Davy, Count Rumford, all concerned themselves with thermochemical investigations of such processes.
Hess (18 4 o) were the first who systematically investigated thermochemical effects in solution, and arrived at conclusions from their experimental data which still possess validity.
Julius Thomsen was the first investigator who deliberately adopted the principle of the conservation of energy as the basis of a thermochemical system.
His thermochemical work was begun in 1853, but most of his experiments were performed in the years 1869-82, the whole being published collectively, under the title Thermochemische Untersuchungen, in four volumes.
Berthelot began (in 1873) a long series of thermochemical determinations.
It is to these two investigators and their pupils that most of our exact thermochemical data are due.
As the principle was abandoned even by its authors, it is now only of historical importance, although for many years it exerted considerable influence on thermochemical research.
From the standpoint of the law of conservation of energy, the relation between chemical and thermochemical action bears the following aspect: - A given amount of any substance under given conditions possesses a perfectly definite amount of intrinsic energy, and, no matter what chemical and physical transformations the substance may undergo, it will, when it returns to its original state, possess the original amount of intrinsic energy.
At the ordinary temperature, which must be allowed for in the thermochemical calculation.
Since thermochemical measurements of this type may be frequently performed with an error due to other causes of much less than i per cent., the error introduced by either of these assumptions is the chief cause of uncertainty in the method.
In view of the not very great accuracy of thermochemical measurements, the precise definition of the heat-unit employed is not a matter of special importance.
It has been proposed to adopt the joule, with the symbol j, as thermochemical unit for small quantities of heat, large amounts being expressed in terms of the kilojoule, Kj =100o j.
(For the exact relation between these heat-units, see Calorimetry.) For ordinary thermochemical work we may adopt the relation 1 cal.
Except for technological purposes, thermochemical data are not referred to unit quantity of matter, but to chemical quantities - i.e.
The notation which Julius Thomsen employed to express his thermochemical measurements is still extensively used, and is as follows: - The chemical symbols of the reacting substances are written in juxtaposition and separated by commas; the whole is then enclosed in brackets and connected by the sign of equality to the number expressing the thermal effect of the action.
- For thermochemical calculations it is of great importance to know the heat of formation of compounds from their elements, even when the combination cannot be brought about directly.
This being the case, we are at liberty to make the assumption that the intrinsic energy of each element (under specified conditions) is zero, without thereby introducing any risk of self-contradiction in thermochemical calculations.
The thermochemical magnitude which is universally determined for organic compounds is the heat of combustion, usually by means of the calorimetric bomb.
An important connexion between heats of combustion and constitution is found in the investigation of the effect of single, double and triple carbon linkages on the thermochemical constants.
The thermochemical properties of the constituents of an explosive will assign an upper limit to the volume, temperature and pressure of the gas produced by the combustion; but much experiment is required in addition.
The two carried out some of the earliest thermochemical investigations, devised apparatus for measuring linear and cubical expansions, and employed a modification of Joseph Black's ice calorimeter in a series of determinations of specific heats.