Page:Outlines of Physical Chemistry - 1899.djvu/247

 know many reactions of this kind, and the equilibrium which they give rise to may be expressed by the equation :

k(a— x) (b — x)(c— x) = k^ai+x) (b l +x)(c l +x)

This equation refers to substances a, b, c,

A l9 B ly o l9 which form two systems capable of being

transformed into each other. The letters a^byC,

a u b l9 c l9 denote the initial active masses, and k and k x

are the activity coefficients of the two opposite transforma- tions. The equation tells us that a quantity x of the bodies of the first system has given rise to a corresponding quantity of substances of the second system, and that at this moment a stationary equilibrium has been established because the two opposite reactions have attained the same speed.

Bemarks. — I. Only substances whose concentration is subject to variation exert a variable influence on the equilibrium or on the speed of a reaction ; gases and dis- solved substances belong to this category. — Insoluble sub- stances, on the contrary, have a particular density and an invariable concentration, and their influence must be

2. To determine the variable active masses it is neces- sary to apply rigorously the definition already given, i.e. to take account of the number of equivalents per unit of volume (per litre).

8. It frequently happens that the two sides of the equation of equilibrium contain the same number of factors (a— x), (b—x), &c. In such cases it is sufficient to take

1 This principle, stated in 1867 by Ouldberg and Waage, may be accepted as an empirical rule.— Besides, an explanation has been given according to which the so-called insoluble substances are not absolutely unable to diffuse in water (or some other medium), but must rather be considered as endowed with an exceedingly small solubility. If this be true, and indeed it seems to be so, then in a mixed system the concentration, or active mass, of such a substance will be constant (i.e. as great as possible), provided the temperature be constant and a residual quantity of the substance be left undis- solved.

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