Electricity and chemistry - Chemistry - page 74

Question
1 Predict the products of the electrolysis of:
a potassium chloride
b lead oxide.

Today we use aluminium in very large quantities. The
annual production in the world is 19.5 million tonnes.
The commercial extraction of aluminium has been
made possible by two scientists, working independently
of each other, who discovered a method using
electrolysis. The two scientists were Charles Martin
Hall (USA), who discovered the process in 1886, and
the French chemist Paul Héroult, who discovered the
process independently in the same year. The process
they developed, often called the Hall-Héroult process,
involves the electrolysis of aluminium oxide (alumina).
The process involves the following stages.

● Electrolysis of
aluminium oxide
Aluminium is the most abundant metallic element in
the Earth's crust. It was discovered in 1825 by Hans
Christian Oersted in Copenhagen, Denmark, but was
fi rst isolated by Friedrich Wöhler in 1827. It makes up
8% of the crust and is found in the minerals bauxite
(Figure 5.5), cryolite and mica, as well as in clay.
In the nineteenth century Napoleon III owned a
very precious dinner service. It was said to be made
of a metal more precious than gold. That metal was
aluminium. The reason it was precious was that it
was very rarely found as the pure metal. Aluminium
is a reactive metal and as such was very diffi cult to
extract from its ore. Reactive metals hold on tightly
to the element(s) they have combined with and
many are extracted from their ores by electrolysis.

• Bauxite, an impure form of aluminium oxide,
is fi rst treated with sodium hydroxide to obtain
pure aluminium oxide, removing impurities such
as iron( iii ) oxide and sand. This improves the
conductivity of the molten aluminium oxide.
• The purifi ed aluminium oxide is then dissolved
in molten cryolite (Na 3 AlF 6 ). Cryolite, a mineral
found naturally in Greenland, is used to reduce
the working temperature of the Hall-Héroult
cell from 2017 °C (the melting point of pure
aluminium oxide) to between 800 and 1000 °C.
Therefore, the cryolite provides a considerable

Figure 5.5 Bauxite mining.