Page:Outlines of Physical Chemistry - 1899.djvu/271

 ��Hydrolytic Equilibrium

It has been pointed out, in speaking of the dissociation brought about by heat, that the thermolysis of ammonium chloride, of phosphorus pentachloride, and of some other substances, is regulated by the equation of equilibrium hu =k l u i u 2 » In the majority of cases u x = u 2, so the equa- tion becomes hu = h x u x 2.

In the case of the dissociation of dissolved substances, we must consider that the water plays a part corresponding The cause of the phenomenon is new, but the effect pro- duced is analogous, so we are entitled to assume that for a binary electrolyte (one with two ions) the equation of equilibrium is

hu = h x u x 2. v

This equation denotes that there is a constant ratio between the product of the concentrations u x of the ionised molecules and the concentration u of the remaining undissociated substance.

The experimental determination of the concentrations u and u x as a rule offers no difficulty, and may be carried

��normal solution the two acids are very unequally dissociated, whilst in extremely dilute solution they have the same degree of dissocia- tion.

1 The solution of one molecule of salt gives rise to the reversible

reaction

+ - + -

MA + 02<=> MOH + HA + Otf,

in which the active mass of the water is constant. The quantity of solvent generally largely preponderates, and the disappearance or formation of a few H 2 molecules is of little consequence. In order to simplify the equation of equilibrium the influence of the solvent is taken into account in the coefficients k and k v (In an analogous manner the coefficients k and k x of the thermal dissociation take into account the influence of the temperature.)

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