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Some of the most important reactions in chemistry — burning a fuel, the rusting of iron, and extracting a metal from its ore — are all examples of oxidation and reduction. At GCSE these two ideas always go together, because whenever something is oxidised, something else is reduced. This lesson, part of Topic C3 of OCR Gateway Science A, defines oxidation and reduction in terms of oxygen, gives the key examples, introduces the term redox, and (for Higher tier) extends the definitions to the loss and gain of electrons.
By the end of this lesson you should be able to define oxidation and reduction in terms of oxygen, identify which substance is oxidised and which is reduced in a reaction, explain redox, and (Higher tier) define oxidation and reduction in terms of electrons using "OIL RIG".
The original meaning of these two words is all about oxygen:
The names make sense once you see them in action: when a substance gains oxygen it is oxidised; when oxygen is removed from a compound, that compound is reduced (its mass is "reduced" as the oxygen leaves).
Combustion is oxidation — a fuel gains oxygen. When magnesium burns, it gains oxygen to form magnesium oxide:
2Mg+O2→2MgO
The magnesium has been oxidised (it has gained oxygen).
The rusting (corrosion) of iron is also oxidation. Iron reacts slowly with oxygen and water to form hydrated iron(III) oxide — rust:
4Fe+3O2→2Fe2O3
The iron has gained oxygen, so it has been oxidised. Corrosion is simply the gradual oxidation of a metal by the oxygen in air and water.
Extracting a metal by reduction with carbon. Many metals are found in nature as oxides, and a metal less reactive than carbon can be extracted by heating its oxide with carbon. For example, iron is extracted from iron(III) oxide in the blast furnace:
2Fe2O3+3C→4Fe+3CO2
Here the iron(III) oxide loses oxygen to become iron — the iron oxide has been reduced. The carbon, meanwhile, gains oxygen to become carbon dioxide, so the carbon has been oxidised.
Exam Tip: Keep the definitions tied to oxygen: "Oxidation is gain of oxygen; reduction is loss of oxygen." Then, in any example, identify which substance gained oxygen (oxidised) and which lost it (reduced).
Notice in the iron-extraction example that both processes happened in the same reaction: the iron oxide was reduced and the carbon was oxidised. This is always true — oxidation and reduction always occur together, because the oxygen that one substance loses is exactly the oxygen another substance gains. A reaction in which both occur is called a redox reaction (from reduction–oxidation).
| Substance | What happens to it | Oxidised or reduced? |
|---|---|---|
| Iron(III) oxide, Fe2O3 | Loses oxygen → iron | Reduced |
| Carbon, C | Gains oxygen → carbon dioxide | Oxidised |
So in 2Fe2O3+3C→4Fe+3CO2, the carbon is the reducing agent (it removes oxygen from the iron oxide) and the iron oxide is being reduced.
Exam Tip: If a question gives you a reaction and asks "which substance is oxidised and which is reduced?", track the oxygen: the one that gains oxygen is oxidised; the one that loses oxygen is reduced. Both answers should be given.
The rusting of iron is the most familiar — and most expensive — example of oxidation in everyday life, so it is worth looking at more closely. Corrosion is the gradual oxidation of a metal by substances in its environment, and for iron and steel that means reaction with both oxygen and water: remove either one and rusting stops. This is why iron rusts quickly outdoors but stays bright in dry air or in water that has had the air boiled out of it.
Knowing that both oxygen and water are needed tells you how to prevent rusting — by keeping them away from the metal surface:
| Method | How it prevents rusting |
|---|---|
| Painting / oiling / greasing | Forms a barrier that keeps oxygen and water off the metal |
| Coating with plastic | A barrier coating, used for items such as garden furniture and bike frames |
| Galvanising (coating with zinc) | A barrier and sacrificial protection — the more reactive zinc is oxidised in preference to the iron |
Galvanising is especially clever. Zinc is more reactive than iron, so even if the zinc layer is scratched, the zinc corrodes (is oxidised) in preference to the iron underneath. This is called sacrificial protection — a more reactive metal is deliberately oxidised to protect a less reactive one. The same idea is used by bolting blocks of magnesium to steel ships' hulls and underground pipes: the magnesium is oxidised instead of the iron.
Exam Tip: Rusting needs both oxygen and water. Barrier methods (paint, oil, plastic) exclude them; sacrificial protection uses a more reactive metal (zinc or magnesium) that is oxidised in preference to the iron.
The idea that a metal can be extracted by reducing its oxide with carbon links directly to the reactivity series. Carbon can only remove the oxygen from the oxide of a metal that is less reactive than carbon itself — such as iron, zinc, tin and lead. These metals hold onto their oxygen less tightly than carbon does, so carbon can "steal" it, reducing the metal oxide while the carbon is oxidised.
Metals more reactive than carbon — such as aluminium, magnesium, sodium and potassium — hold their oxygen too strongly for carbon to remove, so they cannot be extracted this way. Instead they are extracted by electrolysis (the subject of a later lesson), which uses electrical energy to force the reduction. So the position of a metal in the reactivity series decides how it is extracted: reduction with carbon for metals below carbon, electrolysis for metals above it. This is a neat example of redox underpinning a whole branch of industrial chemistry, and it is exactly the kind of synoptic link examiners reward.
Exam Tip: A metal can be extracted by reduction with carbon only if it is less reactive than carbon (e.g. iron, zinc, lead). Metals more reactive than carbon (e.g. aluminium) must be extracted by electrolysis.
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