Page 143 - Basic physical chemistry for the atmospheric sciences

Oxidation-reduction reactions

6
We see from Table . 2 that the reaction
2H20(1)� 0 2 (g) + 4H + (aq) + 4e - (6.27)

y
has a strong oxidizing potential <E!!x = - 1 . 229 V). Consequentl , any
system with a redox potential greater than 1 . 229 V can be reduced by
u
water, with the liberation of oxygen. Th s , oxygen in air is the most
common oxidant in natural environments. This is illustrated by the
following example. U n der standard conditions the F2(s) - F - (aq) sys­
tem has a redox potential of 2.87 V (see Table 6.2). Therefore, it is
reduced by water

2H20(1)� 02(g) + 4H + (aq) + 4e - E!!x = - ( 1 . 2 29 V)
2F2(s) + 4e - � 4F - (aq) e.> e d = 2 . 8 7 V

Net: 2H20(1) + 2Fz(s� 02 + 4H + (aq) + 4F- (aq) men= l . 64 1 V
indicates that the reaction is spontane­
where the positive value of men
ous from left to right.
s
For nonstandard condition , the redox potential of the 0 2 (g) ­
H20(1) system is, from Eq. (6.26)
0 . 0 591 l
E ell = 1 . 22 9 - -- log [Oz(g)] [H + (aq)] 4
c
4
= l . 229 + 0.0 1 4 8 log[02(g)] + 0.059 1 log[H + (aq)]
=
Therefore, since in air [0 (g)] 0 . 2 atm and log[H + (aq)] = - pH
2
E ell = 1 . 22 - 0 . 0 1 0 - 0.059 1 pH
9
c
or,
Ecen = I . 2 2 - 0.059 1 pH (6.28)
Th s , redox potentials in natural environments will generally be less
u
that the value of Ecen given by Eq. (6.28).
In the limit considered above, water acted as a reductant to limit the
oxidation that can occur in nature. Water also acts as an oxidant to
limit the reduction that can occur in nature. In this case, the half­
reaction involving water is

2H 2 0(1) + 2e - � H 2(g) + 20H - (aq) (6.29)
which , from Table 6 . 2 , has a standard electrode potential of - 0 .828
s
V. Under standard condition , Reaction (6.29) can combine spontane-