GALVANIZATION When more than one metal is present, the more reactive metal (the one with a more negative
o (^) E
) reacts first. This fact is the basis of
galvanization
, the protection of one metal
by the sacrifice of a more reactive metal. For
example, the iron in an automobile body is
galvanized
by coating it with zinc, a more reactive metal (compare the
oE
values in Table
11.1). When paint is scratched off the surface, the zinc coating is sacrificed (Zn +
1 /^2
O^2
→
ZnO) to preserve the iron and, therefore,
the automobile body. Indeed, iron can be
protected by placing it in contact with any more reactive metal. PASSIVATION Metal surfaces can also be
passivated
with a protective coating of a metal oxide. One of
the most common forms of
natural
passivation is the protection of aluminum. Aluminum
is much more reactive than iron (see Table 11.1), yet it is used to make airplane bodies and outdoor furniture. Aluminum can be put to
such uses, even though it corrodes, because,
rather than dissolving, it forms a thin layer of Al
O 2
on the outer surface that is impervious 3
to further attack. The underlying aluminum metal is passivated by the protective coating. The galvanized iron in an automobile is also
passivated because, as the sacrificial zinc is
oxidized, it forms a zinc oxide coating, wh
ich then passivates the underlying iron.
11.8
ELECTROLYTIC CELLS One of the major advantages of electrochemistry is that the free energy of the redox electrons can be adjusted by changing the volta
ge between the electrodes with an external
power supply, such as a battery. In Example 11.5, we decided that Cu does not react with strong acids like HCl because
o (^) E
is negative, Cu + 2H
1+^
U
Cu
2+ + H
(^2)
oE
= -0.34V. The
negative value simply means that the donor orb
ital on Cu is at a lower free energy than the
acceptor orbital on H
1+. Electrons do not flow uphill
spontaneously
, but they can be
pushed uphill by connecting an external power supply to the cell. The power supply can provide the 0.34 V the electrons need to make
this uphill climb. Forcing a nonspontaneous
process to occur by the application of
an external power supply is called
electrolysis
, and
the cell in which it is accomplished is called an
electrolytic cell
. Thus, an electrolytic cell
converts electrical energy into chemical energy, while a galvanic cell converts chemical energy into electrical energy.
Galvanic cellChemical energyElectrical energyElectrolyticcellRecharging batteries is a common example of electrolysis. Consider the lead-storage
Chapter 11 Electron Transfer and Electrochemistry