Page:Text-book of Electrochemistry.djvu/213

 198 EQUILIBRIUM BETWEEN ELECTROLYTES, chap.

certain temperatures, the dissociation decreases as the tem- perature rises. The decomposition of ozone into oxygen —

208 = 30.2,

which takes place with evolution of heat, is another example. The fact that the degree of dissociation of electrolytes as a rule decrefises with rising temperature is perhaps connected with the fact that the dielectric constant of water decreases as the temperature rises (by about 0*6 per cent, per degree at 0°, according to Abegg). According to the theory of J. J. Thomson {15) and Nernst {16)^ the degree of dissociation should increase with the dielectric constant. It must be noted, however, that the heat of neutralisation of many acids is smaller than the heat of dissociation of water, and therefore their degree of dissociation must increase with rising temperature, as is the case with succinic acid, and to a greater extent with hydrocyanic acid (heat of neutralisation 3000 cal.).

Electrolytes with a Negative Temperature Co- efficient for the Conductivity. — As the degree of dissocia- tion of several acids, as well as that of some salts, decreases with rising temperature, it may happen that the product aX^ which is equal to X„ i.e, that the equivalent conductivity, and with it the specific, diminishes as the temperature rises ; in other words, that the temperature coefficient becomes negative, although as a rule X^ (for acids) increases by about 1*7 per cent, per degree. It is to be expected that this will most probably be the case with acids which have a high heat of neutralisation. The theory also predicts that this will happen more readily at high than at low tempera- tures. Experiment shows that the temperature coefficient for hypophosphorous acid above 54^ and for phosphoric acid above 74° (both in normal solution) is negative. These results were quite unexpected, for it was formerly supposed that the conductivity of all electrolytes must increase as the temperature was raised.

Neutralisation Volume. — On mixing a solution of an

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