LEWIS. — A NEW SYSTEM OF THERMODYNAMIC CHEMISTRY. 285 



Now in the ideal solution it is easy to show rigorously^ as van't Iloff 

 has done, that the condition of equilibrium at a given temperature is, 



C°C P - • ■ 



",.,!'„ = constant. 



it ... 



where C A , etc., represent the concentrations. But in this solution the 

 concentrations are proportional to the activities, and therefore, 



f° fp . . . 



f£— = K. XXIII 



where K is another constant. Since the activities $ A , etc., are not only 

 the activities in the ideal solution, but also in the original system, it 

 is obvious that equation XXIII expresses a law of extraordinary gen- 

 erality. 



The above quotient, which we have called K, has a value which, for 

 a given reaction at a given temperature, does not depend upon the 

 medium in which the reaction occurs, nor upon the concentrations, nor 

 upon the pressure, nor upon the nature or number of the phases which 

 are concerned in the reaction. In other words K depends only upon 

 the temperature and the specific nature of the reaction. It is there- 

 fore a better measure of the true " affinity " of a chemical reaction 

 than any quantity that has hitherto been used for this purpose. 



The equilibrium ratio, K, changes with the temperature according 

 to a simple law. We may imagine the substances taking part in a 

 given reaction all vaporized in a space so large that each, vapor be- 

 haves like a perfect gas. If the reaction reaches equilibrium under 

 these conditions, it is easy to show that the following equation of van't 

 Hoff is entirely exact, namely, 



CoCl- • • 

 din 



act- • • u 



2 ' 



dT nr- 



where C A , C B , etc., represented the concentrations, and U is the increase 

 in internal energy when the reaction occurs in this extremely attenu- 

 ated gaseous phase. 



Since we are dealing with infinitely attenuated vapors, C A , etc., may 

 be replaced by £ A , etc., whence 



dT ~ MT l AA1V 



