Page:Text-book of Electrochemistry.djvu/108

 When the formula is integrated we obtain —

or — (7i = Coe

Since TqTi for a small temperature interval clianges but little, and the variations of Xi and i are not great, we may write with sufficiently close approximation —

where A represents •^. . =r^ and h - ^0 (=^1 - ^0) is

reckoned in Celsius degrees. The concentration of a saturated solution therefore increases with rise of temperature approxi- mately proportionally to an exponential function, so that the solubility increases almost in the same ratio between 0° and 5° as between 5° and 10°, between 10° and 15°, etc. Attention was first called to this peculiarity by Nordenskiold (1^^),

Change of Homogeneous Equilibrium with the Temperature. — Precisely the same relationships exist in the case of equilibria between a mixture of vapoura and liquids as between a liquid and its vapour. As already pointed out, the volume of the liquid does not enter into the formula on accoimt of its comparative smallness. Let us consider the equilibrium which exists in a mixture of ammonium chloride vapour, ammonia, and hydrochloric acid, the partial pressures of which are ps, pi, and p^. If a change of temperature takes place, the change of equilibrium —

produced is regulated by the above connection. It must, however, be observed that a gram-molecule of ammonium chloride disappears when a gram-molecule of ammonia and one of hydrochloric acid are formed. Van*t Hoff 's application (13) of the Clapeyron formula to this case gives —

dhxpi. dlnpi ^ d Inps _ /i dT dT It' RT'

�� �