Page:Outlines of Physical Chemistry - 1899.djvu/180

 162 OUTLINES OF PHYSICAL CHEMISTRY

constituted by the union of strong acids and strong bases, and that the mixture does not give rise to the forma- tion of any precipitate. Every condensation of matter gives rise to an abnormal thermal effect.

It would be difficult at this part of the course to give an interpretation of these phenomena, and this is reserved for later chapters. Towards the, end of the third part, we shall see to what process the heats of neutralisation must be attributed ; and still further on we shall have occasion to study more closely the nature of the relation- ships which are established between the base and the acid in a salt solution.

The study of heats of neutralisation may now be closed by some remarks on the polybasic acids.

The different basicities of so?ne of these acids lead to the same thermal effect. As an example of this class tri- carballylic acid [C 3 H 6 (COOH)3] may be quoted. The pro- gressive saturation of this acid by one, two, and three equivalents of caustic potash evolves 13*2, 26*8 (or 13*4 x 2) and,89-75 (or 13*25 x 3) Calories.

Other polybasic acids behave in quite a different manner, and, as examples, phosphoric acid and arsenic acid may be specially mentioned.

[P0 4 H 3 in 330 H 2 0, NaOH in 110 H 2 0] = 14*7 Cal. [P0 4 H 3 in 330 H 2 0, 2NaOH in 220 H 2 0] = 263 Cal. [P0 4 H 3 in 830 H 2 0, 3NaOH in 380 H 2 0] = 33*6 Cal.

The evolution of heat is therefore 14*7 Cal. for the first NaOH, 11-6 Cal. for the second, and 7*3 Cal. for the third. It may be added that an excess of the base produces a further evolution of heat (1*2 Cal. for the fourth molecule and 0-8 Cal. for the fifth).

Thomsen has found the following for sulphuric acid : [H 2 S0 4 in 200 H 2 0, NaOH in 100 H 2 0] = 14-75 Cal. [H 2 S0 4 in 200 H 2 0, 2NaOH in 200 H 2 0] = 31-40 Cal.

i.e. 14*75 Cal. for the first molecule and 16*65 Cal. for the

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