Page:Text-book of Electrochemistry.djvu/110

 VI. CHANGE OF HOMOGENEOUS EQUILIBRIUM. 9s

where ^ is a constant and (7^, (7«, etc., denote the concentra- tions (or, more strictly, the osmotic pressures) of the substances P, Q, etc. Further, if fi cal. are absorbed when m mols of substance P, n mols of substance Q, etc., react to form/ mols of substance S^ etc., then, provided that the volume does not change much with the temperature, we obtain for the change in the above special case —

��or —

�� ��when M and M\ are two integration constants, which give the value of In K and log K at temperature Tq} Since fi is expressed in calories, the value of -B is 1*99.

When II is positive, i.e. when in the reaction heat is

absorbed, and Ti is greater than To, ( ^ - m)'^ evidently

negative, and In K greater than Jf, indicating that, with rising temperature, the concentrations Cp, C,, etc., diminish, whilst the concentrations Cg, Cry etc, increase. From this we can draw the following general conclusion, which is applicable both to heterogeneous and homogeneous systems : —

In an equilibrium that system of substances^ the formfation of v:hich is accompanied hy an absorption of heat, increases with risbig temperature.

Some examples illustrating this very important generali- sation may be given. When a substance is melted, heat is absorbed, therefore the melted portion must increase when the temperature is raised.

As in this case the system is a condensed one, if the pressure is kept constant, the transition takes place suddenly when the melting point is passed. In the same way heat is absorbed when vaporisation takes place ; consequently, if in a vessel there is an equilibrium between water and vapour,

' Thia signification of M and .Vi is seen if 2\ be set equal to T^,

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