http://www.ck12.org Chapter 22. Oxidation-Reduction Reactions
22.3 Balancing Redox Reactions
Lesson Objectives
- Balance a redox equation using the oxidation-number-change method.
- Balance a redox equation by breaking it down into oxidation and reduction half-reactions and using the half-
reaction method.
Check Your Understanding
Recalling Prior Knowledge
- How are oxidation numbers determined?
- What is a half-reaction, and how can two half-reactions be written from a single redox reaction?
You learned how to balance equations by inspection in an earlier chapter. Redox reactions can be considerably
more complex, and balancing solely by inspection can sometimes be nearly impossible. Here you will learn two
systematic processes for balancing redox equations.
Oxidation-Number-Change Method
One way to balance redox reactions is by keeping track of the extent to which the oxidation numbers change for each
of the atoms. For the oxidation-number-change method, start with the unbalanced skeleton equation. The example
below is for the reaction of iron(III) oxide with carbon monoxide. This reaction takes place in blast furnaces during
the processing of iron ore into metallic iron (Figure22.6).
Fe 2 O 3 (s)+CO(g)→Fe(s)+CO 2 (g)Step 1: Assign oxidation numbers to each of the atoms in the equation.
+ 3
Fe 2− 2
O 3 (s)++ 2
C− 2
O(g)→0
Fe(s)++ 4
C− 2
O 2 (g)Step 2: Identify the atoms that are oxidized and those that are reduced. In the above equation, the carbon atom
is being oxidized, since its oxidation number increases from +2 to +4. The iron atom is being reduced, since its
oxidation number decreases from +3 to 0.
Step 3: Use a line to connect the atoms that are undergoing a change in oxidation number, and indicate by how many
units each oxidation number is changing.