Page:Outlines of Physical Chemistry - 1899.djvu/172

 154 OUTLINES OF PHYSICAL CHEMISTRY

capacity of its components. The heat evolved by the re- action has, therefore, been augmented by this circumstance, and the thermo-ohemical data obtained make the affinity action seem larger than it in reality is.

In tables of Heats of Formation we are content with denoting the state of the reacting substances and of the products obtained. Corrections, which would be necessary in order to eliminate the thermal effect caused by change of volume and by variations of the specific heat, are, as a rule, not made.

Principle of the Initial and Final States ((?. H. Hess, 1840)

The thermal effect of a reaction depends solely on the initial state and final state of the system. It is the same whatever be the nature and the course of the intermediate stages. It is hardly necessary to mention that all mechanical effects external to the system must be eliminated. In order to clearly see the meaning of this theorem, let us take an example.

If anhydrous lime (GaO) be allowed to react on a dilute solution of hydrochloric acid, a dilute solution of calcium chloride is formed, and 46 Cal. are evolved.

But we can start out from the same initial state and arrive finally at the same dilute solution of calcium chloride by passing through several intermediate states :

The hydration of lime evolves 15 Cal.

The solution of the hydrate formed evolves. . . 3 „ The reaction (lime-water + dilute HC1 solution) evolves 28 „

Adding these together we get 46 Cal., which is the same as in the first operation.

The principle which has just been announced and justified, plays an important role in thermb-chemistry, and permits us to acquire some data the direct determination of which would be attended with the greatest difficulty.

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