Electrolysis — Principles

This lesson covers the principles of electrolysis as required by the AQA GCSE Combined Science Trilogy specification (8464). You need to understand what electrolysis is, the conditions required, the role of electrodes, and what happens at each electrode — including half equations for higher tier.

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What Is Electrolysis?

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

Key Conditions

• The ionic compound must be molten or in aqueous solution so that the ions are free to move.
• In a solid ionic compound, the ions are held in a fixed lattice and cannot move — so solid ionic compounds do not conduct electricity and cannot be electrolysed.

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The Electrolysis Setup

graph TD
    PS["Power Supply<br/>(DC)"] --> |"—"| Cat["CATHODE<br/>(negative electrode)"]
    PS --> |"+"| An["ANODE<br/>(positive electrode)"]
    Cat --> Elec["Electrolyte<br/>(molten or aqueous<br/>ionic compound)"]
    An --> Elec

    subgraph At the Cathode
        CatR["Positive ions (CATIONS)<br/>move here<br/>↓<br/>GAIN electrons<br/>= REDUCTION"]
    end

    subgraph At the Anode
        AnR["Negative ions (ANIONS)<br/>move here<br/>↓<br/>LOSE electrons<br/>= OXIDATION"]
    end

    style PS fill:#f9a825,color:#000
    style Cat fill:#1565c0,color:#fff
    style An fill:#d32f2f,color:#fff
    style Elec fill:#7b1fa2,color:#fff

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Key Terminology

Term	Definition
Electrolyte	The molten or dissolved ionic compound that is decomposed
Electrode	A solid conductor through which electricity enters and leaves the electrolyte
Cathode	The negative electrode (connected to the negative terminal of the power supply)
Anode	The positive electrode (connected to the positive terminal of the power supply)
Cation	A positive ion (e.g. Na⁺, Cu²⁺, H⁺) — moves to the cathode
Anion	A negative ion (e.g. Cl⁻, O²⁻, OH⁻) — moves to the anode

Exam Tip: Remember that Positive ions go to the Negative electrode (cathode) and Negative ions go to the Positive electrode (anode). Opposites attract!

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What Happens at Each Electrode?

At the Cathode (Negative Electrode)

• Positive ions (cations) are attracted to the cathode.
• They gain electrons from the electrode.
• This is reduction (gain of electrons — RIG).

At the Anode (Positive Electrode)

• Negative ions (anions) are attracted to the anode.
• They lose electrons to the electrode.
• This is oxidation (loss of electrons — OIL).

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Electrolysis of Molten Ionic Compounds

When a molten (pure liquid) ionic compound is electrolysed, only two ions are present, making the products straightforward to predict.

Example — Molten Lead Bromide (PbBr₂)

Ion	Charge	Moves To	Electrode Process	Product
Pb²⁺	Positive	Cathode	Gains 2 electrons → Pb	Lead metal
Br⁻	Negative	Anode	Loses 1 electron → Br atoms → Br₂	Bromine gas

Half equations (Higher Tier):

At the cathode (reduction): $\text{Pb}^{2+} + 2e^- \rightarrow \text{Pb}$Pb2++2e−→Pb

At the anode (oxidation): $2\text{Br}^- \rightarrow \text{Br}_2 + 2e^-$2Br−→Br2​+2e−

Example — Molten Sodium Chloride (NaCl)

At the cathode: $\text{Na}^+ + e^- \rightarrow \text{Na}$Na++e−→Na (sodium metal)

At the anode: $2\text{Cl}^- \rightarrow \text{Cl}_2 + 2e^-$2Cl−→Cl2​+2e− (chlorine gas)

Example — Molten Aluminium Oxide (Al₂O₃)

At the cathode: $\text{Al}^{3+} + 3e^- \rightarrow \text{Al}$Al3++3e−→Al (aluminium metal)

At the anode: $2\text{O}^{2-} \rightarrow \text{O}_2 + 4e^-$2O2−→O2​+4e− (oxygen gas)

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Extraction of Aluminium by Electrolysis

Aluminium is extracted from purified aluminium oxide (alumina) dissolved in molten cryolite (Na₃AlF₆):

Feature	Detail
Why cryolite?	It lowers the melting point of aluminium oxide from ~2050°C to ~950°C, reducing energy costs
Cathode product	Molten aluminium — sinks to the bottom and is tapped off
Anode product	Oxygen gas
Anode replacement	The oxygen reacts with the carbon anodes, forming CO₂, so they must be replaced regularly
Cost	Very expensive due to the large and continuous supply of electricity required

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Inert Electrodes

In many electrolysis experiments, inert electrodes (e.g. graphite or platinum) are used. Inert means the electrodes do not react with the electrolyte or the products. Graphite is commonly used because it is cheap, conducts electricity, and has a high melting point.

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Common Mistakes

Mistake	Correction
Saying "positive ions go to the positive electrode"	Positive ions (cations) are attracted to the negative electrode (cathode) — opposites attract
Confusing oxidation and reduction at the electrodes	At the cathode: reduction (gain of electrons). At the anode: oxidation (loss of electrons)
Forgetting that the compound must be molten or dissolved	In a solid, the ions are not free to move and cannot carry the current
Writing "gaining electrons = oxidation"	Gaining electrons = reduction (RIG). Losing electrons = oxidation (OIL)

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Worked Example

Question: During the electrolysis of molten calcium chloride, predict the products at each electrode and write half equations.

Answer:

• At the cathode: Ca²⁺ ions are reduced → calcium metal is produced. Half equation: Ca²⁺ + 2e⁻ → Ca
• At the anode: Cl⁻ ions are oxidised → chlorine gas is produced. Half equation: 2Cl⁻ → Cl₂ + 2e⁻

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Summary

• Electrolysis decomposes a molten or dissolved ionic compound using an electric current.
• Cations (positive ions) move to the cathode (negative electrode) and are reduced (gain electrons).
• Anions (negative ions) move to the anode (positive electrode) and are oxidised (lose electrons).
• For molten compounds, the only products are the metal at the cathode and the non-metal at the anode.
• Aluminium is extracted by electrolysis of aluminium oxide dissolved in cryolite — cryolite lowers the melting point and reduces energy costs.
• Carbon anodes must be replaced regularly because the oxygen produced reacts with them.

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Extended Worked Examples

Worked Example 1 — Predicting Products of Molten Electrolysis

Question: Molten magnesium chloride (MgCl₂) is electrolysed with inert electrodes. State the products and write half-equations.