Electrolysis — Principles

This lesson covers the fundamental principles of electrolysis for Edexcel GCSE Chemistry (1CH0). You need to understand what electrolysis is, know the key terminology, explain how ions move during electrolysis, and identify oxidation and reduction at each electrode.

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What Is Electrolysis?

Electrolysis is the process of using electricity to decompose (break down) an ionic compound that is either molten (melted) or dissolved in water (aqueous).

The word "electrolysis" comes from the Greek:

• Electro = electricity
• Lysis = splitting/breaking down

Key Definition

Electrolysis is the decomposition of an electrolyte using a direct current (d.c.) of electricity.

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Key Terminology

Term	Definition
Electrolyte	A molten or dissolved ionic compound that conducts electricity and is decomposed during electrolysis
Electrode	A solid conductor (usually metal or graphite) through which the electric current enters or leaves the electrolyte
Anode	The positive electrode (connected to the positive terminal of the power supply)
Cathode	The negative electrode (connected to the negative terminal of the power supply)
Cation	A positively charged ion (e.g. Na⁺, Cu²⁺, H⁺, Al³⁺)
Anion	A negatively charged ion (e.g. Cl⁻, O²⁻, OH⁻, Br⁻)
Direct current (d.c.)	Electrical current that flows in one direction only (required for electrolysis)

Exam Tip: Remember: AN OX and RED CAT. Oxidation happens at the ANode; REDuction happens at the CAThode. Or simply: OILRIG — Oxidation Is Loss, Reduction Is Gain (of electrons).

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Why Must the Electrolyte Be Molten or Dissolved?

In a solid ionic compound, the ions are held in a fixed position in a giant ionic lattice. They cannot move, so they cannot carry charge, and electrolysis cannot occur.

When the ionic compound is melted or dissolved in water, the ionic lattice breaks down and the ions become free to move. They can then migrate towards the electrodes when a voltage is applied.

• Cations (positive ions) are attracted to the cathode (negative electrode).
• Anions (negative ions) are attracted to the anode (positive electrode).

This movement of ions is what allows the electrolyte to conduct electricity. The ions are the charge carriers in the liquid.

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The Electrolysis Cell

graph TB
    PS["D.C. Power Supply<br/>+         −"]

    PS -->|"+ terminal"| A["ANODE<br/>(positive electrode)"]
    PS -->|"− terminal"| C["CATHODE<br/>(negative electrode)"]

    subgraph Electrolyte["Electrolyte (molten or aqueous ionic compound)"]
        AN["Anions (−)<br/>migrate to anode →"]
        CA["← Cations (+)<br/>migrate to cathode"]
    end

    A --- AN
    C --- CA

    A -->|"Anions lose electrons<br/>(OXIDATION)"| P1["Product at anode<br/>(e.g. Cl₂, O₂, Br₂)"]
    C -->|"Cations gain electrons<br/>(REDUCTION)"| P2["Product at cathode<br/>(e.g. metal, H₂)"]

    style A fill:#e74c3c,color:#fff
    style C fill:#3498db,color:#fff
    style PS fill:#2c3e50,color:#fff
    style P1 fill:#e67e22,color:#fff
    style P2 fill:#27ae60,color:#fff

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Movement of Ions

When the d.c. power supply is switched on:

1. Cations (positive ions, e.g. Na⁺, Cu²⁺, Pb²⁺) are attracted to the cathode (negative electrode). Opposite charges attract.
2. Anions (negative ions, e.g. Cl⁻, Br⁻, O²⁻) are attracted to the anode (positive electrode). Opposite charges attract.

Remembering the Direction

• Cations go to the Cathode
• Anions go to the Anode

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Oxidation and Reduction at the Electrodes

At the Cathode (Negative Electrode) — REDUCTION

Cations arrive at the cathode and gain electrons from the electrode. This is reduction (gain of electrons).

Examples:

• Pb²⁺(l) + 2e⁻ → Pb(l) — lead ions are reduced to lead metal
• Cu²⁺(aq) + 2e⁻ → Cu(s) — copper ions are reduced to copper metal
• 2H⁺(aq) + 2e⁻ → H₂(g) — hydrogen ions are reduced to hydrogen gas

At the Anode (Positive Electrode) — OXIDATION

Anions arrive at the anode and lose electrons to the electrode. This is oxidation (loss of electrons).

Examples:

• 2Cl⁻(l) → Cl₂(g) + 2e⁻ — chloride ions are oxidised to chlorine gas
• 2Br⁻(l) → Br₂(g) + 2e⁻ — bromide ions are oxidised to bromine gas
• 2O²⁻(l) → O₂(g) + 4e⁻ — oxide ions are oxidised to oxygen gas

The OIL RIG Mnemonic

This is essential for remembering what happens at each electrode:

• Oxidation Is Loss (of electrons) — at the anode
• Reduction Is Gain (of electrons) — at the cathode

Exam Tip: Half equations are frequently tested. When writing them, make sure the charges balance on both sides. Count the total positive and negative charges, then add electrons to balance. The electrons should be on the left for reduction (cathode) and on the right for oxidation (anode).

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Inert vs Reactive Electrodes

Inert Electrodes

Inert electrodes do not react with the electrolyte or the products. They simply provide a surface for the reaction to occur and a path for electrons to flow.

Common inert electrode materials:

• Graphite (carbon) — cheap and readily available, but can be brittle
• Platinum — excellent but very expensive

Reactive Electrodes

Reactive electrodes take part in the electrolysis reaction. For example, when copper electrodes are used in the electrolysis of copper sulfate solution, the copper anode dissolves and copper is deposited on the cathode. This is the basis of copper purification (covered in a later lesson).

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Conditions Required for Electrolysis

For electrolysis to work, you need:

1. An electrolyte — a molten or dissolved ionic compound
2. Two electrodes — dipped into the electrolyte
3. A d.c. power supply — direct current (not alternating current)

If any of these conditions is missing, electrolysis will not occur.

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Summary

• Electrolysis is the decomposition of a molten or dissolved ionic compound using d.c. electricity.
• The electrolyte must be molten or dissolved so that the ions are free to move.
• Cations (positive ions) move to the cathode (negative electrode) where they are reduced (gain electrons).
• Anions (negative ions) move to the anode (positive electrode) where they are oxidised (lose electrons).
• OIL RIG: Oxidation Is Loss, Reduction Is Gain.
• Inert electrodes (graphite, platinum) do not react; reactive electrodes participate in the reaction.
• D.C. (direct current) is required — not a.c.

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Worked Examples: Writing Half-Equations

Half-equations are among the most examined skills in electrolysis questions. Always check that atoms balance and charges balance.