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Extraction of Metals

Corrosion (rusting) of Iron and Steel.

Iron exposed to moist air will react slowly
with oxygen in the air to form iron oxide.
This oxidation process is called rusting.

iron    +   oxygen            iron(III) oxide.
4Fe_(s)    +    3O_2(g)                   2Fe₂O_3(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.

Iron and steel are most commonly protected by
painting (for example bridges, buildings, vehicles),
metal plating (see below)
or plastic coating (for example household equipment).

Metal Plating.

Zinc plating is called galvanizing (this is also sacrificial protection).
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 onto the
surface of the object by reduction at the cathode.
Ag⁺_(aq)  +  e^-       Ag_(s)

As silver ions move from the anode to the cathode
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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