Chemistry Reference
In-Depth Information
H
+
(
aq
) and Cu
2+
(
aq
)
ions are both attracted to the
cathode, the Cu
2+
ions accepting electrons more
readily than the H
+
ions (preferential discharge).
Copper metal is therefore deposited at the cathode
(Figure 5.14).
copper ions
Cu
2+
(
aq
)
+
+
electrons
2e
−
copper atoms
Cu(
s
)
→
→
OH
−
(
aq
) and SO
4
2−
(
aq
) ions are both attracted to
the anode. The OH
−
ions release electrons more
easily than the SO
4
2−
ions, so oxygen gas and water
are produced at the anode (Figure 5.15).
hydroxide ions
4OH
−
(
aq
)
oxygen
O
2
(
g
)
+
+
water
2H
2
O(
l
)
+
+
electrons
4e
−
→
→
anode
cathode
Figure 5.16a
The copper used in electrical wiring has to be very pure.
e
OH
Electrons
collected
by Cu
2
ions.
Copper
coats the
electrode
Cu
2
2e
Cu
e
OH
Electrons
from OH
left behind
O
2
2H
2
O
e
OH
Cu
2
2e
Cu
e
OH
Figure 5.15
Oxygen is given off at the anode and copper is deposited at
the cathode.
Purification of copper
Because copper is a very good conductor of
electricity, it is used for electrical wiring and cables
(Figure 5.16). Pure copper is also used in the
manufacture of cooking utensils owing to its high
thermal conductivity, a property of its metallic
structure (Chapter 3, p. 55).
However, even small amounts of impurities cut
down this conductivity quite noticeably whether
in fine wires or larger cables. The metal must be
99.99% pure to be used in this way. To ensure this
level of purity, the newly extracted copper has to be
purified by electrolysis.
Figure 5.16b
Due to the high density of copper and its cost, steel-cored
aluminium cables are used for electrical transmission through national
grids.
The impure copper is used as the anode and is
typically 1 m square, 35-50 mm thick and 330 kg
in weight. The cathode is a 1 mm thick sheet and
weighs about 5 kg; it is made from very pure copper.
Because copper is itself involved in the electrolytic
process, the copper cathode is known as an 'active'
electrode. The electrolyte is a solution of copper(ii)
ii
)
sulfate (0.3 mol dm
−3
) acidified with a 2 mol dm
−3
solution of sulfuric acid to help the solution conduct
electricity (Figure 5.17).
