CHEMICAL THEORY OF SOLUTIONS. PART I. 41 



Even in the cases wliere ? is given ^ can not be determined 

 at once, because v is generally unknown. But v is in general a 

 small integer, and we are often able to deduce or infer its value 

 from various experimental data. 



The variation of 5^ with the temperature is expressed by 

 the equation 



QlnR ^ D 

 dT BT' 



where D is the heat of reaction for u mois of ©a. As it is 

 quite probable that the heat capacity of @ß is not very different 

 from that of ^Sa, we may as an approximation, assume D to 

 be independent of temperature, then 



In^ 



-§(i-^-) (-) 



where % is the temperature at which ^ becomes unity. This 

 temperature may be called the equijjoise j^oint of the reaction 

 and is characteristic of the homogeneous equilibrium just as the 

 boiling point, melting point, or eutectic point is characteristic 

 of the respective heterogeneous equlibrium. For such reactions 

 as 2 03^302 or 2H20'^2H2 + 02 the equipoise point is very 

 high, while for such reactions as 2 SCL^ S2CI2+CI2 it is re- 

 latively low. When the given temperature is far from the 

 equipoise point we may say that the chemical species on the 

 right or the left side of the equation are stable against the 

 species on the other side according as D, the heat of reaction, 

 is positive or negative. 



{b) Vapour Tension of an Associated Liquid. 

 If we assume that the chemical species ©a and ©p taken 



