310
Electrochemistry III: Electrolysis
go to the negative electrode, and negative ions go to the positive electrode. But
now, at the negative electrode, we must decide whether it is the plus ions of the
solute or the H
+ ions of water that accept the electrons. Our natural reaction
would be to say that we will first form the atom that has the least tendency to
give off electrons. By reference to the table of standard electrode potentials
(Table 17-1), we can see that all ions lying above H
+ will accept electrons less
readily than H
+ at the same concentration. In water, the H
+ concentration is
10~
7 M, a concentration we can use in the Nernst equation (p 275) to show that
the hydrogen electrode potential in water is -0.41 volt. With this knowledge in
hand, we would be disposed to say that all those ions lying above -0.41 volt in
Table 17-1 will accept electrons less readily than H
+ in aqueous solution. This
would imply that metal ions lying below H
+ would plate out as metals from
aqueous solution, but only H 2 gas would be obtained from the electrolysis of
aqueous solutions of metal ions lying above H
+ .
In actual fact this is not true, because a second problem arises. For reasons
not yet clearly understood (despite a great many man-years of research), H
+
ions accept electrons at metal surfaces with much greater difficulty than expected, and this difficulty varies with the kind of metal surface. Only on a
platinum surface covered with finely divided platinum does this difficulty disappear. But even if you start with platinum and begin to plate out another metal
on it, the electrode surface then becomes the "other metal," and it is difficult to
produce H 2 . Extra voltage is required to produce H 2 . This extra voltage is
called "hydrogen overvoltage," and it varies with different metal surfaces. The
net result of "hydrogen overvoltage" combined with a low H
+ ion concentration (10~
7 M) is that most metal ions can be plated out from aqueous solution,
but no H 2 gas is produced; the alkali and alkaline earth metals are the major
exceptions. Two common metals that cannot be plated out from aqueous solution under any circumstances are iron and aluminum.
The following points sum up our discussion of the negative electrode.
1. Metal is plated out in the electrolysis of solutions of metal salts, other
than those of the alkali and alkaline earth metals (and iron and
aluminum). For example, in the electrolysis of a ZnCl 2 solution, we
would obtain metallic zinc,
Zn
2+ + 2e- -> Zn
because the H 2 overvoltage on metallic zinc changes the H-electrode
potential to some value more negative than the Zn-electrode potential
of -0.76 volt.
2. H 2 gas is evolved in the electrolysis of solutions of alkali and alkaline
earth metal salts. For example, in the electrolysis of an NaCl solution
we would obtain H 2 gas,
2H
+ + 2e- -»• H 2 |
Electrochemistry III: Electrolysis
go to the negative electrode, and negative ions go to the positive electrode. But
now, at the negative electrode, we must decide whether it is the plus ions of the
solute or the H
+ ions of water that accept the electrons. Our natural reaction
would be to say that we will first form the atom that has the least tendency to
give off electrons. By reference to the table of standard electrode potentials
(Table 17-1), we can see that all ions lying above H
+ will accept electrons less
readily than H
+ at the same concentration. In water, the H
+ concentration is
10~
7 M, a concentration we can use in the Nernst equation (p 275) to show that
the hydrogen electrode potential in water is -0.41 volt. With this knowledge in
hand, we would be disposed to say that all those ions lying above -0.41 volt in
Table 17-1 will accept electrons less readily than H
+ in aqueous solution. This
would imply that metal ions lying below H
+ would plate out as metals from
aqueous solution, but only H 2 gas would be obtained from the electrolysis of
aqueous solutions of metal ions lying above H
+ .
In actual fact this is not true, because a second problem arises. For reasons
not yet clearly understood (despite a great many man-years of research), H
+
ions accept electrons at metal surfaces with much greater difficulty than expected, and this difficulty varies with the kind of metal surface. Only on a
platinum surface covered with finely divided platinum does this difficulty disappear. But even if you start with platinum and begin to plate out another metal
on it, the electrode surface then becomes the "other metal," and it is difficult to
produce H 2 . Extra voltage is required to produce H 2 . This extra voltage is
called "hydrogen overvoltage," and it varies with different metal surfaces. The
net result of "hydrogen overvoltage" combined with a low H
+ ion concentration (10~
7 M) is that most metal ions can be plated out from aqueous solution,
but no H 2 gas is produced; the alkali and alkaline earth metals are the major
exceptions. Two common metals that cannot be plated out from aqueous solution under any circumstances are iron and aluminum.
The following points sum up our discussion of the negative electrode.
1. Metal is plated out in the electrolysis of solutions of metal salts, other
than those of the alkali and alkaline earth metals (and iron and
aluminum). For example, in the electrolysis of a ZnCl 2 solution, we
would obtain metallic zinc,
Zn
2+ + 2e- -> Zn
because the H 2 overvoltage on metallic zinc changes the H-electrode
potential to some value more negative than the Zn-electrode potential
of -0.76 volt.
2. H 2 gas is evolved in the electrolysis of solutions of alkali and alkaline
earth metal salts. For example, in the electrolysis of an NaCl solution
we would obtain H 2 gas,
2H
+ + 2e- -»• H 2 |