Industrial Electrolysis: Aluminium Extraction and the Chlor-Alkali Process — AQA GCSE Chemistry

Electrolysis is used industrially to extract reactive metals and to make useful chemicals.

Extracting aluminium

Aluminium is more reactive than carbon, so it must be extracted from its ore (aluminium oxide) by electrolysis:

• The aluminium oxide is melted (its melting point is lowered by dissolving it in molten cryolite to save energy).
• Aluminium forms at the cathode (negative electrode) — Al³⁺ + 3e⁻ → Al.
• Oxygen forms at the anode (positive electrode). The oxygen reacts with the hot carbon (graphite) anodes, which burn away and must be replaced regularly — adding to the cost.

Electrolysis is expensive because it uses a lot of electrical energy to melt the ore and drive the reaction.

The chlor-alkali process

Electrolysis of concentrated sodium chloride solution (brine) produces three useful products:

• Hydrogen at the cathode,
• Chlorine at the anode, and
• Sodium hydroxide solution left behind.

These are used to make bleach, plastics (PVC), and many other products.

Exam tips

• Aluminium is extracted by electrolysis because it is more reactive than carbon.
• Cryolite lowers the melting point to save energy.
• The carbon anodes burn away (react with oxygen) and must be replaced.
• The chlor-alkali process gives hydrogen, chlorine and sodium hydroxide from brine.