This article was originally published in Electrochemical and Metallurgical Industry Publication of April 1907. Information within this article is therefore correct as of 1907. The publication of this material aims to provide historical insight on the subject and its place in industry.
At this place we spoke in our last issue of the loss of Mendeleeff, Moissan and Roozeboom. We have now to record the tragic death of Marcellin Berthelot, a few moments after that of his loved wife. Unlike his compatriot, Moissan, Berthelot was not solely a scientist, but he took a part; and a very active part, in the public political life of France. Yet the grand work of his life related to thermochemistry. The two big volumes in which his experimental determinations of heats of reaction are collected, form a "monumentum aere perennius." They will make the name of Berthelot immortal. It would be useless to dwell any further on the value of his experimental work. But a few words may be said on its relation to thermodynamical theory.
When we speak of the heat of a chemical reaction, we really mean the energy of the reaction, measured (arbitrarily) in heat units. We can get this energy completely in the form of heat; or we can get it, at least partly, by means of electrochemical processes, in form of electrical energy; or, for instance, by employing expanding gases, we can get it partly in form of mechanical work. In any case the total energy of reaction is the same, whatever may be the way in which we force the reaction to go on. This is the principle of the conservation of energy. Whenever we want to employ this principle; for instance, for giving the heat balance sheet of a chemical or metallurgical process, we need numerical data for the energies of reaction such as Berthelot has determined. This indicates the enormous field of their applicability. But it also indicates the limitations, although Berthelot has maintained a different view on this point for a long time. The principle of the conservation of energy does not state anything whatever concerning the direction in which a reaction will go on. Berthelot, however, believed in the "principle of maximum work," that if a reaction shall go on at a certain temperature, it is necessary that this reaction shall evolve heat. We know now that pronounced in such generality this rule is wrong. We know now that if we want to find out something on the direction in which a process will go on, we must start from the entropy principle instead of the energy principle; we must know the free energy, not the total energy of reaction. Nevertheless, the strict principle will resolve itself practically into Berthelot's rule for reactions of great intensity and at low temperatures (including our ordinary temperature). It is for this reason that although wrong in its foundation, Berthelot's rule may be usefully employed for many practical purposes.
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