Page:Outlines of Physical Chemistry - 1899.djvu/253

 CHEMICAL ECIUILIBRIUM

��Acids HN0 8.

�Avidity

�HCl.

�HBr.

�HI.

�H,P0 4 *

�|C 2 H 2 4.

�CH 2 C1.C0 2 H

�HF.

�J Tartaric acid

�£ Citric acid

�CH3.CO2H

�HON.

��Heat of neutralisa- tion by NaOH

13-68 Cal.

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��As can be seen, the avidities are not at all proportional to the heats of neutralisation. So that the heat of neutralisation cannot be considered as a measure of the affinity of an acid for a base.

For bases formed from the divalent metals (Mg, Mn, Fe, Zn, Co, Ni, Cu) Thomsen has found that the relative avidity of sulphuric acid varies between 0*7 and 0*81, nitric acid being taken as standard of comparison. Similar irregularities have been noticed for the other polybasic acids.

/3. Study of the change of volume which accompanies

the reaction.

Ostwald's method is to dissolve one gram -equivalent of a base or of an acid in a quantity of water, such that the total weight of the solution is 1 kilogram. The density of the solution is then determined by means of a pycnometer, and from this the volume (at 20°C.) is deduced.

By neutralising one equivalent of base by one equi- valent of acid, a salt solution is obtained which weighs 2 kilograms. A determination of the density of this solu- tion with the pycnometer, and calculation of its volume, shows that this volume is not equal to the sum of the


 * For the first equivalent of soda.

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