GCSEChemistry: Combined ScienceAQARevision Notes4. Chemical Changes4.3 ElectrolysisUsing Electrolysis to Extract Metals

Using Electrolysis to Extract Metals (AQA GCSE Chemistry: Combined Science)

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Extracting metals

Choosing methods for metal extraction

  • The position of the metal on the reactivity series determines the method of extraction
  • Higher placed metals (above carbon) have to be extracted using electrolysis as they are too reactive and cannot be reduced by carbon
  • Lower placed metals can be extracted by heating with carbon which reduces
  • Electrolysis is very expensive as large amounts of energy are required to melt the ores and produce the electrical current
  • The reactivity series of metals is shown below with the corresponding method of extraction

Extracting metals

Metal Extraction method
Most reactive
Potassium

Extracted by electrolysis of the molten chloride or oxide

Large amounts of electricity are required, which makes this an expensive process
Sodium
Lithium
Calcium
Magnesium
Aluminium
Zinc

Extracted by heating with a reducing agent such as carbon or carbon monoxide in a blast furnace

A cheap process as carbon is cheap and can also be a source of heat
Iron
Copper
Silver  Found as pure elements
Gold
Least reactive

Examiner Tip

Questions on this topic often ask you to explain why in some cases electrolysis is used and in other cases reduction by heating with carbon is used. Make sure you can explain when each process is used and why.

The extraction of aluminium

How is aluminium extracted?

  • Aluminium is a reactive metal, above carbon in the reactivity series  
  • Its main ore, is bauxite, which contains aluminium oxide
  • Aluminium is higher in the reactivity series than carbon, so it cannot be extracted by reduction using carbon
  • Instead, aluminium is extracted by electrolysis  

The electrolytic cell for extraction of aluminium

extraction of aluminium using electrolysis

Diagram showing the extraction of aluminium by electrolysis

  • Bauxite is first purified to produce aluminium oxide, Al2O3
  • Aluminium oxide is then dissolved in molten cryolite 
    • This is because aluminium oxide has a melting point of over 2000°C which would use a lot of energy and be very expensive
    • The resulting mixture has a lower melting point without interfering with the reaction
  • The mixture is placed in an electrolysis cell, made from steel, lined with graphite
  • The graphite lining acts as the negative electrode, with several large graphite blocks as the positive electrodes
  • Aluminium is produced at the cathode
  • Oxygen is produced At the anode 

Why does the anode need to be replaced?

  • The carbon in the graphite anodes reacts with the oxygen produced to produce CO2

C (s) + O2 (g)   →   CO2 (g)

  • As a result the anode wears away and has to be replaced regularly
  • A lot of electricity is required for this process of extraction, this is a major expense

Examiner Tip

Questions on this topic often ask you to explain why cryolite is used in the process so make sure you are able to explain its use, providing reasons for your answer.

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    Stewart

    Author: Stewart

    Expertise: Chemistry Lead

    Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Exam Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.