Balancing Redox Reactions

Balancing redox reactions can be pretty simple for some system, but some redox reactions can be exceptionally challenging. To balance any redox reaction, we can follow a systematic set of steps, and every Gen Chem book happily provides a set of steps. In my experience, most books use rules that require a little bit of faith and function like a black-box. There are a LOT of places that mistakes can be made when balancing redox reactions, so I prefer to use rules that have built-in places to check my answer before I go through the whole process. Here they are:

Balancing redox rules (in acidic or neutral aqueous solutions):

1. Assign oxidation numbers to all atoms in the equation

2. Identify the oxidation and reduction half reaction

3. In each half reaction, balance all atoms except hydrogen and oxygen

4. In each half reaction, add electrons to the reactant or product side to balance the change in oxidation state. Note: you are not adding electrons to balance charge

5. In each half reaction, add water molecules to balance any oxygen atoms

6. In each half reaction, add H⁺(aq) to balance any hydrogen atoms

7. At this point, the half reactions should be balanced, check the charge balance to confirm

8. Multiply each half reaction by an appropriate integer to balance the electrons involved in the oxidation and reduction processes

9. Add the half reactions together

10. Again, the resulting reaction should be balanced, check the charge balance to confirm

11. Cancel out any spectator species, sit back and pat yourself on the back for writing such a lovely balanced redox equation.

These rules work well, BUT rely upon some assumptions. First, since we're using water and H⁺(aq), the reaction must be taking place in aqueous solution that is neutral or acidic. That's OK for most redox reactions in Gen Chem, but once in a while we'll run into a rxn that takes place in basic aqueous solution. How do we handle that? Think about it... basic solutions have excess (relatively speaking) hydroxide ions. Hydroxide ions react with H⁺(aq) ions to form water. If a reaction is taking place in basic aqueous solution, balance it according to the above rules and then add:

12. Add enough OH^-1(aq) to each side to react with all the H⁺(aq) that is present

13. Check charge balance

14. Cancel any excess water

As with any process, practice is the key, so practice balancing redox rxns, then practice a little more, and when you think you have it all figured out, practice a couple more times. We'll do some of that in class...