ii.$ Step $2:$ Show an increase in oxidation number per atom of the oxidised species and hence, the net increase in oxidation number. Similarly, show a decrease in the oxidation number per atom of the reduced species and the net decrease in oxidation number. Determine the factors which will make the total increase and decrease equal. Insert the coefficients into the equation.
$iii.$ Step $3:$ Balance oxygen atoms by adding $H_2O$ to the side containing less $O$ atoms, one $H_2O$ is added for one $O$ atom.
Balance $H$ atoms by adding $H^+$ ions to the side having less $H$ atoms.
$iv.$ Step $4:$ If the reaction occurs in basic medium, then add $OH^–$ ions equal to the number of $H^+$ ions added in step $3,$ on both the sides of equation. The $H^+$ and $OH^–$ ions on same side of reactions are combined to give $H_2O$ molecules.
$v.$ Step $5:$ Check the equation with respect to both, the number of atoms of each element and the charges. It is balanced.
Note: For acidic medium, step $4$ is omitted.
$ii.$ Step $2:$ Show an increase in oxidation number per atom of the oxidised species and hence, the net increase in oxidation number. Similarly, show a decrease in the oxidation number per atom of the reduced species and the net decrease in oxidation number. Determine the factors which will make the total increase and decrease equal. Insert the coefficients into the equation.
$iii.$ Step $3:$ Balance oxygen atoms by adding $H_2O$ to the side containing less $O$ atoms, one $H_2O$ is added for one $O$ atom.
Balance $H$ atoms by adding $H^+$ ions to the side having less $H$ atoms.
$iv.$ Step $4:$ If the reaction occurs in basic medium, then add $OH^–$ ions equal to the number of $H^+$ ions added in step $3,$ on both the sides of equation. The $H^+$ and $OH^–$ ions on same side of reactions are combined to give $H_2O$ molecules.
$v.$ Step $5:$ Check the equation with respect to both, the number of atoms of each element and the charges. It is balanced.
Note: For acidic medium, step $4$ is omitted.
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