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Although the heat evolution, or diminution of total energy, gives an approximate measure of the stability of a compound, it is really the content of free energy which determines its stability. Of the total energy diminution, part is in general convertible into work by suitable means, whilst the other part appears as heat. The part convertible into work is the free energy. For example, the reaction:
Zn + CuSO4 Aq. = ZnSO4 Aq. + Cu,
as it occurs in the ordinary way evolves heat, but when it occurs in the Daniell cell part of the energy change is obtainable as electrical energy, which in turn is (theoretically) completely convertible into work. It is this free energy change which provides an accurate measure of the tendency of the system Zn + CuSO4, Aq. to pass spontaneously into the system Cu + ZnSO4, Aq., i.e., which provides a measure of the relative stability of the two systems. Only those changes can occur spontaneously which are attended by a diminution of free energy. The corresponding statement for the total energy, to the effect that only those reactions occur spontaneously which are attended by evolution of heat, was stated by Thomsen and Berthelot, although the latter had the correct idea in mind when he called it the principle of maximum work. This principle is very often, as we have seen, approximately true and is a useful guide. The statement is true at the absolute zero, and in many reactions between solids and liquids it holds approximately. The correct statement of the principle forms what is known as Nernst's Heat Theorem (1906): this enables us in many cases to calculate equilibrium constants from heats of reaction.
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