Hermann von HelmholtzHermann von Helmholtz was a German physicist and a physician. He made great contributions to many fields of science. From the point of view of information philosophy, his most important work was on the classical mechanical foundation of thermodynamics. Helmholtz' s article on the conservation of energy (he called it "force") appeared in 1847, a few years before Clausius. Several years later, in 1882 his article On the thermodynamics of chemical processes defined his idea of available or "free" energy. It is the amount of mechanical work that can be done by heat transfer from a hot body to a cool one at constant volume.
Previous investigations on the work- value of chemical processes relate almost exclusively to the quantities of heat appearing or vanishing during the formation and decomposition of compounds. Now alterations in the state of aggregation and in the density of the substances concerned are indissolubly connected with most chemical changes. Of these last, however, we know certainly that they are capable of producing or consuming work in two different forms, to wit: first in the form of heat, secondly in the form of other kinds of work convertible without limit. A store of heat is not, as is well known from Carnot's law—more precisely expressed by Clausius—convertible without limit into other work-equivalents. We can only bring this about in any case, and even then only partially, by allowing the untransformed residue of the heat to pass over into a body of lower temperature. We know that in the case of fusion, vaporization, expansion of gases, &c, heat can even be extracted from surrounding bodies at the same temperature, to pass over into work of other forms. Since such changes, as already said, are indissolubly connected with most chemical processes, this circumstance clearly proves that, even in the case of chemical changes, the origin of these two forms of work-equivalents must be examined, and they must be considered from the point of view of Carnot's law. It has long been known that there are chemical processes which occur spontaneously and proceed without external force, and in which cold is produced. Of these processes the customary theoretical treatment, which deals only with the heat developed as the measure of the work-value of the chemical forces of affinity, can give no satisfactory account*... If we now take into consideration that chemical forces can produce not merely heat but also other forms of energy, the latter even without the necessity of any change of temperature in the interacting substances being sot up corresponding to the magnitude of the effect, as, for example, in the case of the production of work by a galvanic battery ; then it appears to me unquestionable that, even in the case of chemical processes, a distinction must be made between the parts of their forces of affinity capable of free transformation into other forms of work, and the parts producible only as heat.In what follows I shall, for the sake of brevity, distinguish these two parts of the energy as the " free " and as the " bound " energy. We shall see later that processes spontaneously originating and proceeding without the help of any external force, when the system is at rest and is maintained at a constant uniform temperature, can take place only in such a direction as to cause diminution of free energy. In this category will also be reckoned chemical processes originating spontaneously and proceeding at constant temperature. On the assumption of the unrestricted applicability of Clausius's law, it would therefore be the value of the free energy, not that of the total energy made known by development of heat, which especially determines the direction in which chemical affinity can become active.
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