Chemistry · Higher
Electrolysis
Breaking down ionic compounds using electricity, electrode products, and aluminium extraction.
Electrolysis uses electricity to decompose ionic compounds that are molten or dissolved in water. Positive ions go to the cathode; negative ions go to the anode.
Key ideas
• **Electrolyte** – ionic compound that is molten or in solution (ions free to move)
• **Cathode** – negative electrode; positive ions (cations) are attracted and gain electrons (reduction)
• **Anode** – positive electrode; negative ions (anions) are attracted and lose electrons (oxidation)
In molten binary salts (e.g. PbBr₂): metal at cathode, non-metal at anode.
Worked example
In molten lead bromide, what forms at the cathode?
Solution
**Lead** (Pb²⁺ ions gain electrons → Pb).
Aqueous solutions (simplified)
In water, H⁺ and OH⁻ are also present.
Often:
• At cathode – the less reactive of the metal ion / hydrogen is produced
• At anode – oxygen (from OH⁻) unless a halide is present in reasonable concentration (then halogen may form)
Rules can vary with concentration – learn the examples your specification requires.
Aluminium extraction
Aluminium oxide is dissolved in molten cryolite (lowers melting point).
• Cathode: Al³⁺ + 3e⁻ → Al
• Anode: 2O²⁻ → O₂ + 4e⁻ (carbon anodes gradually burn away forming CO₂)
Practice questions
Try these without looking at the answers first.
Q1.Which electrode are positive ions attracted to?
Show answer
Cathode (negative)
Q2.What is an electrolyte?
Show answer
Molten or dissolved ionic compound that conducts
Q3.Why is cryolite used in aluminium extraction?
Show answer
Lowers the melting point of aluminium oxide