Redox reactions
In an oxidation-reduction (redox) reaction, electrons move from one substance to another.
- Oxidation is the loss of electrons. The oxidation number increases.
- Reduction is the gain of electrons. The oxidation number decreases.
A memory aid is OIL RIG: oxidation is loss, reduction is gain.
The two always happen together, because the electrons one substance loses must go somewhere.
- The reducing agent is the substance that is oxidized. It gives electrons away.
- The oxidizing agent is the substance that is reduced. It takes electrons.
Worked example: 2 Mg + O2 → 2 MgO
| Atom | Before | After | Change |
|---|---|---|---|
| Mg | 0 | +2 | Lost 2 electrons: oxidized |
| O | 0 | -2 | Gained 2 electrons: reduced |
Magnesium is oxidized and is the reducing agent. Oxygen is reduced and is the oxidizing agent.
Rusting, burning, batteries and cellular respiration are all redox processes.
Common mistake
Thinking reduction means the oxidation number gets bigger because something was “gained”. Electrons are negative, so gaining them makes the number go down.