|
Application
of electrolysis
Francine
Taylor-Campbell, Contributor
YOU
SHOULD BE ABLE TO:
- Describe
industrial applications of electrolysis,
metallic extraction and purification,
electroplating and anodizing.
POINTS
TO NOTE
- Electrolysis
is used commercially to extract
reactive metals such as sodium and
aluminium from their ores.
- Electroplating
and corrosion protection uses the
principle of electrolysis.
- 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.
Anodizing
Anodizing
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 deposit 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.
Francine
Taylor-Campbell teaches at Jamaica
College. Send questions and comments
to kerry-ann.hepburn@gleanerjm.com
|
