Oxidation, Reduction and Redox

This lesson covers oxidation, reduction and redox reactions as required by the Edexcel GCSE Chemistry specification (1CH0), Topic 4. You need to understand the definitions of oxidation and reduction in terms of oxygen transfer and electron transfer, identify what is oxidised and reduced in reactions, and write half equations (Higher tier).

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Definitions of Oxidation and Reduction

There are two ways to define oxidation and reduction at GCSE level.

Definition 1: In Terms of Oxygen

Term	Definition	Example
Oxidation	Gain of oxygen	Magnesium + oxygen → magnesium oxide (Mg gains O)
Reduction	Loss of oxygen	Copper oxide + carbon → copper + carbon dioxide (CuO loses O)

Definition 2: In Terms of Electrons (Higher)

Term	Definition	Memory Aid
Oxidation	Loss of electrons	Oxidation Is Loss (OIL)
Reduction	Gain of electrons	Reduction Is Gain (RIG)

graph LR
    subgraph "OIL RIG"
    A["<b>O</b>xidation<br/><b>I</b>s<br/><b>L</b>oss<br/>(of electrons)"] --- B["<b>R</b>eduction<br/><b>I</b>s<br/><b>G</b>ain<br/>(of electrons)"]
    end

    style A fill:#e74c3c,color:#fff
    style B fill:#3498db,color:#fff

Exam Tip: Learn OIL RIG — it is the single most important mnemonic for this topic. Oxidation Is Loss (of electrons); Reduction Is Gain (of electrons). This applies to all redox reactions.

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Redox Reactions

A redox reaction is a reaction in which both oxidation and reduction occur simultaneously. One substance is oxidised while another is reduced. You cannot have one without the other.

Why Are They Simultaneous?

When one substance loses electrons (oxidation), those electrons must go somewhere — they are gained by another substance (reduction). Electrons are transferred from one species to another.

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Identifying Oxidation and Reduction: Oxygen Transfer

Example 1: Reduction of Copper Oxide by Carbon

Word equation: copper oxide + carbon → copper + carbon dioxide

Symbol equation: 2CuO(s) + C(s) → 2Cu(s) + CO₂(g)

Analysis:

• Copper oxide is reduced — it loses oxygen (CuO → Cu)
• Carbon is oxidised — it gains oxygen (C → CO₂)

Example 2: Burning Magnesium in Air

Word equation: magnesium + oxygen → magnesium oxide

Symbol equation: 2Mg(s) + O₂(g) → 2MgO(s)

Analysis:

• Magnesium is oxidised — it gains oxygen

In this reaction, oxygen itself is reduced, but at foundation tier you just need to identify that magnesium is oxidised.

Example 3: Iron Oxide Reduced by Carbon

Word equation: iron(III) oxide + carbon → iron + carbon dioxide

Symbol equation: 2Fe₂O₃(s) + 3C(s) → 4Fe(s) + 3CO₂(g)

Analysis:

• Iron(III) oxide is reduced — it loses oxygen
• Carbon is oxidised — it gains oxygen
• This is a redox reaction (both oxidation and reduction occur)

Example 4: Hydrogen Reducing Copper Oxide

Word equation: copper oxide + hydrogen → copper + water

Symbol equation: CuO(s) + H₂(g) → Cu(s) + H₂O(l)

Analysis:

• Copper oxide is reduced — it loses oxygen
• Hydrogen is oxidised — it gains oxygen
• Carbon is not involved — hydrogen acts as the reducing agent

Exam Tip: The substance that is oxidised is called the reducing agent (it reduces the other substance by giving it electrons or taking its oxygen). The substance that is reduced is called the oxidising agent. This can be confusing — just remember: the reducing agent is the one being oxidised.

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Identifying Oxidation and Reduction: Electron Transfer (Higher)

Example 1: Magnesium Burning in Oxygen

Symbol equation: 2Mg(s) + O₂(g) → 2MgO(s)

Electron transfer analysis:

• Each magnesium atom loses 2 electrons: Mg → Mg²⁺ + 2e⁻ (oxidation)
• Each oxygen atom gains 2 electrons: O + 2e⁻ → O²⁻ (reduction)

Example 2: Iron Displacing Copper

Ionic equation: Fe(s) + Cu²⁺(aq) → Fe²⁺(aq) + Cu(s)

Electron transfer analysis:

• Iron loses electrons: Fe → Fe²⁺ + 2e⁻ (oxidation — iron is the reducing agent)
• Copper ions gain electrons: Cu²⁺ + 2e⁻ → Cu (reduction — copper ions are the oxidising agent)

Example 3: Zinc Displacing Copper

Ionic equation: Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)

• Zn → Zn²⁺ + 2e⁻ (oxidation)
• Cu²⁺ + 2e⁻ → Cu (reduction)

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Half Equations (Higher)

A half equation shows what happens to one element in a redox reaction. It shows either the oxidation or the reduction separately.

Writing Half Equations

Step 1: Write the reactant and product for one element. Step 2: Balance the atoms. Step 3: Add electrons to balance the charges.

Example: Iron + Copper Sulfate

Oxidation half equation (iron):

Fe → Fe²⁺ + 2e⁻

(Iron atom loses 2 electrons to form an iron(II) ion)

Reduction half equation (copper):

Cu²⁺ + 2e⁻ → Cu

(Copper(II) ion gains 2 electrons to form a copper atom)

Check: The number of electrons lost in the oxidation half equation equals the number gained in the reduction half equation (both 2e⁻). This must always balance.

Example: Chlorine Displacing Bromide

Reduction half equation (chlorine):

Cl₂ + 2e⁻ → 2Cl⁻

(Each chlorine molecule gains 2 electrons to form 2 chloride ions)

Oxidation half equation (bromide):

2Br⁻ → Br₂ + 2e⁻

(Two bromide ions each lose 1 electron to form a bromine molecule)

Exam Tip: When writing half equations, always check that: (1) the atoms balance, (2) the charges balance, and (3) the electrons are on the correct side — electrons on the RIGHT for oxidation (loss), electrons on the LEFT for reduction (gain).

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Linking Redox to Other Topics

Redox reactions appear in many areas of GCSE chemistry:

Topic	Redox Connection
Displacement reactions	More reactive metal is oxidised; less reactive metal ion is reduced
Extraction of metals	Metal ore is reduced to obtain the metal; carbon (or electricity) provides the electrons
Electrolysis	Reduction at the cathode (metal ions gain electrons); oxidation at the anode (non-metal ions lose electrons)
Reactions of metals with acid	Metal is oxidised (loses electrons); hydrogen ions are reduced (gain electrons)
Combustion	Fuel is oxidised (gains oxygen); oxygen is the oxidising agent
Rusting	Iron is oxidised to form iron oxide

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Oxidising and Reducing Agents

Term	Role in the Reaction	What Happens to It
Oxidising agent	Causes another substance to be oxidised (takes electrons from it)	Is itself reduced (gains electrons)
Reducing agent	Causes another substance to be reduced (gives electrons to it)	Is itself oxidised (loses electrons)

Common oxidising agents: oxygen, halogens (Cl₂, Br₂), acidified potassium manganate(VII) Common reducing agents: carbon, hydrogen, reactive metals (Mg, Zn, Fe)

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Summary