An oxidation-reduction (redox) reaction is one where one species loses electrons and another gains them. For example, the reaction

Cu⁺²(aq) + Zn(s) -> Zn⁺²(aq) + Cu(s)

When a species gains electrons, it is being reduced. (Think of the charge on the species reducing.) When a species loses electrons, it is being oxidized. In the above reaction, the zinc is being oxidized and the copper is being reduced.

• Oxidation: Zn(s) -> Zn⁺² + 2e^-
• Reduction: Cu⁺²(aq) + 2e^- -> Cu(s)

In all redox reactions, reduction and oxidation occur together: one cannot happen without the other. The electrons flow from one species to the other: there is no net charge gain or loss.

The species that causes another to be oxidized is called an oxidizing agent. It takes up the electrons that the oxidized species gives off and is therefore reduced. The reverse is true for a reducing agent. In the above reaction

• Zn is the reducing agent: it is oxidized
• Cu⁺² is the oxidizing agent: it is reduced

Example: In the reaction below, identify the oxidizing and reducing agents

Fe⁺³(aq) + Cu(s) -> Fe⁺²(aq) + Cu⁺(aq)

Solution:: Identify which species gained or lost electrons in the reaction. The charge on the iron went down, so it gained an electron. The charge on the copper went up, so it lost an electron. The iron was therefore reduced and the copper oxidized, and so

• Cu(s) was oxidized, it acted as a reducing agent
• Fe⁺³(aq) was reduced, it acted as an oxidizing agent

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