23.3. Electrolysis http://www.ck12.org
FIGURE 23.13
Apparatus for the production of hydrogen and oxygen gases by the
electrolysis of water.Oxidation (anode): 2Cl−(aq)→Cl 2 (g)+2e− E^0 =− 1 .36 V
Reduction (cathode): 2H 2 O(l)+2e−→H 2 (g)+2OH−(aq) E^0 =− 0 .83 V
Overall reaction: 2Cl−(aq)→O 2 (g)+2H 2 (g) E^0 cell=− 2 .06 VSince the hydroxide ion is also a product of the net reaction, the important chemical sodium hydroxide (NaOH) is
obtained by evaporating the water after the hydrolysis is complete.
Electrometallurgy
The extraction of metals from ores and the purification of metals is often accomplished usingelectrometallurgy.
Most metals are found in nature in an oxidized form (usually an oxide or a sulfide) called an ore. If the ore is either
dissolved in an aqueous solution or melted, passing an electric current through the material causes the pure metal
to be deposited on one of the electrodes. This process was used by the English chemist Humphrey Davy to isolate
metallic sodium from a solution of sodium hydroxide, and several other elements were discovered in the next several
years using this same technique.
We saw earlier that sodium is most commonly produced by electrolysis of molten sodium chloride. Many other
metals are also isolated and purified using electrometallurgy techniques, including gold, silver, copper, aluminum,
lead, and many alkali and rare earth metals. For some metals, this is the only financially viable method of extraction
and purification. In addition to the production of a desired metal, these techniques are also being studied for use in
the processing of nuclear wastes and the cleanup of environmental contamination by toxic metals.
Aluminum is one example of a substance that was very rare in its pure form before electrometallurgy techniques
were developed but is now extremely common. Aluminum ore (bauxite) is predominantly aluminum oxide (Al 2 O 3 ).
Refinement of the metal by simple heating, as is done with iron ore, is ineffective, because the melting point of the
aluminum ore is so high (>2000°C). Instead, the bauxite is first mixed with sodium hydroxide at high temperatures
to remove the other contaminants from the aluminum oxide. This material is then recrystallized and separated from
other ore residues. Then, the purified aluminum oxide is dissolved in a molten mixture of calcium fluoride and
cryolite (an aluminum fluoride-sodium fluoride salt that melts at 1012°C). The lower melting point of the cryolite
allows the aluminum ore to dissolve at a temperature that is much lower than its own melting point. The resulting
solution is then electrolyzed using carbon electrodes at a temperature of 950-980Udeg;C. The aluminum cations are
reduced to neutral aluminum at the cathode (Al^3 +→Al^0 ), and the oxygen anions are oxidized back to molecular
oxygen at the anode (2 O^2 −→O 2 ).
One problem with current methods of aluminum production is the generation of environmental contaminants. Hy-
drogen fluoride gas is an extremely corrosive and toxic byproduct that must be trapped and neutralized. Other