Page 73 - Bruno Linder Elementary Physical Chemistry

August 18, 2010 11:36 9in x 6in b985-ch06 Elementary Physical Chemistry

58 Elementary Physical Chemistry

1J = 1C × 1V)

w other =∆G T,P = −n E o
= −2 × 96,485 coulomb × 1.18 volt

= −227,046.60 J ≈−227 kJ (6.30)
The foregoing illustrates that the Gibbs free energy, a state function,
can be obtained from non-PV work (which is not a state function),
analogous to the determination of the enthalpy ∆H P,T from PV work.

Note: Although the potential of a single electrode cannot be measured,
one electrode can be assigned the value 0 and the other electrode can
then be assigned a value obtained from the measured total potential.
The electrode chosen to be assigned zero is the hydrogen electrode.

In the above example, we used the published electrode potentials to
evaluate w other. Such potentials were originally obtained by measuring
voltage changes under maximum resistance between the electrodes. This
is done to obtain results under quasi-static (reversible) conditions.

6.2.9. Cells at Equilibrium
It should be emphasized that in the above example, the current was
produced by the Cu–Zn reaction, but the reaction was not in a state of
equilibrium. If the reaction is in equilibrium, ∆G = 0 and therefore, w other
must also be zero. Thus, there would be no current flow.

Note: One can rationalize the above by recalling that, in general,
∆G T,P ≤ w other . Thus, if the reaction is to move in a forward
direction, ∆G T,P must be less than w other and if it is in a reverse
direction, ∆G T,P must be greater than w other . When the system is in
equilibrium, w other must be zero.

o
Finally, since ∆G =∆G + RT ln K and thus, equal to zero (why?), it
o
follows that 0 = −n E + RT ln K,and so
o
ln K = −n E /RT (6.31)