Steps for balancing redox reactions:

1) Assign oxidation numbers for all numbers.  See Assigning Oxidation Numbers

2) Identify which atoms change in oxidation number.

3) Write skeletal (without coefficients) half-reactions for the atoms that change oxidation number.

Half-reaction - The oxidation OR reduction part of the reaction that occurs in one half-cell.

4) Balance all atoms that are NOT hydrogen (H) or oxygen (O)

5) Balance the oxygen and hydrogen atoms using the following rules:

• In Acidic Solution:

  O:  balance each excess oxygen with ONE H₂O
  H:  balance each hydrogen with one H⁺

• In Basic Solution:

  O:  balance each excess oxygen with TWO OH^-
  H:  balance each 2 hydrogens with one H₂O

6) Balance the total charge by adding electrons.

• In the oxidation half-reaction, the electrons will be a product (RIGHT)
• In the reduction half-reaction, the electrons will be a reactant (LEFT)

7) If necessary, multiply each half-reaction by to get the lowest common multiple so that the total electrons for reduction (right) equal the total electrons for oxidation (left).  Add the two half-reactions together.

8) Cancel equal moles of any substance.  Electrons should also cancel out if balanced correctly.

9) CHECK to make sure that the MASS and CHARGE are balanced.

Example in Acidic Solution:

Assign oxidation numbers:

Write skeletal half-reactions for atoms that change oxidation number:

Balance all atoms that are not O or H:

Balance O by adding H₂O;  balance H by adding H⁺:

Balance total charge by adding electrons:

Multiply each half-reaction by to get the lowest common multiple so that the total electrons for reduction equal the total electrons for oxidation, and add the two half-reactions together:

• NOTE:  Remember to distribute the factor that you are multiplying to every term in the reaction.

Cancel equal moles of any substance.  Electrons should also cancel out if balanced correctly:

CHECK to make sure the mass and charge are balanced:

• NOTE:  In acidic solution redox reactions, H₂O and H⁺ are added (no OH^-).

Example in Basic Solution:

Assign oxidation numbers:

Write skeletal half-reactions for atoms that change oxidation number:

Since all atoms that are not O or H are already balanced, balance O by adding two OH^- to;  balance H by adding H₂O for every 2 hydrogens:

Balance total charge by adding electrons:

Multiply each half-reaction by to get the lowest common multiple so that the total electrons for reduction equal the total electrons for oxidation, and add the two half-reactions together:

• NOTE:  Remember to distribute the factor that you are multiplying to every term in the reaction.

Cancel equal moles of any substance.  Electrons should also cancel out if balanced correctly:

CHECK to make sure the mass and charge are balanced:

• NOTE:  In basic solution redox reactions, H₂O and OH^- are added (no H⁺).

Next:  "Galvanic (Voltaic) Cells"