Page:Text-book of Electrochemistry.djvu/212

 XII.

��HEAT OF NEUTRALISATION.

��Then the process would become exaxjtly like the former. The heat of dissociation, /i, of the succinic acid can be calculated from the change of the dissociation constant with the temperature, by means of the same equation as we have applied to water. If /x thus found be multiplied by 0*985, and the product subtracted from the heat of neutralisation of strong acids, we must obtain the correct value for the neutralisation of succinic acid. I (14) have calculated the heat of neutralisation of several acids in this way, and have obtained values which agree with those determined calori- metrically by Thomsen.

Heat of Neutralisation at 21-5°.

��Add.

� � � �Observed.

�CalcaUted.

�Hydrobromic acid

�Nitric acid. ..

�Acetic acid. .

�Propionic acid

�Butyric acid. .

�Succinic acid. ..

�Dichloracetic acid.

�Phosphoric acid. .

�Hypophosphorous acid

�Hydrofluoric acid

� �

��The heat of neutralisation of most acids is evidently greater than the heat of dissociation of water, which I have calculated to be 13,212 cal. at 21-5°. The cause of this is that the heat of dissociation of most acids is negative, i.e.

J 1 xr

,y is negative, or, in other words, the dissociation constant,

and consequently also the degree of dissociation, decreases with rising temperature. This may appear strange since in the ordinary dissociations the degree increases with rising temperature. But cases of ordinary dissociation are known, e.g, the decomposition of hydrogen selenide and hydrogen telluride into the elements, in which, at any rate at

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