Page 133 - Basic physical chemistry for the atmospheric sciences

Oxidation-reduction reactions 1 1 1>

the standard half-cell (or electrode) potential for Reaction (6. 14) is
Ei ed = 0.34 V .
If zinc is made the electrode of one half-cell and hydrogen i s the
other half-cell, the standard cell potential is found by measurement to
V
be 0. 76 , and electrons flow in the wire f r om the zinc electrode to the
hydrogen electrode. Comparing this with the situation shown in Figure
6. 1 , we see that the silver electrode has been replaced by a hydrogen
electrode (and AgN0 by H 2S04) and the copper electrode by a zinc
3
electrode [and CuS0 by Zn(N03) z] . Therefore, in place of Reaction
4
(6. 1 ) , we now have for the oxidation half-cell reaction
Zn(s)� Zn2 + (aq) + 2e- (6. 1 6 )
and , for the reduction half-cell reaction [replacing Reaction (6.2)]

2H + (aq) + 2e - � H2(g) (6. 1 7 )
Therefore, the spontaneous overall cell reaction is
Zn(s) + 2H + (aq)� Zn 2 + (aq) + Hz(g) (6. 1 8 )

for which � ell = 0. 76 V . Since, by definition, Reaction (6. 1 7 ) does not
generate any potential, the magnitude of the standard half-cell (or
V
electrode) potential for Reaction (6. 1 6) is �x = 0. 76 .
a
b
The question now arises s to the sign to e attached to the magni­
tudes of the electrode potentials for copper and zinc derived above.
l
Clear y , they should be given opposite signs, because in the Cu-H2
cell the electrons in the wire move from the hydrogen electrode to the
copper electrode; whereas, in the Zn-H2 cell they move from the zinc
to the hydrogen electrode. Whether the negative sign is attached to
the copper or to the zinc electrode potential i s , of course, a matter of
convention. The convention that has been adopted is that if an elec­
trode f o rms part o f the half-cell in which the reduction reaction takes
place when it is coupled with a hydrogen half-cell, the electrode poten­
tial is assigned a positive value. Conversely, if an electrode f o rms part
o f the half-cell in which the oxidation reaction takes place when it is
coupled with a hydrogen half-cell, the electrode potential is assigned
a negative value. Applying this convention to the Cu-H2 cell, we see
from Reaction (6. 1 4) that Cu has a positive electrode potential (i. e . ,
)
f,�le<.1 = 0 . 3 4 V ; applying it to the Zn-H2 cell, we see from Reaction
i
(6. 1 6 ) that zinc has a negative electrode potential ( . e . , �x = - 0 .76 V).
If we write the oxidation half-cell reaction (6. 1 6 ) in the form of its
reverse reduction half-reaction