Page 171 - MODERN ELECTROCHEMISTRY

ION-SOLVENT INTERACTIONS 111

A well-known thermodynamic expression for the change in entropy of a
process is

with as the free-energy change.
Furthermore, when an electrochemical cell works in a thermodynamically revers-
ible way (see Vol. 2, Chapter 7),

where n is the number of electrons for one step of the overall reaction.
It follows from Eq. (2.56) that

Now, this of a cell reaction must be composed of the entropies of at least two
different ions in solution (because two electrode reactions are involved, one at each
electrode), so that Eq. (2.58) cannot lead directly to an individual ionic entropy.
However, in 1941, Lee and Tai considered the potential and temperature coeffi-
cients of the following cells:

The suffix (ecm) for cells 1 and 3 represents the term “electrocapillary maximum”
and can be regarded (Vol. 2, Chapter 6) as a potential at which the electrode has zero
charge.
Lee and Tai assumed that the potential at which the excess charge on the electrode
is zero also indicates an interfacial potential difference of zero. This would not be
consistent with the viewpoint of workers in the late 1990s (Vol. 2, Chapter 6), but let
it be assumed to be so for now and follow Lee and Tai’s reasoning.
30
Contemplating then cell (3), if the Hg electrode does not contribute to the
temperature coefficient of the potentials measured, then E 3 of the cell (being equivalent
to ) must yield here the entropy difference of the ion undergoing a
reversible equilibrium reaction at Pt (right-hand electrode, cell 3) with in the gas
phase and in solution at unit activity.

30
In fact, Lee and Tai made measurements on cells (1) and (2) and obtained data on cell (3) by observing
that