Electrolysis

Electrolysis is a key chemical process used to break down substances using electricity. It’s important in industry for extracting reactive metals and purifying materials, and it also helps us understand how ions behave when electricity is passed through solutions.

Practicals on this page:
Electrolysis

What Is Electrolysis?

Electrolysis is the breakdown of an ionic compound using electricity.

It only works when:

• The compound is molten (melted) or dissolved in water (aqueous)
• Ions are free to move and carry the electric current

During electrolysis:

• Positive ions (cations) move to the negative electrode (cathode)
• Negative ions (anions) move to the positive electrode (anode)

This movement of ions allows chemical changes to occur at each electrode.

Electrolysis of Molten Ionic Compounds

When ionic compounds are molten, they contain only the metal and non-metal ions.

Example:
Molten lead bromide (PbBr₂)

• Pb²⁺ goes to the cathode → gains electrons → lead metal forms
• Br⁻ goes to the anode → loses electrons → bromine gas forms

This type of electrolysis is used in industry to extract reactive metals that can’t be reduced with carbon.

Extracting Metals Using Electrolysis

Metals that are more reactive than carbon (e.g. aluminium, potassium) must be extracted by electrolysis.

Example: Aluminium extraction from bauxite (aluminium oxide)

Aluminium oxide is dissolved in molten cryolite to lower the melting point

Electrolysis separates aluminium (at the cathode) and oxygen gas (at the anode)

This process requires a lot of energy, making it expensive.

Electrolysis of Aqueous Solutions

Aqueous solutions contain ions from the compound and ions from water (H⁺ and OH⁻).

This means more than one possible product can form at each electrode.

At the cathode (where reduction happens):

• If the metal is more reactive than hydrogen, hydrogen gas is produced
• If the metal is less reactive than hydrogen, the metal is deposited

At the anode (where oxidation happens):

• If the solution contains halide ions (Cl⁻, Br⁻, I⁻), the halogen is produced
• If no halide ions are present, oxygen gas is released from OH⁻

Examples of Electrolysis of Aqueous Solutions

Sodium chloride (NaCl) solution:

• Cathode: H⁺ → H₂ gas (Na is more reactive than hydrogen)
• Anode: Cl⁻ → Cl₂ gas (a halide is present)

Copper sulfate (CuSO₄) solution:

• Cathode: Cu²⁺ → Copper metal (less reactive than hydrogen)
• Anode: No halides, so OH⁻ → O₂ gas

Half Equations

Half equations show what happens to the ions at each electrode during electrolysis. They include the gain or loss of electrons.

Reduction (gain of electrons) – occurs at the cathode
Example:

Oxidation (loss of electrons) – occurs at the anode
Example:

Writing half equations helps you understand the electron transfer in redox reactions.

Also see Chemical Changes, Rates of Chemical Change, Energy Changes

PRACTICAL - Electrolysis

In this practical, you investigate the products formed when an aqueous ionic solution is electrolysed using inert electrodes (like graphite or carbon rods).

A common example is the electrolysis of copper(II) chloride solution:

• At the cathode (negative electrode): copper metal is deposited
• At the anode (positive electrode): chlorine gas is produced

You’ll:

• Set up the circuit with a power supply and two inert electrodes
• Place them into the electrolyte solution
• Collect and observe the products at each electrode (e.g. bubbles, solid deposits)

This helps you understand ion movement, displacement, and how to identify products based on the reactivity series and presence of halide ions.

Links for Learning

BBC Bitesize: Electrolysis
Study Rocket: Electrolysis
Revise Chemistry: Electrolysis
Revise Science: Electrolysis
Study Mind: Explaining Electrolysis