1 of 22© Boardworks Ltd 2011 Extracting Metals by Electrolysis.

1 of 22 © Boardworks Ltd 2011

Extracting Metals by Electrolysis



Extracting metals from ores

Most metals that we use are found combined with other elements, as compounds in ores. These metals must be extracted from their ores before they can be made useful.

The method of extraction which is most appropriate depends on the reactivity of the metal being extracted. This can be discovered using the reactivity series.

Metals above carbon in the reactivity series must be extracted using electrolysis.

incr

ea

sin

g r

eac

tivi

ty

potassiumsodiumcalcium

magnesiumaluminium

zinciron

copper

gold

lead

silver

(carbon)

(hydrogen)

platinum


What is electrolysis?

Electrolysis is a process that uses electricity to separate the elements in a compound. Electrolysis is expensive and so it is only used to extract reactive metals that cannot be extracted in other ways.

Aluminium is a reactive metal that is found in the ore bauxite combined with oxygen as aluminium oxide (Al2O3). Electrolysis breaks down the Al2O3 into aluminium and oxygen. As the aluminium loses oxygen, reduction takes place.

What is the word equation for the extraction of aluminium?

aluminium oxide aluminium oxygen +


Ionic substances

Electrolysis involves breaking down ionic substances into simpler substances using electricity.

An ionic substance contains charged particles called ions. An ion is an atom that has gained or lost electrons and so carries a positive or negative charge, which is shown after its formula.

In electrolysis, the ionic substance must be dissolved in water or melted so that the ions are free to move.

Ions with a positive charge have lost electrons, and ions with a negative charge have gained electrons.

Al3+

AlAluminium

atom

Aluminium ion

loses 3 electrons


Oxidation and reduction

heat

In electrolysis, the substance that the current passes through and splits up is called the electrolyte.

Positive ions move to the negative electrode

and gain electrons.

This is reduction.

Negative ions move to the positive electrode and lose electrons.

This is oxidation.

The electrolyte contains positive and negative ions.

What happens to these ions during electrolysis?


OILRIG

An easy way to remember what happens to the electrons during oxidation and reduction is to think OILRIG.

Equations written to show what happens to electrons during oxidation and reduction are called half-equations, i.e.:

Oxidation Is Loss of electrons

Reduction Is Gain of electrons

magnesium + oxygen magnesium oxide

2Mg (s) O2 (g) 2MgO (s)+

oxidation: Mg Mg2+ + 2e-

reduction: O2 +

4e- 2O2-


Extracting aluminium

Aluminium is one of the most useful metals in the world.

Aluminium ore (bauxite) is more reactive than carbon and has a very high melting point (2050 °C).

Electrolysis is used to extract aluminium from its ore. Why is it not possible to extract aluminium by heating its ore with carbon?

In electrolysis, the ore is dissolved in a compound called cryolite (Na3AlF6), which effectively lowers the melting point to 1,000 °C.


Aluminium from bauxite


Redox equations – aluminium

What processes occur at the electrodes during the electrolysis of aluminium oxide (Al2O3)?

What is the overall equation for this extraction by electrolysis?

At the negative electrode:

Al3+ + 3e- Al (reduction)

At the positive electrode:

2O2- O2 + 4e- (oxidation)

aluminium oxide aluminium + oxygen

2 Al2O3 (l) 4 Al (l) + 3 O2 (g)


Economics of electrolysis

The extraction of aluminium from its ore by electrolysis requires large amounts of energy in the form of expensive electricity to complete the many stages in the process.

To reduce costs, aluminium extraction plants are often located near sources of cheap electricity such as hydroelectric dams.

This energy is needed both to melt the aluminium oxide and to split up the compound.


Extracting aluminium summary



How is copper purified?

Copper can occur as a native metal, but more often it is found combined with other elements, such as in the ore malachite.

Copper is an excellent conductor and does not corrode quickly which makes it a good material for wiring and plumbing. Only pure copper can be used for electric wires because even very low levels of impurities will reduce the conductivity.

The copper extracted from compounds by reduction with carbon is impure. Electrolysis can be used after reduction to remove the impurities and obtain pure copper.


Purifying copper using electrolysis


Labelling copper purification


Redox equations – copper

What happens at the electrodes during the purification of copper by electrolysis?

At the negative electrode:

Cu2+ + 2e- Cu (reduction)

This process is carried out on a huge scale in industry. The copper formed on the negative electrodes is 99.99% pure.

At the positive electrode:

Cu Cu2+ + 2e- (oxidation)

The precious metals recovered from the impurities are also sold off and help to make this industrial process profitable.


Purifying copper – true or false?



Glossary


Anagrams


Multiple-choice quiz

1 of 22© Boardworks Ltd 2011 Extracting Metals by Electrolysis.

Documents

aluminium aluminium

electrolysis slide

extraction of aluminium

aluminium summary slide

aluminium ore bauxite

aluminium extraction

redox equations aluminium

bauxite slide