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Extracting Metals by Electrolysis


Methods of extracting metals

There are two main methods of

extracting metals from their ores:

reduction with carbon

electrolysis (using electricity).

The method of extraction

which is most appropriate

depends on the reactivity of

the metal being extracted.

Most metals 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 reactivity series and metal extraction

The reactivity of a metal determines how it is extracted.

Metals above carbon in the

reactivity series must be

extracted using electrolysis.

Metals less reactive than

carbon can be extracted from

their ores by reduction.

Copper, silver, gold and

platinum can occur in their

pure form (natively) and do not

need to be extracted.

incre

asin

g r

ea

cti

vit

y

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 as

aluminium oxide (Al2O3).

What is the word equation for the extraction of aluminium?

Electrolysis breaks down the Al2O3

into aluminium and oxygen. As the

aluminium loses oxygen, reduction

takes place.


Ionic substances

Electrolysis involves breaking down ionic compounds 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.

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, such

as metal ions, have lost electrons.

Ions with a negative charge have

gained electrons.

Aluminium

atom

Aluminium

ion

loses 3

electrons


Oxidation and reduction

heat

In electrolysis, electrodes are used to pass a current through

a substance. 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?


2O2- O2 + 4e- Al3+ + 3e- Al

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:

Oxidation: Reduction:


Extracting aluminium

Aluminium is more reactive than

carbon and it’s ore has a very high

melting point (2050°C). This

means aluminium cannot be

extracted by reduction with carbon.

Electrolysis is used to extract aluminium from its ore.

In electrolysis, the ore is dissolved

in a compound called cryolite

(Na3AlF6), which lowers the melting

point to 1,000°C.

Why is it not possible to extract

aluminium by heating its ore

with carbon?


Extracting 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:

At the positive electrode:

aluminium oxide aluminium + oxygen

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

Al3+ + 3e- Al (reduction)

2O2- O2 + 4e- (oxidation)


Economics of electrolysis

Extracting aluminium from its ore using electrolysis requires

large amounts of electrical energy. This is very expensive.

To reduce energy use,

aluminium extraction plants

are often located near

sources of renewable

electricity, such as

hydroelectric dams.

The energy is needed to melt the aluminium oxide in the

ore and to extract the aluminium by electrolysis.


Extracting aluminium summary


Glossary


Multiple-choice quiz

extracting metals by electrolysis -...

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