What Are Redox Reactions?
Chemical reactions are happening all around us — iron rusting, batteries powering our phones, food being digested. Many of these involve a special category of reactions called redox reactions. The name "redox" combines two words: reduction and oxidation, because these two processes always occur together.
A chemical reaction in which oxidation and reduction occur simultaneously. One species loses electrons while another gains them.
The key idea is that redox reactions involve the transfer of electrons from one substance to another. You cannot have oxidation without reduction — they are always a pair. Think of it like a game of catch: if one player throws the ball (loses electrons), another player must catch it (gains electrons).
Imagine you have a £10 note and you give it to a friend. You have lost £10 and your friend has gained £10. Electron transfer in redox reactions works the same way — one atom gives up electrons and another receives them. The total number of electrons is always conserved.
Redox reactions underpin some of the most important processes in science and technology, from photosynthesis and cellular respiration in biology, to electroplating and metal extraction in industry.
Oxidation and Reduction — Defining the Terms
Students often confuse oxidation and reduction. The most reliable way to remember them is through electron transfer:
The loss of electrons by a species during a chemical reaction. A species that is oxidised loses electrons and its oxidation state increases.
The gain of electrons by a species during a chemical reaction. A species that is reduced gains electrons and its oxidation state decreases.
A classic memory aid used by chemists worldwide:
OIL RIG — Oxidation Is Loss, Reduction Is Gain (of electrons). Write this on your notes page and memorise it. It will save you in every redox question you ever encounter!
Historically, oxidation was defined as the addition of oxygen to a substance (for example, iron rusting or wood burning). Reduction was defined as the removal of oxygen. These older definitions are still valid and useful for simpler reactions, but the electron transfer definition is more universal and works for all redox reactions — even those that don't involve oxygen at all.
Summary of definitions:
| Process | Electrons | Oxidation State | Old definition |
|---|---|---|---|
| Oxidation | Lost | Increases | Gains oxygen / loses hydrogen |
| Reduction | Gained | Decreases | Loses oxygen / gains hydrogen |
Many students mix up oxidation and reduction. Remember: oxidation = loss. Even though "oxidation" sounds like it should involve gaining oxygen (which it does in the older definition), in terms of electrons, it means losing them. Trust OIL RIG!
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