304 



SCIEXCE PROGRESS. 



process occurs when any strongly dissociated acid acts on 

 any strongly dissociated base, so that, in all such cases, the 

 heat evolution should be approximately the same. This re- 

 sult is fully borne out by the experiments of Thomsen, who 

 found that the heat of neutralisation of one gram-molecule 

 of a strong base, by an equivalent quantity of a strong acid, 

 was nearly constant, and equal to 13,700 or 13,800 calories. 



In the case of weaker acids, the dissociation of which is 

 less complete, divergences from this constant will occur, lor 

 some of their molecules have to be separated into their ions. 

 For instance, sulphuric acid, which in the fairly strong solu- 

 tions used by Thomsen is only about half dissociated, gives 

 a higher value for the heat of neutralisation. Since the 

 salts formed are almost completely dissociated, it follows 

 that heat must be evolved when a molecule of sulphuric 

 acid is dissociated into its ions. The heat of formation of 

 a molecule from its ions is, of course, not the same as that 

 associated with the formation of the molecule from its ele- 

 ments in the usual way, for the energy of the ion is very 

 different from that of the atom in its normal state. 



If we know the temperature coefficient of dissociation, 

 it is possible to calculate the heat of formation of a gram- 

 molecule from its ions by means of a thermodynamical 

 relation, which was first applied by Clausius to the case 

 of the evaporation of a liquid. In this way Arrhenius * has 

 deduced the heats of formation of many bodies, from which 

 the following are selected : — 



1 Zeits. fur physikal. Chemie, iv.. p. 96, 1889; ix., p. 339, 1892. 



