GCSE EDEXCEL Methods of Extraction

Extracting iron and copper

Ores

Unreactivemetals such as gold are found in the Earth's crust as the uncombined elements. However, most metals are found combined with other elements to form compounds.

An ore is a rock that contains enough of a metal or a metal compound to make extracting the metal worthwhile:

• low-grade ores contain a small percentage of the metal or its compound
• high-grade ores contain a larger percentage

Most metals are extracted from ores found in the Earth's crust. It is more expensive and wasteful to extract a metal from a low-grade ore, but most high-grade ores have already been used.

Extraction methods

The extraction method used depends upon the metal's position in the reactivity series. In principle, any metal could be extracted from its compounds using electrolysis. However, large amounts of electrical energy are needed to do this, so electrolysis is expensive.

If a metal is less reactive than carbon, it can be extracted from its compounds by heating with carbon.

Metal oxide + carbon → metal + carbon dioxide

For example, molten copper can be produced from copper oxide by heating with carbon:

Copper oxide + carbon → copper + carbon dioxide

2CuO(s) + C(s) → 2Cu(l) + CO₂(g)

Copper oxide is reduced as carbon is oxidised, so this is an example of a redox reaction.

Remember:

• oxidation is the gain of oxygen by a substance
• reduction is the loss of oxygen by a substance
• a redox reaction involves the loss and gain of oxygen

Note: the impure copper is purified by electrolysis.

The table summarises the extraction methods used for different metals.

Extracting iron

As iron is below carbon in the reactivity series it can be displaced from its compounds by heating with carbon. Iron is extracted from iron ore in a large container called a blast furnace. Iron(III) oxide is reduced to molten iron when it reacts with carbon. The overall reaction is:

Fe₂O₃(s) + 3CO(g) → 2Fe(l) + 3CO₂(g)

The iron oxide is reduced and the carbon is oxidised.

These reactions happen because carbon is more reactive than iron, so it can displace iron from iron compounds. Extracting a metal by heating with carbon is cheaper than using electrolysis.

Question

Write a balanced equation for the reaction between tin(IV) oxide and carbon, forming molten tin and carbon dioxide.

SnO₂(s) + C(s) → Sn(l) + CO₂(g)

Extracting aluminium

Aluminium is more reactive than carbon so it must be extracted from its compounds using electrolysis. Even though aluminium is more abundant than iron in the Earth's crust, aluminium is more expensive than iron. This is mainly because of the large amounts of electrical energy used in the extraction process.

Electrolysis of aluminium oxide

The electrolyte

Aluminium ore is treated to produce pure aluminium oxide. The electrolytes used in electrolysis are ionic compounds:

• in the molten state, or
• dissolved in water

Aluminium oxide is insoluble in water, so it must be molten to act as an electrolyte. However, the melting point of aluminium oxide is high. A lot of energy must be transferred to break its strong ionic bonds, and this is expensive. To reduce costs, powdered aluminium oxide is dissolved in molten cryolite. This ionic compound melts at a lower temperature than aluminium oxide, reducing costs.

The electrolysis process

The diagram shows an electrolysis cell used to extract aluminium. Both electrodes are made of graphite, a form of carbon with a high melting point and which conducts electricity.

A cross section through an electrolysis cell

During electrolysis:

• at the cathode, aluminium ions gain electrons and form aluminium atoms
• at the anode, oxide ions lose electrons and form oxygen gas

The oxygen reacts with the carbon anodes, forming carbon dioxide. So the anodes gradually wear away. They must be replaced frequently, adding to the cost of producing aluminium.

Question

Higher - Explain, with the help of a half equation, how oxide ions are oxidised during the electrolysis of aluminium oxide.

The half equation is: 2O^2- → O₂ + 4e^-.

It shows that oxide ions lose electrons, and oxidation is loss of electrons.

Question

Explain, with the help of a half equation, how aluminium ions are reduced during the electrolysis of aluminium oxide.

The half equation is: Al³⁺ + 3e^- → AlIt shows that aluminium ions gain electrons, and reduction is gain of electrons.