Page 158 - Science Class 08

At the anode (+) of the salt of the metal that is to form the
2+
Cu Õ Cu + 2 e–
2-
2+
Cu + SO Õ CuSO 4
4
In this way, copper breaks in the solu on to
form copper sulphate and it loses two
electrons on the anode. These electrons flow
to the ba ery and from there to the cathode.
Copper ions is taken out from the solu on at
the cathode. Thus, the concentra on of CuSO₄
in the solu on remains constant.
IMPORTANCE OF ELECTROPLATING
Electropla ng has so many uses as follows:
v Electropla ng is used to give metal objects
a be er appearance or to protect them
from corrosion, wear or rust.
v Tableware, trays, decora ve pieces and
jewellery are plated with gold or silver to
make them more a rac ve.
v Copper is coated with chromium to protect
it from corrosion. Chromium has a shiny
appearance. It does not corrode. It resists
scratches. However, chromium is expensive
and it may not be economical to make the
whole object out of chromium. So, the
object is made from a cheaper metal and
only a coa ng of chromium over it is done.
Chromium pla ng is also done on car parts,
bath taps, kitchen gas burners, bicycle
handle bars, wheel, rims and many others.
v Tin cans, used for storing food, are made
by electropla ng n onto iron. Tin is less
reac ve than iron. Thus, food in n does
not come to contact with iron and is
protected from ge ng spoilt.
v Iron used in bridges are also coated with
zinc to protect them from rus ng.
ELECTROPLATING IN INDUSTRIES
In industries, electropla ng is done at large
scale. First, a container is filled with a solu on
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coa ng. For example, if copper is to form the
coa ng, the solu on will consists of copper
sulphate (a salt of copper) mixed with water.
This solu on is called the electroly c bath. The
object to be plated is immersed in the bath. A
metal bar, composed either of the metal that is
to form the coa ng or of metal that is not
affected by the electroly c bath, is also
immersed in the bath. The en re apparatus is
called an electroly c cell.
The object to be coated is connected to the
nega ve terminal of an electric ba ery, and
becomes a cathode. The metal bar is connected
to the posi ve terminal of the electric power
source and is called an anode.
When electric power is applied, electrolysis of
the electroly c bath occurs. The bath gives up
its metal content to the surface of the cathode.
This coa ng forms an alloy with the metal of
the cathode and adheres to the cathode a er
the cathode has been removed from the bath.
As the electropla ng process con nues, the
metal salts in the bath are used up. If the
anode is a bar of the coa ng metal, the bar
dissolves in the bath at the same rate that the
bath gives up its metal to the cathode. If the
anode is made of another metal salts of the
coa ng metal must be added to the bath as
metal becomes deposited in the cathode.
The longer the process con nues, the greater
the thickness of the coa ng on the cathodes.
Science-8