THE HYDRATE THEORY OF SOLUTIONS 209 



with the surprising contrast that whilst the dissociation of the 

 salt into metal and gas involves the absorption of 105,600 

 calories, the dissociation into ions involves an absorption of 

 4,400 calories only. 



KCl solid = K metal + CI gas . . - 105,600 cal. 

 KC1 solid = K ion + CI ion . . - 4,400 cal. 



— 101,200 cal. 



In the case of hydrogen chloride the figures are even more 



remarkable : 



HCl gas = H gas + CI gas . . - 22,000 cal. 



HC1 gas = H ion + CI ion . . . + 17,300 cal. 



since the heat changes, although of similar magnitude, are 

 actually opposite in sign. 



The difficulty was accentuated when Arrhenius {Zeit. phys. 

 Chern. 1889, 4, 104), by making use of van't HofFs equation, was 

 enabled to calculate from the temperature-variations of the 

 coefficient of ionisation the thermal change involved in the 

 ionisation of the dissolved salt. In the case of potassium 

 chloride this amounted to a liberation of +250 calories, 1 so that 

 the equations for the dissociation of the dissolved (but non- 

 ionised) salt are 



KCl diss = K metal + CI gas ... - 100,950 cal. 

 KCl diss = K ion + CI ion . . . + 250 cal. 



- 101,200 cal. 



An alternative expression of the heat changes involved in 

 ionisation is given by combining the above equations, e.g. : 



K metal + Ci gas = K ion + CI ion . + 101,200 cal. 



H gas + CI gas = H ion + CI ion . + 39,300 cal. 



From these equations it appears that the conversion of an 

 element from its ordinary form into the ionised form involves 

 the liberation of large quantities of heat, the sum of the heats 

 of formation of a pair of ions being identical with the heat of 

 formation of the dissolved salt as determined experimentally. 2 

 From what source are these large quantities of energy 



1 In a subsequent paper (Zeit. phys. Chem. 1892, 9, 340) a small negative value 

 was given, but the earlier value appears more probable in view of the fact that 

 the coefficient of ionisation decreases when the temperature is raised. 



2 For a full discussion of the heats of formation of the ions see Bousfield and 

 Lowry, Trans. Faraday Soc. 1907, 3, 129. 



14 



