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Industrial
applications of electrolysis
By
Francine Taylor-Campbell, Contributor
YOU
SHOULD BE ABLE TO:
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Describe industrial applications of
electrolysis; metallic extraction
and purification, electroplating and
anodising.
POINTS
TO NOTE
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Electrolysis is used commercially
to extract reactive metals such as
sodium and aluminium from their ores.
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Electroplating and corrosion protection
uses the principle of electrolysis.
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Pure metals can be obtained using
the process of electrorefining.
EXTRACTION
OF SODIUM USING THE DOWN'S CELL
(Please
refer to your text for the cell diagram)
In
this process molten sodium chloride
and calcium chloride are used. The
purpose of calcium chloride is to
lower the temperature to prevent sodium
chloride from boiling. At the cathode:
Na+ ions are discharged. Na+(l) +
e == Na(l)
At
the anode: Chlorine is liberated.
2Cl-(l) === Cl2(g) + 2e
NOTE:
Steel hood is used to keep the sodium
and chlorine apart and to prevent
them from reacting.
ANODISING
Anodising
is a process of producing corrosion
resistant articles by coating. The
anode in this cell is an aluminium
object and the electrolyte solution
is one which can liberate oxygen at
the anode such as dilute sulphuric
acid.
At
the anode: OH- ions are discharged.
4OH-(aq)
== 2H2O(l) + O2(g) + 4e
The
oxygen liberated at the anode reacts
with the aluminium object to form
a protective oxide coating of aluminium
oxide (Al2O3), which is resistant
to corrosion.
ELECTROPLATING
This
is the process of coating an object
with a thin layer of another metal
by electrolysis. This can be used
to prevent corrosion and to make the
object more attractive. In this process
the object to be coated is the cathode
and the metal to be used for coating
(plating) is the anode. For example
to add a nickel plate to an object,
a nickel sulphate solution could be
used as the electrolyte. At the cathode:
The object or metal to be coated (plated)
acts as the cathode. For example in
nickel plating, nickel ions are discharged
and deposits on the cathode or object
thereby coating it.
Ni2+(aq)
+ 2e == Ni (s)
At
the anode: If the object is to be
nickel plated then the anode must
be made of nickel. The nickel will
therefore dissolve to form nickel
ions.
Ni
(s) cathode === Ni2+ (aq) + 2e NOTE:
If the object is to be chrome plated
or silver plated then a chromium or
silver electrolyte solution is used
and the anode is made of chromium
or silver.
PURIFICATION
OF COPPER
The
purity of copper can be improved by
electrorefining. In this case the
same principle of electroplating or
electrodeposition is used. The electrolyte
is a solution containing copper ions
such as copper sulphate. The cathode
is a strip of pure copper and the
anode is a lump of the impure copper.
At the anode: Copper atoms from the
impure copper dissolve to form copper
ions. This causes the anode to decrease
in size. Cu(s) == Cu2+(aq) + 2e
Impurities
fall off the anode and are collected
in the cell.
At
the cathode: Cu2+ ions migrate towards
the cathode where they are deposited
on the cathode as solid copper. Cu2+
(aq) + 2e === Cu(s)
The
cathode becomes thicker (increases
in size). In next week's lesson we
will examine some questions on electrolysis.
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Francine Taylor-Campbell teaches at
Munro College in St. Elizabeth and
is an assistant examiner with CXC.
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