In a recent work on chemical equilibrium, M. van't Hoff has shown that the majority of his experimental laws can be summarized in the following statement:
All equilibria between two different states of matter (systems) are displaced by a lowering of temperature toward that of the two systems whose formation develops the heat.
It has seemed to me that this law could be generalized still more by extending what it says of the temperature to condensation, and, moreover, it can be given an identical form to that of the laws of all equilibria which produce mechanical work by their displacement, and which depend consequently on the theorem of Carnot. The statement which I propose to make includes reversible chemical phenomena in the class of reciprocal phenomena to which M. Lippmann has recently added reversible electrical phenomena.
Every system in stable chemical equilibrium submitted to the influence of an exterior force which tends to cause variation, either in its temperature or its condensation (pressure, concentration, number of molecules in the unit of volume) in its totality or only in some one of its parts can undergo only those interior modifications which, if they occur alone, would produce a change of temperature, or of condensation, of a sign contrary to that resulting from the exterior force.
These modifications are generally progressive and partial.
They are sudden and complete when they can be produced without changing the individual condensation of various homogeneous parts which constitute the system in equilibrium, while changing the condensation of the system as a whole.
They are zero when their production cannot produce changes analogous to those due to the exterior force.
Finally, if these modifications are possible, nevertheless they are not essential. In the case where they do not occur, and where the system remains unaltered, the equilibrium, from being stable, becomes unstable and can then undergo only modifications which tend to bring it nearer the condition of stability.
Some examples of equilibria, for the most part well known, will show the generality of the applications of this law which includes equally phenomena of fusion, vaporization, and solution, which cannot in any way be distinguished from chemical phenomena, properly called.
1. Heating the total system produces the endothermic modification, such as fusion and volatilization of the whole body; polymerization of C2N, etc.; reversible dimorphic transformation of AgI; NH4O, NO5; dissociation of CO2; CaO, CO2; Bi2O3, 3NO5, 4HO; reversible endothermic combination of CS2 and very probably also of NO5, etc.; endothermic solution of most salts; endothermic crystallization of some salts: NaO, SO3; CaO, HO, well known to have a solubility decreasing with temperature.
2. Partial heating of a system produces modifications which tend to cool the part heated altogether, such as the propagation of heat by conduction, the production of thermoelectric currents, the change of concentration by diffusion, the transfer of metal from one point to another of a strip placed in a solution of one of its salts.
3. The increase in condensation of a whole system maintained at constant temperature produces modifications which tend to reduce the condensation of the system, such as the fusion of ice, the solidification of paraffin, the dimorphic transformation of AgI, the combination of the dissociation products of CO2.
4. The increase of condensation of part of a system produces modifications tending to diminish the condensation of the part which is altered, such as condensation of water vapor, combination of CaO + CO2 at red heat, diffusion of unequally concentrated solutions, transfer of metal on a strip placed in a solution of one of its salts of variable concentration, lowering of the melting point of an alloy or a mixture of salts during its progressive solidification.
5. The modifications of equilibrium are usually progressive: for example, in the dissociation of CO2, and in general in all systems whose elements are not simply juxtaposed, but of which some form homogeneous mixtures between themselves.
6. The modifications of equilibrium are total when they can be produced without altering the condensation of any of the parts of the system, while changing the total condensation of the whole system. Such is the condensation of water vapor, the fusion of ice, the dimorphic transformation of AgI, the dissociation of CaO, CO2 and of solid CuO, the solution of salts. These systems for an infinitely small change of condensation of one of their parts pass from an extreme limit of their equilibrium state to the extreme opposite limit.
7. The modifications of equilibrium are zero when they cannot produce an effect analogous to that due to the exterior force. Dissociation is independent of pressure for mixtures which combine without change of volume, hydriodic acid, for example. The limit of equilibrium is independent of temperature when their transformations do not liberate heat, which is the case in esterification.
8. Finally, all modifications of equilibrium are only possible, but are not necessarily produced, as is shown in superfusion, superheating, supersaturation, the rapid cooling of dissociated carbonic acid. The unstable systems thus obtained can only be modified to bring nearer the conditions of stable equilibrium. The transformation of these unstable equilibria is generally effected with a liberation of heat, conforming to the principle of maximum work because, as M. van't Hoff has remarked, ordinary temperature differs little from absolute zero, for which stable equilibrium corresponds to liberation of all the heat contained in the body.
Lippmann, Annales de Chimie et de Physique, 5th series, vol. XXIV, p. 172. [original note]