Page 142 - Basic physical chemistry for the atmospheric sciences

1 2 8 Basic physical chemistry

6.9 shows that if the concentration of one of the reactants is decreased
{[Cr(s)] went from I M to 0.5 M}, and the concentration of one of the
products is increased {[Cr 3 + (aq)] went from 1 M to 2 M}, the cell
potential is decreased (from 0.74 V to 0.73 V in this case). This is
because an increase in the concentration of one of the products and a
decrease in the concentration of one of the reactants reduces the
n
forward reactio . Left to itself, a chemical system will move toward
its equilibrium state. When it reaches this state Kc = Q (see Section
1 . 3 ) and, from Eqs. (6.22) and (6.26), Ecen = 0 . We have also seen in
Section 2 . 4 that when a chemical system is at equilibrium dG = O .
Hence, when the species in an electrochemical cell are in equilibrium,
the Gibbs free energy is a minimum and the cell generates zero electric
potential difference (i.e . , "the battery has run down ") .

6.8 Redox potentials; Eh-pH diagrams
In geochemistry, the ability of an environmental system to oxidize or
reduce, as measured by its electrode potential , is often called its redox
potential. It is given the symbol Eh (where "h" indicates that the
reference half-cell is hydrogen). The Eh is analogous to p H , in that it
is a measure of the ability of a system to supply or to take up elec­
trons, while the pH of a system measures its ability to supply or take
up protons. If a system has a propensity for supplying electrons (i.e . ,
if it provides a good reducing environment for an oxidant), it will have
a low value of Eh (i.e. , be high up the list in Table 6 . 2 ) . If a system
has a propensity for consuming electrons (i. e . , if it provides a good
oxidizing environment for a reductant , it will have a high value of Eh
)
(i.e . , be low down in the list in Table 6 .2) .
There are limits to the ranges of values of Eh and pH in natural
environments. For example, some of the most acidic solutions in
nature, with pH values below zero, occur near active volcanoes due
i
to the em s sions of acidic gases. However, these acidities are quickly
reduced by reactions with soils and rocks . Complete neutralization is
generally not obtained because of the buffering capacity of C02 from
the air (see Section 5 . 1 3 ) . Thus, the pH values of natural systems
generally lie in the range 5 to 6, with 4 as a rough lower limit. Very
basic solutions , with pH values up to about 1 1 , can form if COrfree
s
water is in contact with carbonate rocks or certain silicate . Again ,
however, because C02 acts as a buffer, the pH values of surface
waters generally do not rise above about 9.