Page 132 - Basic physical chemistry for the atmospheric sciences

1 1 8 Basic physical chemistry
2
Cu + (aq) + 2Ag(s)� Cu(s) + 2Ag + (aq)
t
proceeds spontaneously from right to lef ; therefore, its standard cell
potential is - 0.46 .
V
The standard cell potential depends on the electronic propensity of
the species involved in the overall reaction. Thus , the value of �el l for
Reaction (6.3) depends on the tendency of Cu(s) to give up electrons
[see half-reaction (6. 1 ) ] and the tendency of Ag + (aq) to gain electrons
[see half-reaction (6. 2 )] . If we could define standard half-cell (or elec­
trode) potentials [e.g. , for the half-reaction (6. 1 ) and for the half­
reaction (6. 2 )], they would provide measures of the strengths of vari­
s
ous oxidants and reductant . This can be done by (arbitrarily) as­
signing one specific half-reaction a standard half-cell potential of 0.
The half-reaction chosen for this purpose is that for the hydrogen
half-cell
(6. 1 3 )
H2(g)� 2H + (aq) + 2e -
for which �ell = 0 (for the forward or reverse reaction) when the
e
reactants and products are in their standard stat s .
Standard half-cell potentials can b e measured for other half-reac­
tions by measuring the magnitude of the standard cell potential (�cu)
when each half-cell is combined with the hydrogen half-cell (we need
not concern ourselves here with the technical aspects of such mea­
2
surements). For example, when the Cu-Cu + half-cell [see Reaction
(6. 1 ) ] is combined with the hydrogen half-cell , the standard cell poten­
tial is 0.34 V and electrons flow in the external (wire) part of the circuit
f r om the hydrogen electrode to the copper electrode. Comparing this
with the situation shown in Figure 6. 1 , we see that the silver electrode
has been replaced by a h y drogen electrode (and AgN0 by H2S0 ) ,
4
3
and the flow of electrons through the wire has been reversed . There­
fore, in place of Reaction (6. 1 ) , (6. 1 3 ) is the oxidation half-cell reac­
tion. The reduction half-cell reaction [which replaces reaction (6.2)] is
the reverse of Reaction (6. 1 ) , that is,
2
Cu + (aq) + 2e - � Cu(s) (6. 1 4 )
From the addition of Reactions (6. 1 3 ) and (6. 1 4 ) , we can see that the
spontaneous overall cell reaction is now

H2(g) + Cu2 + ( aq)� 2H + ( aq) + Cu(s) (6. 1 5 )
for which �ell = . 3 4 V . Since, by definition, Reaction (6. 1 3 ) does not
0
generate any electric potential difference, the absolute magnitude of