Oxidation-Reduction Reactions

Learning Goals

You will learn how to balance oxidation-reduction reactions in neutral, acidic, and basic solutions.

Synopsis

Oxidation-reduction reactions are extremely important in our daily life. A battery is an electrochemical cell in which a redox reaction is occurring and produces the needed voltage for some application. Such cells are called either voltaic cells or galvanic cells.

It is very important that you review the material in Chapter 5 about oxidation-reduction reactions so that you can perform the first step below. The process of balancing an oxidation-reduction reaction can be summarized as follows:

1. First recognize the reaction as an oxidation-reduction reaction by determining which species are reduced and which are oxidized.
2. Dissect the overall reaction into an oxidation reaction and a reduction reaction (called half reactions).
3. Use necessary stoichiometric coefficients to obtain mass balance of each reaction.
4. Achieve charge balance of each reaction by adding electrons appropriately.
5. Multiply both sides of each reaction by the appropriate number to achieve charge balance for each reaction.
6. Add together the two modified half-reactions.
7. Check the resulting equation for mass and charge balance.

For a neutral solution the above process is very straightforward. In an acidic or basic solution, step 3 requires special treatment. Let's look first at the procedure if the solution is acidic:

3a. Balance all atoms except oxygen and hydrogen. Then balance the oxygen atoms by adding H₂O molecules (enough to balance the oxygen atoms) to the side deficient in oxygen. Now add enough hydrogen ions (H⁺) to the side deficient in hydrogen atoms to balance the hydrogen atoms.

You then proceed with steps 4-7 as above.

Now consider the modification of step 3 if the solution is basic:

3b. Balance all atoms except oxygen and hydrogen. Now balance the oxygen atoms. For every oxygen atom needed, add 2 hydroxyl ions (OH^-) to the side needing oxygen atoms, and for every 2 hydroxyl ions added, add one H₂O molecule to the other side.

Now proceed with steps 4-7 as above.

1. Balance the following oxidation-reduction reaction that is carried out in an acidic solution. Clearly show what is reduced and what is oxidized. Br^- + MnO_4- -->Br₂ + Mn²⁺
2. Balance the following oxidation-reduction reaction that is carried out in a basic solution. Clearly show what is reduced and what is oxidized. Cl₂(g) --> Cl^-(aq) + ClO^-(aq)

On-Line Activity

1. Here is a site for more information on balancing redox reactions in a basic solution: basic
2. Here is a site for more information on balancing redox reactions in an acidic solution: acidic
3. Here is a site for more information on determining oxidation states: states