Preliminary discussion on reaction equilibrium

Our present purpose is to give an illustration of the way in which 0 varies during the course of the reaction and in particular to show that it has a minimum value at a certain composition. 

For simplicity it will be supposed that the substances are present 

If the system before reacting consisted of, say, 1 mol each of A and JB, then at the given moment 

In this expression the /4 s are properties of the pure components, and, if the temperature is constant, they do not change during the course of the reaction. Thus, at constant temperature and pressure, G is a function of the single variable which is capable of variation between 1 and 0. 

If the pressure is chosen as being, say, unity, the equation can be rearranged to give 

---