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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