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Extraction of Metals
Corrosion (rusting) of Iron and Steel.
Iron exposed to moist
air will react
with oxygen in the air to form iron oxide.
This oxidation process is called rusting.
iron + oxygen
4Fe(s) + 3O2(g) 2Fe2O3(s)
Rusting requires both oxygen and water.
Salt or acid will accelerate rusting.
Rusting may be prevented by
1) coating the surface (by painting for example)
to prevent contact with the air.
2) sacrificial protection of a more reactive metal.
and steel are most commonly protected by
painting (for example bridges, buildings, vehicles),
metal plating (see below)
or plastic coating (for example household equipment).
Zinc plating is called galvanizing (this is also sacrificial
Chromium plating is used on water taps and some
car wheels to give a highly polished protective surface.
The metal plating process
(also called electroplating)
uses electrolysis of a solution containing ions of the plating metal.
The anode is made from the pure plating metal.
The metal object that needs plating is used as the cathode.
Most metals can be plated.
Common plating metals are gold, nickel and silver
as well as chromium and zinc referred to above.
Silver plating could be done in the cell below.
When electricity is passed through the cell
silver is dissolved at the anode by oxidation.
Ag+ ions go into the silver nitrate solution.
Ag(s) - e- Ag+(aq)
Silver is deposited
surface of the object by reduction at the cathode.
Ag+(aq) + e- Ag(s)
ions move from the anode to the
the anode gets smaller as the object becomes silver plated.
This is a redox reaction.
The rate at which the silver
ions enter the electrolyte from the
anode is the same as the rate at which the silver ions
leave the electrolyte at the cathode. The concentration of
the silver nitrate solution therefore remains unchanged.
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