244 OUTLINES OF PHYSICAL CHEMISTRY
When the original substance itself is a gas or a vapour, one more variable must be taken into consideration, and the equation becomes
Ten = k l u l u 2 ,
has a constant value. Examples of this case are phos- phorus pentachloride, ammonium chloride, amylene hydro- chloride, nitrogen peroxide, dkc. 1 For all these substances the dissociation is accompanied by an increase in volume, and the equilibrium is influenced by the pressure.
Suppose that by increasing the external pressure we reduce the gaseous products to a fraction of their original volume : the concentrations u, u u and u 2 will increase in
the same proportion, the ratio u ^ will no longer have its
equilibrium value, and some of the dissociated products will recombine.
The case of hydriodic acid (studied by Lemoine) is very different. For the dissociation we have the equation
A constant value of the ratio — ^ corresponds to each
temperature. But as the decomposition does not alter the
number of molecules, the degree of dissociation is quite
independent of the pressure. The phenomenon represents
a simple reversible reaction rather than a true case of
dissociation.
��1 The dissociation of PC1 5 , NH 4 C1, and C 5 H n Cl, has been studied by determining their vapour densities at fairly high temperatures. In these experiments it is evident that no part of the substance was left unvaporised.
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