Redox reactions: electron transfer and changing oxidation states
An oxidation–reduction (redox) reaction is a chemical reaction in which electrons are transferred and the oxidation states of some elements change. Oxidation and reduction always occur together: an electron cannot be lost unless another particle accepts it.
Oxidation is loss of electrons and an increase in oxidation state.
Reduction is gain of electrons and a decrease in oxidation state.
How to recognize a redox reaction
- Assign oxidation states to the relevant elements in reactants and products.
- Find atoms whose oxidation states change.
- An increase marks oxidation; a decrease marks reduction.
- Check that the number of electrons lost equals the number gained.
For example, in Zn + CuSO4 → ZnSO4 + Cu, zinc changes from 0 to +2, while copper changes from +2 to 0. Zinc loses two electrons and copper ions accept the same two electrons. The sulfate ion does not change and acts as a spectator.
Oxidizing and reducing agents
| Participant | What it does | What happens to it |
|---|---|---|
| Oxidizing agent | Accepts electrons from another substance | Is reduced |
| Reducing agent | Donates electrons to another substance | Is oxidized |
The names describe what an agent does to the other substance. Therefore, the oxidizing agent itself is reduced, and the reducing agent itself is oxidized.
Half-reactions and electron balance
A redox process can be separated into two half-reactions:
Oxidation: Mg → Mg2+ + 2e−
Reduction: Cu2+ + 2e− → Cu
Overall: Mg + Cu2+ → Mg2+ + Cu