Page 74 - MODERN ELECTROCHEMISTRY

18 CHAPTER 1

Later in this book we will discuss these new tools, how they work, and what they bring
to electrochemistry. They have provided the ionics electrochemist with a new kind of
microscope.
Although this is still introductory material, three rather general points may be
made.
1. Until the 1980s, the major methods of investigation in ionics were nonspec-
troscopic. For example, conductance results were used to infer the existence of
complex ions. Alternatively and typically, the change of the dielectric constant of a
solution as a function of the concentration of ions (measurements at various
frequencies) was interpreted in terms of structural hypotheses about ion–solvent
interactions.
The new optical and spectroscopic methods are more discerning, more definite in
what they reveal. For example, in solutions of in water and in dimethylfor-
mamide, one used to speak of ion-pair formation, By now it is known that
there are several kinds of ion pairs. For example, the ions may be in direct contact or
they may be separated by a solvent molecule. The concentration of the free ions (if
they give vibration spectra) can be followed. In general, an enormous increase in detail
(corresponding to an increase in knowledge of the variety of particles present in the
nonaqueous systems) has become available.
2. Amazing bonds have been revealed by the new methods. For example,
Perelygin found that thiocyanates of the alkali metals form groups in acetonitrile.
Valence theory is sometimes hard put to interpret the unusual forms found:

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3. Few measurements of the so-called “driving force,” the Gibbs free energy,
have become available as yet; however, for a number of reactions in organic
nonaqueous solutions it is entropy driven, that is, is driven to a negative
value over the positive (endothermic) by a positive and its influence as
in the basic thermodynamic equation:
Necessity does seem to be the mother of invention. This nonaqueous electrochem-
istry has great practical value, for example, in new high-energy-density batteries and
fuel cells—just the things needed for electricity storage and production (respectively)
on board nonpolluting electric cars.

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Clearly, a standard free energy difference cannot be a drivingforce. However, the larger the negative value
of the more will be the tendency of a reaction, with reactants and products in their standard states, to
proceed.