One more important application of thermody namics to chemical phenomena has been made and requires mention in the present sketch. Thermodynamics, in studying a transformation of some material system, endeavors to ascertain the maximum mechanical work that might be produced by the transformation. the 'maximum work' meaning the work that might he obtained by the use of some ideal mechanical device, frictionless and permitting, so to speak, of no leakage of energy. The importance of knowing this maximum of work is very great. Any natu ral change taking place of itself—whether it he the falling of a stone, the expansion of a com pressed gas, the combustion of coal, or any other change, mechanical or chemical—may be used to produce mechanical work; and no material sys tem is capable of changing unless it possesses the capacity for producing work—or. as Helm holtz terms it, 'free energy.' In other words. it may be said that it is because a system can pro duce mechanical work that it is capable of changing spontaneously. The of coal (i.e. the chemical transformation of the system, carbon and oxygen), once started, can go on of itself because it can be used to produce mechani cal work, or, what is the same, because the sys tem carbon and oxygen possesses a certain amount of free energy. When, therefore, the free energy of a system has been used, without loss, to produce mechanical work, and that work has been measured, we have a measure of the cause of the given transformation. The cause of chemical transformations is generally termed 'chemical affinity.' Obviously, then, the maxi mum work that can he produced by a chemical transformation is a measure of the chemical affinities involved in it, and this is why the de termination of maximum work has great impor tance for chemical theory. But it may also be valuable for purely practical purposes. Take, for instance, again the combustion of coal. It is well known that steam engines are very waste ful of energy. In connection with the problem of a more economic use of coal the question must naturally arise, How much mechanical work could possibly be obtained altogether by burn ing a certain amount of coal, that an ideally perfect device were employed for the purpose? The direct measurement of the maxi mum work, although theoretically possible, could not be actually carried out. But the maximum work of a reaction can be readily calculated, with the aid of thermodynamics, if the concentra tions of the reacting substances and their prod ucts, when in the state. of chemical equilibrium, are known. In the case of the combustion of coal the equilibrium-concentrations have been determined by indirect measurement, and on the basis of this Nernst has calculated approxi mately the maximum work of the combustion for three different temperatures: If 12 grants of carbon were burned at the absolute zero (-273° C.), the equivalent of 97,650 calories might be
obtained; at 18° C. the maximum work would be equivalent to 91,470 calories; at 1000° C. the equivalent of only 70,625 calories can be ob tained; this in spite of the fact that the heat given off by the combustion is practically the same at all temperatures, viz. 97,650 calories. Only at the absolute zero of temperature could the heat produced by the combustion of coal be entirely transformed into mechanical work.
In conclusion, a few words must be said with reference to an erroneous principle that has gained somewhat wide acceptance among chem ists—viz. Berthelot's principle, according to which it is the heat produced by a reaction, and not t-he maximum possible mechanical work, that measures the cause of the reaction; and of two re actions that might take place in a given system, the one accompanied by the greatest evolution of heat must necessarily take place. This principle holds good often, but not always, and so cannot be looked upon as a law of nature. The most important argument against it is that, were it unlimited in its application, as Bernelot claims it to be, reversible reactions would be•hnpossible; for one of a pair of reversible reactions not only does not develop heat, but necessarily absorbs heat; and hence, if Berthelot's principle were correct, that reaction could not take place at all, and its opposite reaction would be complete. See REACTION, CHEMICAL.
The principal names connected with experi mental thermo-chemistry are those of Hess, Julius Thomsen, Berthelot. and Stohmann. The first to apply the principles of thermodynamics to chemical phenomena was Horstmann. The problem was next taken up by Dr. Gibbs, of Yale University, whose thorough and original treatment of the subject remained unknown for a number of years. The importance of Van 't Hoff's thermo-chemical work may be readily seen from the present sketch. Finally, Le Chatelier, Planck, Riecke, and Duhem have made note worthy contributions to the mathematical treat ment of the subject.
BIBLIOGRAPHY. Thomson, Thermochemische Bibliography. Thomson, Thermochemische Untersurhungen (Leipzig, 1882-86) ; Bernelot, Therniochimic (Paris, 1897) ; id., Traite pra tigue de calorimetric chintique (lb., 1893) ; Muir and Wilson, The Elements of Thermal Chemistry (London, 18S5) ; Jahn, Die der Thermochemie (Vienna, 1892) Planck, Grund riss der allgemeinen Thermochemie (Breslau, 1893) ; Naumann, Thermochemie (Brunswick, 1892). Consult also the literature of theoretical and physical chemistry recommended in the article CHEMISTRY.