Balancing REDOX Equations

Mr. ChapmanChemistry 30

In previous classes, we learned how to balance chemical equations by counting atoms.

Following the Law of Conservation of Mass, we learned that the number of atoms on each side of the chemical equation must be equal.

Many REDOX reactions cannot be balanced as easily.

Balancing Equations

Consider the following REDOX reaction:

Cu(s) + Ag+(aq) → Cu2+

(aq) + Ag(s)

• At first glance, this equation appears to be balanced. There are one copper and one silver atom on each side of the chemical equation.

• Why isn’t this equation considered balanced?

Balancing REDOX Reactions

Looking at the previous equation, it is clear that the charges are not balanced. On the left side of the equation, we have a charge of +1, but on the right side we have a charge of +2.

Charges represent gain or loss of electrons, and, like atoms, electrons are conserved during a chemical reaction – they are not created or destroyed.

Gain and Loss of Electrons

There are two techniques that we use to balance REDOX reactions:

1. Oxidation Number method2. Half Reaction method

• In this class, we will only be learning to use the half-reaction method.

Two Techniques to Balance

This technique involves breaking down the REDOX reaction into its two separate components: oxidation and reduction.

Because neither reaction can actually occur without the other (oxidation and reduction always go together), we call each component a half-reaction.

Half-Reaction Method of Balancing

1. The overall equation is broken down into two half-reactions. If there are any spectator ions, they are removed from the equations.

2. Each half reaction is balanced separately – first for atoms and then for charge. Electrons are added to one side of the equation or the other in order to balance charge. Example: If the reactant side of the equation has a total charge of +3, the product side must also equal +3.

Steps to Balancing by Half-Reactions

3. Next, the two equations are compared to make sure electrons lost equal electrons gained. One of the half reactions will be an oxidation reaction, the other will be a reduction reaction.

4. Finally, the two half reactions are added together, and any spectator ions that were removed are placed back into the equation.

Steps to Balancing by Half-Reactions

Balance the following REDOX reaction:

Au3+(aq)  +  I−(aq)    Au(s)  +  I2(s)

Practice Balancing REDOX