THEORIES OF ELECTROLYSIS. 303 



solutions, however, the value of k does not keep so satis- 

 factorily constant, and no good explanation of the discrepancy 

 has yet been given. 



If we put a equal to \ in the equation, we find for k a 

 value \J2v. Thus, 2k measures the concentration at which 

 the electrolyte is just half dissociated. 



Ostwald considers that this constant, k, eives the "lono-- 

 sought numerical value of the chemical affinity," since it 

 depends only on the nature of the substance. 



In the case of substances like ammonia and acetic acid, 

 where the dissociation is small, 1 - a is nearly equal to 

 unity, and the equation becomes 



V 



or a = Jvk, 



so that the molecular conductivity should be proportional to 

 the square root of the dilution. This result has been fully 

 confirmed by experiments on solutions of acetic acid. 



The theory gives an immediate explanation of Hess' law 

 of thermoneutrality, which expresses the fact that, in general, 

 no heat change occurs when two neutral salt solutions are 

 mixed. Since the salts, both before and after mixture, exist 

 mainly as dissociated ions, it is obvious that large thermal 

 effects can only appear when the state of dissociation of the 

 products is very different from that of the reagents. Let us 

 now consider the case of the neutralisation of a base by an 

 acid in the light of the dissociation theory. In dilute solu- 

 tion, such substances as hydrochloric acid and potash are 

 almost completely dissociated, so that, instead of representing 

 the reaction as 



HC1 + KOH = KC1 + H 2 0, 

 we must write 



+ - + - . + 



H + C14-K + OH = K + CI+ H 2 0. 



The ions K and CI suffer no change, but the hydrogen of 



the acid and the hydroxyl of the potash unite to form water, 



which is only very slightly dissociated owing to the fact 



that, being the solvent, it is present at a great concentration. 



The heat liberated, then, is almost exclusively that produced 



by the formation of water from its ions. An exactly similar 



