ELECTRO-AFFINITY 91 



This electro-affinity determines, on the one hand, the 

 direction in which a reaction will proceed in case one of the 

 above elements comes into contact with a normal solution 

 of one of the binary compounds, e. g. magnesium will dis- 

 place zinc from its halogen compounds, and chlorine will 

 displace bromine or iodine from combination with a 

 metal. So too, the limit to which the displacement will 

 take place may be worked out numerically from the above 

 data. 



Secondly, the data explains that binary compounds, of 

 which one has a strong affinity for positive, the other for 

 negative electricity, such as sodium chloride, have a ten- 

 dency to break up into ions according to the equation : 



and that this is most to be expected when the attraction 

 between positive and negative electricity is small, in media 

 of high dielectric constant such as water. 



Compounds of markedly negative elements such as 

 chlorine would therefore Jead to formation of positive 

 hydrogen ions in such a medium, that is, show an acid 

 character, and those of markedly positive elements such as 

 sodium with hydroxyl to formation of negative hydroxyl 

 ions, that is, show a basic character. 



2. Evolution of Heat in Simple Reactions- 



Effects of affinity may, as already explained, be followed 

 quantitatively in association with the work which a reac- 

 tion can perform. Since data are lacking we are driven 

 to the accompanying evolution of heat, which, at the abso- 

 lute zero would be equal to the work done, but at ordinary 

 temperature will differ from that more or less. 



A direct comparison may be made if from the heat of 

 formation of binary electrolytes the heat of formation of 

 the ions, or heat of ionization be calculated *. Again, taking 



1 Ostwald, Lehrbuch, 1893, ii. 953. 



