CHE 4 Balancing RedOx Reactions, Page 2

Electrochemistry Preliminary, Page 2

Step 3

Balance two half-reactions

From the previous page, we identify two half-reactions:

Rules to follow in order to balance each equation:

Step	To Do
1	Balance atoms other than H and O
2	Balance O by adding H₂O
3	Balance H by adding H⁺
4	Balance charge by adding e^-

Using the above example, the half-reactions are balanced in the following way using the table above.

Reduction Rxn	Oxidation Rxn

Explanations

Reduction Reaction Oxidation Reaction

Step 1 -- There is only Cl atom to adjust. The left-hand side contains one Cl. The right-hand contains one Cl, and therefore they are balanced.

Step 1 -- There is only one Cr atom on the both sides of the equation. Therefore, it is balanced in terms of atoms other than O and H.

Step 2 -- By adding one H₂O to the right-hand side of the eqution make Oxygen atoms balanced.

Step 2 -- On the right-hand side, there are two Oxygen atoms more than those of the left-hand side. So, add two H₂O to the left.

Step 3 -- Since two Hydrogens are introduced in Step 2, 2H⁺ must be added to the left-hand side of the equation.

Step 3 -- Since two H₂O (4 H's) are added at Step 2, four H⁺ must be added to the right-hand side.

Step 4 -- Overall charge on the right-hand side is -1 due to the charge on the Cl^- ion. The left-hand side has +1 before adding e^-. In order to make the charge on the left-hand side to come out to be the same as the right is to add 2e^- on the left.

Step 4 -- The charge on the left-hand side is -1. The righ-hand side is +2. Therefore, one should add three e^- on the right-hand side to match the overall charges on the both side.

Reduction Reaction	Oxidation Reaction
Step 1 -- There is only Cl atom to adjust. The left-hand side contains one Cl. The right-hand contains one Cl, and therefore they are balanced.	Step 1 -- There is only one Cr atom on the both sides of the equation. Therefore, it is balanced in terms of atoms other than O and H.
Step 2 -- By adding one H₂O to the right-hand side of the eqution make Oxygen atoms balanced.	Step 2 -- On the right-hand side, there are two Oxygen atoms more than those of the left-hand side. So, add two H₂O to the left.
Step 3 -- Since two Hydrogens are introduced in Step 2, 2H⁺ must be added to the left-hand side of the equation.	Step 3 -- Since two H₂O (4 H's) are added at Step 2, four H⁺ must be added to the right-hand side.
Step 4 -- Overall charge on the right-hand side is -1 due to the charge on the Cl^- ion. The left-hand side has +1 before adding e^-. In order to make the charge on the left-hand side to come out to be the same as the right is to add 2e^- on the left.	Step 4 -- The charge on the left-hand side is -1. The righ-hand side is +2. Therefore, one should add three e^- on the right-hand side to match the overall charges on the both side.

Hint for adding electrons: Since an electron, e^-, has a minus charge, you should add to the side of the equation that is more positive!

March, 2000, Nikita Matsunaga