How to Balance Redox Reactions

The Half-Reaction Method

First, separate the equation into two half-reactions: the oxidation portion, and the reduction portion. This is called the half-reaction method of balancing redox reactions, or the ion-electron method. Each half-reaction is balanced separately and then the equations are added together to give a balanced overall reaction. We want the net charge and number of ions to be equal on both sides of the final balanced equation.

For this example, let's consider a redox reaction between KMnO₄and HI in an acidic solution:

MnO₄^- + I^- → I₂ + Mn²⁺

Separate the Reactions

Separate the two half-reactions:

I^- → I₂

MnO₄^- → Mn²⁺

Balance the Atoms

To balance the atoms of each half-reaction, first balance all of the atoms except H and O. For an acidic solution, next add H.

Balance the iodine atoms:

2 I^- → I₂

The Mn in the permanganate reaction is already balanced, so let's balance the oxygen:

MnO₄^- → Mn²⁺ + 4 H₂O

Add H⁺ to balance the water molecules:

MnO₄^- + 8 H⁺ → Mn²⁺ + 4 H₂O

The two half-reactions are now balanced for atoms:

MnO₄^- + 8 H⁺ → Mn²⁺ + 4 H₂O

Balance the Charge

Next, balance the charges in each half-reaction so that the reduction half-reaction consumes the same number of electrons as the oxidation half-reaction supplies. This is accomplished by adding electrons to the reactions:

2 I^- → I₂ + 2e^-

5 e^- + 8 H⁺ + MnO₄^- → Mn²⁺ + 4 H₂O

Next, multiply the oxidation numbers so that the two half-reactions have the same number of electrons and can cancel each other out:

5(2I^- → I₂ +2e^-)

2(5e^- + 8H⁺ + MnO₄^- → Mn²⁺ + 4H₂O)

Add the Half-Reactions

Now add the two half-reactions:

10 I^- → 5 I₂ + 10 e^-

16 H⁺ + 2 MnO₄^- + 10 e^- → 2 Mn²⁺ + 8 H₂O

This yields the following equation:

10 I^- + 10 e^- + 16 H⁺ + 2 MnO₄^- → 5 I₂ + 2 Mn²⁺ + 10 e^- + 8 H₂O

Simplify the overall equation by canceling out the electrons and H₂O, H⁺, and OH^- that may appear on both sides of the equation:

10 I^- + 16 H⁺ + 2 MnO₄^- → 5 I₂ + 2 Mn²⁺ + 8 H₂O

Check Your Work

Check your numbers to make certain that the mass and charge are balanced. In this example, the atoms are now stoichiometrically balanced with a +4 net charge on each side of the reaction.

In summary:

• Step 1: Break reaction into half-reactions by ions.
• Step 2: Balance the half-reactions stoichiometrically by adding water, hydrogen ions (H⁺) and hydroxyl ions (OH^-) to the half-reactions.
• Step 3: Balance the half-reactions charges by adding electrons to the half-reactions.
• Step 4: Multiply each half-reaction by a constant so both reactions have the same number of electrons.
• Step 5: Add the two half-reactions together. The electrons should cancel out, leaving a balanced complete redox reaction.