Extraction of Metals (SQA National 5 Chemistry): Revision Note

Methods of metal extraction

• Most metals are found in the Earth's crust not as pure elements, but chemically bonded to other elements in compounds

• A rock that contains enough of a metal compound to be worth extracting is called an ore

Extraction of metals

• To get the pure metal, it must be extracted from its ore

• In the ore, the metal exists as a positive ion

• The extraction of a metal from its ore is always a reduction reaction

• The positive metal ions must gain electrons to become neutral metal atoms

• For example:

Al³⁺ + 3e^- → Al

• The method used for extraction depends entirely on the metal's reactivity

• Very reactive metals form very stable compounds, which are difficult to break down, and therefore require more energy to extract

Extraction by heat

• This method is suitable for the least reactive metals, such as silver, gold, and mercury

• The compounds of these metals are unstable and easily decompose

• Heating the ore provides enough energy to break the bonds and release the metal

• For example, the extraction of silver from silver oxide:

silver oxide → silver + oxygen

2Ag₂O (s) → 4Ag (s) + O₂ (g)

• Each silver ion (Ag⁺) gains one electron:

Ag⁺ + e^- → Ag

Extraction by heating with carbon (or carbon monoxide)

• This method is suitable for metals in the middle of the electrochemical series, such as copper, lead, tin, iron, and zinc

• These metals are too reactive to be extracted by heat alone

• They must be heated in a furnace with a reducing agent

  • A reducing agent is a substance that removes oxygen

  • Carbon (in the form of coke) or carbon monoxide (CO) are commonly used reducing agents

• For example, the extraction of iron in a blast furnace:

iron(III) oxide + carbon monoxide → iron + carbon dioxide

Fe₂O₃ (s) + 3CO (g) → 2Fe (l) + 3CO₂ (g)

• Each iron(III) ion (Fe³⁺) gains three electrons:

Fe³⁺ + 3e^- → Fe

The blast furnace

Extraction by electrolysis

• This method is suitable for the most reactive metals, such as potassium, sodium, calcium, magnesium, and aluminium

• These metals form very stable compounds that cannot be reduced by carbon

• Electrolysis, which uses a powerful direct electric current to break down the compound, is required

  • This process is very expensive due to the high energy demand

• For example, the extraction of aluminium from aluminium oxide:

  • The molten aluminium ore is decomposed by electricity

  • The positive aluminium ions (Al³⁺) are attracted to the negative electrode

  • At the negative electrode, each aluminium ion gains three electrons to become a neutral aluminium atom:

Al³⁺ + 3e^- → Al

Extraction of aluminium

Extraction summary