146 OUTLINES OF PHYSICAL CHEMISTEY
The energy lost by the atoms or molecules may mani- fest itself in the form of heat, electricity, light, or mechanical motion. Thermo-chemistry treats of the heat phenomena which accompany chemical reactions.
From what has been said it would seem, a priori, that
THE THERMAL EFFECT OF A REACTION GIVES US A MEASUBE OF THE AFFINITIES OF WHICH IT IS THE RESULT. And
particularly, the heat of formation of a compound from its elements would be proportional to the affinities which unite the atoms.
In fact, if in a reaction the atoms rearrange themselves so as to enter into new unions, the amount of chemical energy transformed into heat will be the greater according as the new attractions entering into play are greater than the old attractions.
It is necessary, however, to notice that the changes which take place in the relationship between the molecules — that is to say, in the physical state of the matter, the density, the cohesion, the crystalline form, &c. — give rise to thermal effects which are generally very considerable. As yet, we have not succeeded in determining what part of the general thermal effect (calories in a thermo-chemical obser- vation) is due solely to the influence of the atomic affini- ties, and what part is due to the changes in the state of the molecules.
Furthermore, even in the case of direct combinations the heat evolved by the union of the isolated atoms is abso- lutely unknown. In the direct formation of hydrochloric acid gas from hydrogen and chlorine, or in the synthesis of water from its elements, the equations
are not realised, but true double decompositions take place, H 2 + Cl 2 = 2HC1 and 2H 2 + 2 = 2H 2 0.
The true heat of formation of two molecules of hydro- chloric acid from isolated atoms of hydrogen and chlorine
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