How to Balance Redox Reactions

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Balancing Redox Reactions - Half-Reaction Method

This is a diagram that describes the half-reactions of a redox reaction.
This is a diagram that describes the half-reactions of a redox reaction or oxidation-reduction reaction. Cameron Garnham, Creative Commons License

To balance redox reactions, assign oxidation numbers to the reactants and products to determine how many moles of each species are needed to conserve mass and charge. 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+

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Balancing Redox Reactions - Separate the Reactions

Batteries are a common example of a product that uses redox reactions.
Batteries are a common example of a product that uses redox reactions. Maria Toutoudaki, Getty Images

Separate the two half reactions:

I- → I2

MnO4- → Mn2+

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Balancing Redox Reactions - Balance the Atoms

Balance the number and type of atoms before dealing with charge.
Balance the number and type of atoms before dealing with charge. Tommy Flynn, Getty Images

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

2

O to balance the O atoms and H

+

to balance the H atoms. In a basic solution, we would use OH

-

and H

2

O to balance the O and 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 4 waters molecules:

MnO4- + 8 H+ → Mn2+ + 4 H2O

The two half-reactions are now balanced for atoms:

MnO4- + 8 H+ → Mn2+ + 4 H2O

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Balancing Redox Reactions - Balance the Charge

Add electrons into the equation to balance charge.
Add electrons into the equation to balance charge. Newton Daly, Getty Images

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

Now multiple the oxidations numbers so that the two half-reactions will have the same number of electrons and can cancel each other out:

5(2I- → I2 +2e-)

2(5e- + 8H+ + MnO4- → Mn2+ + 4H2O)

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Balancing Redox Reactions - Add the Half-Reactions

Add half reaction after balancing mass and charge.
Add half reaction after balancing mass and charge. Joos Mind, Getty Images

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 final equation:

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

Get 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

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Balancing Redox Reactions - Check Your Work

Check your work to make sure it makes sense.
Check your work to make sure it makes sense. David Freund, Getty Images

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.

Review:

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-reactions 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.