Oxidation and Reduction

Some of the most important reactions in all of chemistry — burning a fuel, iron rusting on a gate, pulling a metal out of its ore — are examples of oxidation and reduction. At GCSE these two ideas always travel together, because whenever one substance is oxidised, another is reduced at the same time. This lesson, part of Topic C3 of OCR Gateway Combined Science, 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 what redox means, and (Higher tier) define oxidation and reduction in terms of electrons using "OIL RIG".

This lesson builds AO1 recall of the oxidation, reduction and redox definitions, AO2 application when you assign oxidation and reduction (by oxygen or, at Higher, by electrons) in a given equation, and AO3 analysis when you decide which species has been oxidised and which reduced.

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Oxidation and Reduction in Terms of Oxygen

The original meaning of these two words is all about oxygen:

• Oxidation is the gain of oxygen by a substance.
• Reduction is the loss of oxygen from a substance.

The names make sense once you watch them in action: gain oxygen and you are oxidised; take oxygen away from a compound and that compound is reduced (its mass is "reduced" as the oxygen leaves).

Examples of Oxidation

Combustion is oxidation — a fuel gains oxygen. When magnesium burns, it gains oxygen to become magnesium oxide:

$2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$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:

$4\text{Fe} + 3\text{O}_2 \rightarrow 2\text{Fe}_2\text{O}_3$4Fe+3O2​→2Fe2​O3​

The iron has gained oxygen, so it has been oxidised. Corrosion is simply the slow oxidation of a metal by the oxygen in air and water.

Example of Reduction

Extracting a metal by reduction with carbon. Many metals occur naturally as oxides, and a metal less reactive than carbon can be won by heating its oxide with carbon. Iron, for instance, is extracted from iron(III) oxide in the blast furnace:

$2\text{Fe}_2\text{O}_3 + 3\text{C} \rightarrow 4\text{Fe} + 3\text{CO}_2$2Fe2​O3​+3C→4Fe+3CO2​

Here the iron(III) oxide loses oxygen to become iron — so the iron oxide has been reduced. The carbon, in turn, gains oxygen to become carbon dioxide, so the carbon has been oxidised.

Exam Tip: Keep the definitions pinned to oxygen: "Oxidation is gain of oxygen; reduction is loss of oxygen." Then in any example, work out which substance gained oxygen (oxidised) and which lost it (reduced).

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Redox: Both Happen Together

Notice in the iron-extraction example that both processes occurred in the one reaction: the iron oxide was reduced and the carbon was oxidised. This is always the case — oxidation and reduction always happen together, because the oxygen one substance loses is exactly the oxygen another substance gains. A reaction where both occur is a redox reaction (from reduction–oxidation).

Substance	What happens to it	Oxidised or reduced?
Iron(III) oxide, $\text{Fe}_2\text{O}_3$Fe2​O3​	Loses oxygen → iron	Reduced
Carbon, $\text{C}$C	Gains oxygen → carbon dioxide	Oxidised

So in $2\text{Fe}_2\text{O}_3 + 3\text{C} \rightarrow 4\text{Fe} + 3\text{CO}_2$2Fe2​O3​+3C→4Fe+3CO2​, the carbon acts as the reducing agent (it removes oxygen from the iron oxide), and the iron oxide is the substance being reduced.

Exam Tip: If a question gives a reaction and asks "which substance is oxidised and which is reduced?", follow the oxygen: the substance that gains oxygen is oxidised; the one that loses oxygen is reduced. Give both answers.

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Corrosion and How It Is Prevented

The rusting of iron is the most familiar — and most expensive — example of oxidation in everyday life, so it is worth a closer look. 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: take away either one and rusting stops. That 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 shows you how to prevent rusting — keep them off the metal:

Method	How it prevents rusting
Painting / oiling / greasing	Forms a barrier that keeps oxygen and water away from the metal
Coating with plastic	A barrier coating, used for things like 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 neat. Zinc is more reactive than iron, so even if the zinc layer gets scratched, the zinc corrodes (is oxidised) in preference to the iron beneath. This is sacrificial protection — a more reactive metal is deliberately oxidised to protect a less reactive one. The same trick 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.

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Reactivity and Extracting Metals by Reduction

The idea that a metal can be extracted by reducing its oxide with carbon links straight to the reactivity series. Carbon can only strip 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 their oxygen less tightly than carbon does, so carbon can take it, reducing the metal oxide while the carbon is oxidised.

Metals more reactive than carbon — such as aluminium, magnesium, sodium and potassium — grip their oxygen too tightly 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 a metal's place in the reactivity series decides how it is extracted: reduction with carbon for metals below carbon, electrolysis for metals above it. This is a tidy example of redox underpinning a whole branch of industrial chemistry.

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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Higher Tier: Oxidation and Reduction in Terms of Electrons

Higher tier: The oxygen definitions are really a special case of a broader idea based on electrons. In terms of electron transfer:

• Oxidation Is Loss of electrons.
• Reduction Is Gain of electrons.

The mnemonic is OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).

When magnesium burns, for example, the magnesium atom loses two electrons to become a $\text{Mg}^{2+}$Mg2+ ion, so it is oxidised:

$\text{Mg} \rightarrow \text{Mg}^{2+} + 2e^-$Mg→Mg2++2e−

The oxygen gains those electrons to become $\text{O}^{2-}$O2− ions, so it is reduced:

$\text{O}_2 + 4e^- \rightarrow 2\text{O}^{2-}$O2​+4e−→2O2−

The electron definition is more powerful because it works even when there is no oxygen involved at all.

Displacement Reactions as Redox

Higher tier: A displacement reaction, where a more reactive metal pushes a less reactive metal out of solution, is a redox reaction. Zinc, for instance, displaces copper from copper(II) sulfate solution:

$\text{Zn} + \text{CuSO}_4 \rightarrow \text{ZnSO}_4 + \text{Cu}$Zn+CuSO4​→ZnSO4​+Cu

Following the electrons:

• The zinc atom loses two electrons to become $\text{Zn}^{2+}$Zn2+ — it is oxidised ($\text{Zn} \rightarrow \text{Zn}^{2+} + 2e^-$Zn→Zn2++2e−).
• The copper ion gains two electrons to become copper metal — it is reduced ($\text{Cu}^{2+} + 2e^- \rightarrow \text{Cu}$Cu2++2e−→Cu).

The sulfate ions ($\text{SO}_4^{2-}$SO42−​) are spectators. So a displacement reaction is a transfer of electrons from the more reactive metal to the ions of the less reactive metal — a redox reaction.

Exam Tip: Remember OIL RIG: Oxidation Is Loss, Reduction Is Gain of electrons. In a displacement reaction the more reactive metal is oxidised (loses electrons) and the less reactive metal ion is reduced (gains electrons).

Worked Example — Identifying oxidation and reduction by electrons