304 PROPERTIES OF ELECTRICALLY CONDUCTING SYSTEMS 



conclusions may at the present time be reached relative to the manner 

 in which the thermal quantities are dependent upon the various factors 

 governing the condition of a solution. 



Water, itself, is ionized, and the energy of the ionization reaction 

 corresponds very satisfactorily with the heats of neutralization of strong 

 acids and bases. According to the Ionic Theory, the heats of neutraliza- 

 tion of different strong acids and bases should be the same at low con- 

 centrations, since the neutralization process under these conditions con- 

 sists essentially in a combination of the hydrogen and hydroxyl ions to 

 form water. The most reliable determination of the heats of neutraliza- 

 tion was made by Wormann. 33 The mean value of the heat of neu- 

 tralization for hydrochloric and nitric acids with sodium and potassium 

 hydroxides at 18 was found to be approximately 13700 calories. The 

 heat of ionization of water is related to the ionization constant of water 

 by means of the equation: 



(97) d\ogK_ U 



dT ~ RT 2 ' 



where U is the energy change accompanying the ionization of one mol 

 of water. Noyes and his associates 3 * have measured the ionization 

 constant of water at a series of temperatures up to 218. The heat of 

 ionization derived from their results is in good agreement with the value 

 found by Wormann for the heat of neutralization. Thus at 18 Noyes 

 finds that the value 14055 is in agreement with his experimental values. 

 Direct determinations of the heat of neutralization of strong acids and 

 bases at higher temperatures do not appear to exist, so that a compari- 

 son in these regions cannot be made. At higher temperatures the ioniza- 

 tion constant of water passes through a maximum, as a consequence of 

 which it follows that the heat of ionization changes sign. 



Equation 97 is likewise applicable to the ionization process of elec- 

 trolytes in water. If the ionization values are known at different tem- 

 peratures, the energy change accompanying the ionization process may 

 be calculated, assuming that the energy change accompanying the process 

 remains constant. The equation holds true even though the conditions 

 for dilute systems are not fulfilled, provided the concentrations enter- 

 ing in the equation represent the real concentrations of the molecular 

 species in question. Thermal data of sufficient precision are not avail- 

 able to make it possible to determine to what extent the results of con- 

 ductance measurements at different temperatures are in agreement with 

 thermal data. In a general way, however, the results appear to be in 

 agreement. In the case of the weak acids and bases, the order of 



* Wormann, Ann. D. Phys. 18, 775 (1905). 

 Carnegie Publications, No. 63 (1907). 



