Rules for Balancing Redox Reactions by Ion Electron Method (Half-Reaction Method)

Rules for Balancing Redox Reactions by Ion Electron Method (Half-Reaction Method)

1. Ion-electron method/half-reaction method (IEM or HRM)
2. Oxidation number change method
3. Aggregate redox species method (or ARS method)

Jette and LaMev developed the method for balancing redox-reactions by ion electron method in 1927.

1.    Convert molecular equation into ionic equation.

2.    Split ionic equation into two half-reactions or equations (one for the species that is oxidized and its products and for the species that is reduced and its products).

3.      Balance the atoms other than O and H by inserting suitable coefficients in each half reaction separately.

4.   (a).   For reactions in an acidic medium, add one H₂O for each O atom to balance O and one H⁺  for each H atom to balance H. The O atoms are balanced first.

      (b) For reactions in a basic medium, to balance O atom add two OH^– per O atom to the side   that is deficient in O while add one H₂O on the other side at the same time. To balance H  atom, add one H₂O per H atom to the side where H is less while add one OH^– on the  other side at   the same time.

                                                            OR

    For reactions in a basic medium, balance O and H atoms as balanced in acidic medium. Then   add OH^– ions to both sides of the equation equal to the number of H⁺ ions.  (If there is no H⁺   then do not add OH^– ions). Where H⁺ and OH^– appear on the same side of the equation,   combine the ions to give H₂O.

5.  Balance the charge in each half equation by inserting e^– (electrons) as a reactant or product.

6.    Equalize the loss and gain of electrons in both half equations by multiplying one or both half reactions by appropriate co-efficients.

7.    Add the two half equations after canceling electrons. If H⁺, OH^– or H₂O appears on both sides of the final equation, cancel out the duplications.

(To verify equation is atomically and electrically balanced, count net charges on both sides of the equation, if charges are same then equation is balanced).

Balance the following equation in acidic medium by I.E.M. 

Solution

Splitting of ionic equation into two half-reactions

After writing oxidation number of those species that undergo any change, split ionic equation into two half reactions:

 

Balancing of Equation No. I

Overall Redox Reaction

To balance e^–, oxidation half-equation is multiplied by 5 (so that 10e^– are produced) and the reduction half-equation is multiplied by 2 (so that the same 10e^– are consumed) and then add resultant equations after canceling e^–.

 

Solution

First molecular equation splits into ionic form and then oxidation number of only those species have been written who have undergone any change (the species that do not undergo any change in oxidation no. do not appear in equation).

 

Balance the following equation in basic medium by I.E.M

 

Solution

Splitting of ionic equation into two half-reactions

After writing oxidation number of those species that undergo any change, split ionic equation into two half reaction:

Overall Redox Reaction
To equalize the number of electrons, eq. I is multiplied by 2 and eq. II by 3 and then resultant equations are added.

 

Balance the following equation in Basic medium by I.E.M.

Solution
Splitting of ionic equation into two half-reactions
After writing oxidation number of those species that undergo any change, split ionic equation into two half reactions: