Half Reactions and Oxidation Numbers
Assign oxidation numbers to the following:
a) Hg (e) PbI 2 (i) CrO 3-2
b) Ag +1 (f) K 2r 2 7 (j) Al2O 3
c) FeO (g) - NiO (k) CaCO 3
d) KClO 3 (h) BrO 3 (l) NaNO 3
In order for one chemical species to be oxidized, another must be reduced. Because of this, we can talk about
oxidation and reduction as processes that are done to something or processes that are done by something.
The species that is oxidized is called a reducing agent.
The species that is reduced is called an oxidizing agent.
Although oxidation and reduction happen simultaneously, it’s sometimes useful to write them separately. This
involves the use of “half-reactions.”
2 K + I 2 2KI
This can be written as two half reactions:
The oxidation of potassium: 2 K 2 K + 2 e-
The reduction of iodine: I + 2 e 2 I -
The potassium is oxidized, therefore it is a reducing agent
The iodine is reduced, therefore it is an oxidizing agent.
The potassium is oxidized by iodine. (* note: the agent must be a reactant)
The iodine is reduced by potassium.
**Remember, the word oxidation comes from the word oxygen. Oxygen is a strong oxidizing agent, which
means that in many reactions, oxygen will be reduced.
For the f