Popular Science Monthly/Volume 23/October 1883/The Aim of Thermo-Chemical Investigations
THEORETICAL chemistry is based upon the molecular theory, according to which all matter is made up of molecules, and these molecules of atoms. The physical state of bodies depends upon the arrangement and motions of the molecules; the other physical and chemical properties depend upon the kind and number of the atoms in the molecule, upon their arrangement and relative motions. Every action which causes a change in the internal structure of the molecules is a chemical action. These changes may be of various sorts: either there takes place simply a change of position among the atoms of the molecule and there is then formed a body which is isomeric or metameric with the original substance; or the molecule is broken up into several molecules, and the process is then one of dissociation, or of simple decomposition; or, further, several molecules unite together to form a single molecule, in which case the process is spoken of as one of condensation or addition; or, finally, molecules act upon each other with interchange of atoms and formation of new molecules, which is the phenomenon of mutual decomposition, the most common sort of chemical action.
From the doctrine of the conservation of matter, it follows that the mass of the bodies which take part in any chemical reaction does not change, and is fully accounted for in the products of the reaction: this principle forms the foundation for quantitative chemical investigations into the composition of substances.
From the principle of the conservation of energy, it follows, in like manner, that no energy is lost or created, and that, consequently, the whole of the energy originally present in the bodies, which act upon each other, appears again in the products of the chemical reaction, although often in a different manner than before. This principle forms the foundation of all quantitative thermo-chemical investigations.
The energy of a molecule is always the same at the same temperature; any increase or diminution of the energy of the molecules, without change of their internal structure, manifests itself as an elevation or a lowering of the temperature of the substance in question.
By chemical action, the structure of the molecule is changed, the atoms become grouped together in a different way, new relations manifest themselves among them, and the energy of the new molecule becomes different from that of the original one. The simplest case is that where the only change is in the grouping and the motions of the atoms of the same molecule, that is where an isomeric compound is formed; in such a case the atoms of the molecule pass from one condition of equilibrium to another, and, according as the new condition of equilibrium answers to a greater or less stability than the original, there occurs either an evolution or an absorption of energy, and the temperature of a corresponding body changes; in the first case it rises, in the second case it falls.
The chemical action, the passage from one isomeric condition to the other, is therefore accompanied by an evolution, or by a disappearance of heat, according as the attractions of the atoms are more fully or less fully satisfied in the body in its new condition.
The case is similar in other chemical actions; if the molecules of the substances produced by the reaction contain at the same temperature a smaller total energy than the molecules of the bodies acting upon each other, then the chemical action is accompanied by an evolution of heat; in the opposite case, an absorption of heat takes place. The greater the difference, the greater also will be the change of temperature accompanying the process, so that, if the action takes place rapidly, the temperature may reach that of a red heat, as a result of which the chemical action assumes the character of combustion in the more limited sense of the word.
The aim of quantitative thermo-chemical investigation is now, in the first place, to measure those quantities of heat which are evolved or absorbed in chemical actions. It is true that these values furnish no direct information as to the magnitude of the forces which are concerned in the chemical action, partly because they are merely an expression for the difference between the energy of the molecules decomposed and that of the molecules formed, partly because they are often affected by other actions which accompany the chemical process; they furnish, however, the material for theoretical investigations—for the higher aim of thermo-chemistry is to establish the dynamical laws of chemical action and to afford an insight into the mysterious region of the constitution of chemical compounds—that is, of the molecules.
Up to the present time, an almost impenetrable veil has enveloped the internal structure of the molecules and the true nature of the atoms: we know, at the most, the relative number of the different atoms in the molecule, the relative mass of the molecules, and of the individual atoms, and the presence of certain groups of atoms (radicals) in the molecules; but we know almost nothing about the nature of the forces which dominate in the molecule, and which cause the formation and decomposition of compounds.Experience teaches that the different atoms exert an influence upon each other, which seems to be independent of their mass and which appears now as attraction, now as repulsion, and that the combining capacity of the atoms does not extend beyond a certain limit; still, up to the present time, no satisfactory explanation has been offered for these, the chief phenomena of chemistry. Chemical processes consequently do not as yet admit of a mathematical discussion in their entire extent, as is the case, for example, with the phenomena of physics and astronomy; for the general mathematical discussion of chemical phenomena we lack that which is most important as a basis, namely, a knowledge of the fundamental laws which govern the actions of the atoms. With each decade, however, chemistry approaches nearer and nearer the exact sciences, and already many laws of wider or narrower application are being established on the basis of experiment. The extremely rich and varied material of chemistry now arranges itself in large groups, the members of which follow certain common rules or laws, with reference to their formation and decomposition, and whose properties may be, to a certain extent, deduced from the composition of their molecules.
There is, however, probably no doubt that the mutual action of the atoms, their attractions, and their unequal combining capacity—in short, that "affinity" follows the general dynamic and static laws of mechanical phenomena, and that, in chemistry, as in mechanics, the right of the stronger prevails; with this assumption, general dynamic and static laws may be developed for the phenomena of chemistry, although the real nature of "affinity" is still entirely unknown.
- Translated from the introduction to Thomson's "Thermochemische Untersuchungen," Leipsic, 1882, by W. R. Nichols.