Question

Discuss the advantage and limitations of the phase rule .

Answer 1

Advantages of phase rule :

1) It is applicable to both physical and chemical equilibria .
2) It requires no information regarding molecular /micro-structure ,since it is applicable to macroscopic systems .
3) It is a convenient method of classifying equilibrium states in terms of phases ,components and degrees of freedom .
4) It helps us to predict the behaviour of a system under different conditions of the governing variable .

5) It indicates that different systems with same degree of freedom behave similarly .
6) It does not take any cognizance of either the nature or quantities of components present in the system .
7) It helps in deciding whether under the given set of conditions :
a) Various substances would exist together in equilibrium or
b) Some of the substances present would be interconverted or
c) Some of the substances present would be eliminated .

Limitations of phase rule :

1) It can be applied to systems in equilibrium only .
2) It is of little value in case of systems which attain the equilibrium state very slowly
3) It applies only to a single equilibrium system and provides no information regarding any other possible equilibria in the system .
4) It requires utmost care in deciding the number of phases existing in an equilibrium state ,since it considers only the number of phases rather than their amounts .Thus ,even if a trace of the phase is present ,it accounts towards the total number of phases .
5) It conditions that all phases of the system must be present simultaneously ,under the same conditions of temperature and pressure .
6) The solid ,liquid phases should not be so finely sub-divided as to bring about deviation from their normal values of vapour pressure .