|Lecture Notes: 9 December||
In these reactions we see a transfer of electrons from one atom or molecule to another. First let's look at some terms.
Notice that the methane is oxidized by the oxygen. We say that the carbon and hydrogen are both oxidized to give the new covalent products, water and carbon dioxide.
As another example we can look at a key oxidation reaction in glycolysis, the central pathway of metabolism. Don't worry about these reactions - they will not be on an exam. They are presented for your interest.
Balancing Redox Equations
There are two common methods for balancing redox reactions: the oxidation number method and the half-reaction method. The half-reaction method works very well for ionic reactions, it is relatively easy to give partial credit, and it is the only method I will use in this class. If you know how to do the other method you are welcome to do so, but be careful to make sure you show your work or I won't be able to give partial credit!
The Half-Reaction Method
In the half-reaction method what we do is first break an equation into two parts and then balance the parts individually. Presented stepwise:
- Separate the reaction into two half-reactions.
- Balance each half-reaction separately:
- Balance atoms other than O & H by inspection.
- Balance O by adding H2O to the opposite side.
- Balance H:
- In acid solution add H+ as appropriate.
- In base solution add H2O to the side needing H, balance O by adding equal numbers of OH- to the opposite side.
- Balance the charge by adding electrons (e-) - add to same side as excess of positive charge, or opposite side if excess negative charge.
- Balance the charges of the two half-reactions by multiplying appropriately, then add to two equations together and cancel items appearing on both sides.
Example. Balance the following equation as it occurs in acid solution:
MnO4- + Cl- Mn2+ + Cl2
© R A Paselk
Last modified 10 December 2009