analogous manner the effect produced by the forces which act between the atoms. Every change, accomplished with- out the intervention of a foreign energy, tends towards the production of that substance or system of substances Every simple or complex action of a purely chemical nature is accompanied by an evolution of heat (Thomsen,
In general this is the case, and we may say that this rule is in agreement with observed facts. Thus at low temperatures water is only decomposed by those metals whose heat of oxidation is higher than that of hydrogen. And in the same way the action of metals on dilute acids is determined; the metal dissolves with evolution of hydrogen, only if it can form a salt whose heat of forma- tion is greater than that of the acid employed.
The replacement of one metal by another in a salt solution likewise may only take place exothermically. The reactions
CuS0 4 .aq + Fe = FeS0 4 .aq + Cu
take place with evolution of heat (85 to 87 Cal.).
Direct substitution reactions generally conform to the law of maximum work. By combination with hydrogen and the metals, chlorine evolves more heat than bromine, and this latter more than iodine. Hydriodic acid and the iodides are decomposed by bromine and by chlorine, but hydrobromic acid and the bromides are only decomposed by chlorine.
On the other hand, iodine is substituted for chlorine
��1 'Tout ohangement accompli sans l'intervention d'une Anergic etrangere tend vers la production du corps ou du systeme de corps qui digage le plus de chaleur.'
- ' Jede einfaohe oder zusammengesetzte Wirkung rein ohemisoher
Natur ist von einer Warmeentwickelung begleitet.'
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