Thermodynamic criteria of chemical equilibrium in general

In the consideration of Chemical Equilibrium from the Kinetic standpoint we saw that equilibrium could be conveniently divided into two classes — 

From the kinetic standpoint we were able to grasp the important idea involved in the term Active Mass, and we saw how this idea led, in the case of homogeneous systems, to the generalisation known as the Law of Mass Actioh, and in the case of heterogeneous systems to the generalisation called the Law of Partition or Distribution 

We must, in fact, first postulate two things (i) that the temperature is supposed to be maintained constant, and (2) that the reaction under consideration can proceed without a volume change Now scarcely any reaction takes place without a volume change, but m condensed systems, as van t Hoff called them, that is in systems either liquid or solid (homogeneous or heterogeneous) we can imagine the reaction to occur practically without a volume change 

The criterion of equilibrium being reached in such a reaction, 1 e in one in which the temperature and volume are maintained constant, is simply that at the equilibrium point the Free Energy is a minimum, and therefore if we consider the system when equilibrium is reached and imagine the reaction to go to a small extent, namely, the transformation of hi molecules from one side of the equilibrium to the other, then the work done or free energy change is zero This is wntten algebraically (S/)tv = o 1 

1 Those familiar with the principle of maxima and minima in the differential calculus will see that the above equation represents mathematically a maximum point on a continuous curve as well as a minimum More strictly we should write for the equilibrium criterion (5/]xv = o, but the physical or chemical significance of the equality relation, vie (8/1 tv = o, will cause no confusion 

---