Oxidation and Reduction

This lesson covers oxidation and reduction (redox reactions) as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand the definitions of oxidation and reduction in terms of oxygen transfer and electron transfer, identify which species is oxidised and which is reduced in a reaction, and recognise oxidising and reducing agents.

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Oxidation and Reduction — Oxygen Transfer

The simplest definitions of oxidation and reduction involve oxygen:

Term	Definition (Oxygen Transfer)
Oxidation	The gain of oxygen
Reduction	The loss of oxygen

Remembering the Definitions

A useful mnemonic for the oxygen-transfer definitions is:

Oxidation Is Gain (of oxygen), Reduction Is Loss (of oxygen)

But be careful — when we move to electron transfer, the mnemonic changes.

Example 1: Burning Magnesium

2Mg + O₂ → 2MgO

• Magnesium gains oxygen → magnesium is oxidised.
• Oxygen is the oxidising agent (it provides the oxygen / causes the oxidation).

Example 2: Reduction of Copper Oxide with Carbon

2CuO + C → 2Cu + CO₂

• Copper oxide loses oxygen → copper is reduced.
• Carbon gains oxygen → carbon is oxidised.
• Carbon is the reducing agent — it causes the copper oxide to be reduced.
• Copper oxide is the oxidising agent — it provides oxygen to the carbon.

Exam Tip: Oxidation and reduction always happen together in the same reaction. If one substance is oxidised, another must be reduced. That is why these reactions are called redox reactions (reduction + oxidation).

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Oxidation and Reduction — Electron Transfer

A more precise and fundamental definition involves electrons:

Term	Definition (Electron Transfer)
Oxidation	The loss of electrons
Reduction	The gain of electrons

OIL RIG

The most important mnemonic in chemistry for redox:

Oxidation Is Loss (of electrons)

Reduction Is Gain (of electrons)

graph LR
    A["OIL<br/>Oxidation<br/>Is<br/>Loss<br/>(of electrons)"] --- B["RIG<br/>Reduction<br/>Is<br/>Gain<br/>(of electrons)"]

    style A fill:#e74c3c,color:#fff
    style B fill:#2980b9,color:#fff

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Applying Electron Transfer to Reactions

Example 1: Reaction of Sodium with Chlorine

2Na + Cl₂ → 2NaCl

What happens to each element:

• Sodium loses one electron each: Na → Na⁺ + e⁻ → oxidation
• Chlorine gains one electron each: Cl + e⁻ → Cl⁻ → reduction

Sodium is the reducing agent (it gives away electrons, causing chlorine to be reduced). Chlorine is the oxidising agent (it takes electrons, causing sodium to be oxidised).

Example 2: Reaction of Magnesium with Hydrochloric Acid

Mg + 2HCl → MgCl₂ + H₂

• Magnesium loses 2 electrons: Mg → Mg²⁺ + 2e⁻ → oxidised
• Hydrogen ions gain electrons: 2H⁺ + 2e⁻ → H₂ → reduced

Example 3: Displacement — Iron in Copper Sulfate

Fe + CuSO₄ → FeSO₄ + Cu

• Iron loses 2 electrons: Fe → Fe²⁺ + 2e⁻ → oxidised
• Copper ions gain 2 electrons: Cu²⁺ + 2e⁻ → Cu → reduced

Iron is the reducing agent; copper sulfate (Cu²⁺) is the oxidising agent.

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Oxidising Agents and Reducing Agents

Term	What It Does	What Happens to It
Oxidising agent	Causes another substance to be oxidised (causes loss of electrons)	It is reduced (gains electrons)
Reducing agent	Causes another substance to be reduced (causes gain of electrons)	It is oxidised (loses electrons)

This is a common source of confusion: the oxidising agent is itself reduced, and the reducing agent is itself oxidised.

Exam Tip: A very common exam question is: "Identify the substance that is oxidised and the substance that is reduced." Use OIL RIG — look for which species loses electrons (oxidised) and which gains electrons (reduced). Also know that the reducing agent is the one that gets oxidised.

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Identifying Redox in Different Types of Reaction

Reaction Type	Oxidation	Reduction
Metal + oxygen	Metal gains oxygen (oxidised)	—
Metal oxide + carbon	Carbon gains oxygen (oxidised)	Metal oxide loses oxygen (reduced)
Metal + acid	Metal loses electrons to form ions (oxidised)	H⁺ ions gain electrons to form H₂ (reduced)
Displacement	More reactive metal loses electrons (oxidised)	Less reactive metal ions gain electrons (reduced)
Electrolysis	At the anode (+), negative ions lose electrons (oxidised)	At the cathode (−), positive ions gain electrons (reduced)

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Bringing Both Definitions Together

The two definitions of oxidation and reduction are consistent with each other:

Process	Oxygen Definition	Electron Definition
Oxidation	Gain of oxygen	Loss of electrons
Reduction	Loss of oxygen	Gain of electrons

When a metal reacts with oxygen, the metal atoms lose electrons to form positive ions while the oxygen atoms gain electrons to form oxide ions. The oxygen definition and the electron definition describe the same process from different perspectives.

Exam Tip: At GCSE, you should be able to use both definitions. If the question involves oxygen transfer, use the oxygen definition. If it involves ions and electrons, use OIL RIG. Many marks are available for correctly identifying oxidation and reduction in equations.

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Redox in Everyday Life

Redox reactions are not just laboratory curiosities — they occur everywhere:

Process	Oxidation	Reduction
Rusting	Iron is oxidised (Fe → Fe₂O₃)	Oxygen is reduced
Combustion	Fuel is oxidised	Oxygen is reduced
Respiration	Glucose is oxidised	Oxygen is reduced
Batteries	Metal electrode is oxidised	Another substance is reduced
Photosynthesis	Water is oxidised	Carbon dioxide is reduced

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Summary