Page:Outlines of Physical Chemistry - 1899.djvu/233

 �NATUEE OF SALT SOLUTIONS ' /r\- ; B L

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lised by one of hydrochloric acid evolves 13m Call r Ah*cMTY yet the theory states that the final product cofct^ins only dissociated salt : Na + CI. So that the successive ~ "heats """ evolved, which would seem to indicate a gradual condensa- tion of matter, are leading to the maximum disintegration — to the absolute freedom of the ions. The contradiction

Of all the exothermic effects enumerated above, the new theory only gives a satisfactory explanation of one — namely, the heat of neutralisation. The formation of an equivalent of salt by the interaction of a strong base and a strong acid in dilute aqueous solution is accompanied by an almost constant evolution of 13 to 14 Calories, and ArrhAnius explains that the following takes place :

solution of solution of solution of

base acid salt

+ -

The ions M and E (metal and acid radical) after the

reaction being free, as before, the only change which has

- +

taken place is the union of the ions OH and H to form

neutral water. Now water is not an electrolyte 2 and is,

therefore, not dissociated, but is formed with evolution of

heat. As there is no other thermal effect, this formation

of water always corresponds to about the same evolution

of heat — about 18 Calories.

The constancy of the heat of neutralisation is thus

explained, but the whole of the thermo-chemical data does

not seem to be in accord with Arrhdnius's theory.

1 This contradiction appears quite as well if we consider the heat of formation of sodium chloride from its elements ; 97*9 Cal., leaving out of account the energy dissipated in decomposing the molecules

Na n and Cl 2. The solution of the salt absorbs only a small quantity. t of heat. And yet according to the theory this solution actually sets the sodium and the chlorine free, and that without any chemical }'. intervention on the part of the solvent.

2 Specific conductivity of water slowly distilled in a good ap- paratus : 2 x 10~ 6. Specific conductivity of water distilled in vacuo : about eight times less.

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