Page 115 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN005M-206 June 15, 2001 20:25

Electrochemistry 193

By an anologous process the CO adduct of an iron(II) B. Aluminum Production
IV
porphyrin [(Cl 8 TPP)Fe (CO)] is reduced to H 2 C(O) at
The Hall process for the electrolytic production of alu-
−0.87 V. The oxidation of Fe(CO) 5 in MeCN yields minum utilizes an iron container that is lined with carbon,
II
Fe (MeCN) 2+ in a two-electron process (Fig. 7e),
which serves as the cathode. The electrolyte within this
container is molten cryolite (Na 3 AlF 6 ), which dissolves
−2e −
II
Fe(CO) 5 + 4 MeCN Fe (MeCN) 2+ + 5CO the aluminum oxide (Al 2 O 3 ) that is the raw material for
4
the process,
E p,a , +0.97VvsSCE. (182)
Na 3 AlF 6 (l) + Al 2 O 3 (s) 3 NaAlOF 2 . (185)
II
On the basis that Fe (s) is oxidized to Fe (MeCN) 4+
A series of carbon anodes are immersed into the molten
at ∼0.0 V (Fig. 7a), the carbonyls of Fe(CO) 5 stabi- solution. Electrolysis produces elemental alumium at the
lize the iron against oxidation by about 22 kcal mol −1 cathode (liquid metal at the operating temperature),
−1
[ E × 23.06 kcal mol −1 (eV) ; 0.97 × 23.06]. The
−
IV
(Cl 8 TPP)Fe (CO) molecule is oxidized at +0.75 V vs Cathode: Na 3 AlF 6 (l) + 3 e − Al (l) + 3 NaF , (186)
2
II
+0.32 V for (Cl 8 TPP)Fe ; a stablization by the CO of and molecular oxygen at the anode,
−1
about 10 kcal mol .
These examples of the electrochemical character of Anode: NaAlOF 2 + 2 NaF − Na 3 AlF 6 (l)
2
organometallics are limited, but illustrate that their oxi- + 1/2 ·O 2 ·+ 2e . (187)
−
dation and reduction is closely similar to that for organic
molecules. Thus, the electron transfer is never carbon cen- Hence, the electrolytic cell reaction only consumes Al 2 O 3
to produce Al (l) and ·O 2 · (g),
tered and often involves residual water [H-atom addition
via reduction and (HO·) addition or H-atom abstractron
Cell: 2 Na 3 AlF 6 + 3 NaAlOF 2 2Al + 3Na 3 AlF 6
via oxidation] or solvent components.
+ 3/2 ·O 2 · (Na 3 AlF 6 + Al 2 O 3 )
Net: Al 2 O 3 Al + 3/2 ·O 2 ·. (188)
As the Al 2 O 3 is consumed by electrolysis, more of it is
VI. INDUSTRIAL ELECTROSYNTHESIS
continuously added, and liquid aluminum metal is period-
ically drained from the bottom of the cell.
A. Electroplating and Electrorefining
1. Copper Refining
C. Chloro-Alkali Production
Although metallic copper is produced via the smelting of
The industrial production of chlorine (Cl 2 ) and sodium
copper ores, it contains enough impurities (and the atten-
hydroxide (NaOH) involves the electrolysis of molten
dant increase in electrical resistance) to preclude its use as
sodium chloride or of brine solutions. With the molten
an electrical conductor. Hence, all copper for use in elec-
salt the metallic sodium that is produced at the carbon
trical wire and cable must be purified via electrorefining
cathode is vaporized and collected in a condenser as the
with an impure copper plate as the anode, a pure copper
solid metal. A common system for brine uses a cell with
sheet as the cathode, and copper sulfate/sulfuric acid as
a liquid mercury cathode that flows in one direction along
the electrolyte:
the floor with the brine flowing in the opposite direction
past a series of graphite anodes. The electrolysis reactions
Anode: Cu (s) (impure) + 6H 2 O produce chlorine gas at the anode, which is collected,
II
−
Cu (OH 2 ) 2+ + 2e , (183) − −
6 Anode: 2 Cl (aq) Cl 2 (g) + 2e , (189)
II
Cathode: Cu (OH 2 ) 2+ + 2e −
6 and sodium amalgam [Na 2 (Hg)] at the cathode,
Cu (s) (pure) + 6H 2 O. (184) Cathode: 2 Na(OH 2 ) + Hg (l) + 2e −
+
6
Na 2 (Hg) (l) + 6H 2 O, (190)
Because most copper ores contain traces of gold and silver
(which become a part of the impure smelted copper), the which flows from the cell and is reacted with water to give
sediment that builds up in the bottom of the electrolysis a concentrated sodium hydroxide solution,
cell concentrates these valuble metals. The periodic refin-
Na 2 (Hg) (l) + 14 H 2 O 2 Na(OH 2 ) +
ing of the sediment often yields sufficient material to pay 6
−
for the entire purification process. + 2HO (aq) + H 2 (g). (191)