Balancing Redox reaction by Oxidation Number Method
The various steps involved in the balancing of redox equations according to this method are :
1. Write skeleton equation and indicate oxidation number of each element and thus identify the elements undergoing change in oxidation number.
2. Determine the increase and decrease of oxidation number per atom. Multiply the increase (or) decrease of oxidation number of atoms undergoing the change.
3. Equalise the increase in oxidation number and decrease in oxidation number on the reactant side by multiplying the respective formulae with suitable integers.
4. Balance the equation with respect to all atoms other than O and H atoms.
5. Balance oxygen by adding equal number of water molecules to the side falling short of oxygen atoms.
6. H atoms are balanced depending upon the medium in same way as followed in ion electron method.
Let us balance the following equations by oxidation number method.
MnO2 + Cl- -> Mn2+ + Cl2 + H2O (in acidic medium)
MnO2 + Cl- - > Mn2+ + Cl2 + H2O
MnO2 (4+) + Cl (-1) -> Mn (+2) + Cl2 + H2O
O.N. Decreases by 2 per Mn : MnO2 -> Mn
O.N. increases by 1 per Cl : Cl -> Cl2
Equalise the increase / decrease in O.N by multiply MnO2 by 1 and Cl-1 by 2.
MnO2 + 2 Cl- -> Mn2+ + Cl2 + H2O
Balance other atoms except H and O. Here they are all balanced.
Balance O atoms by adding H2O molecules to the side falling short of oxygen atoms.
MnO2 + 2Cl- -> Mn2+ + Cl2 + H2O + H2O
Balance H atoms by adding H+ ions to the side falling short of H atoms
MnO2 + 2Cl- + 4H+ -> Mn2+ + Cl2 + 2H2O
Problems for practice
Balance the following equations
1. Mg + NO3- -> Mg2+ + N2O + H2O (in acidic medium)
2. Cr3+ + Na2O2 -> CrO4- + Na+
3. S2- + NO3- -> NO + S
4. FeS + O2 -> Fe2O3 + SO2 (molecular form)
5. Cl2 + OH- -> Cl- + ClO3- + H2O