Question

What is the mathematical form of the Gibbs phase rule equation? Explain the meaning of each one of the term involved in it.

Answer 1

GIBB'S PHASE RULE: Gibb's phase rule states that, in every heterogenous system in equilibrium ,the sum of the number of phases and degree of freedom is greater than the number of component by two provided the equilibrium between any number of phases is not influenced by gravity, electrical or magnetic forces or by surface action, and is only influenced by temperature, pressure and concentration. It is expressed in the form,

                                                 P+F= C+2

                                             Or F = C-P+2

Where, P = Number of phases, C= Number of components and F =Degree of freedom.

1} PHASES:
A phases is defined as any homogenous ,physically distinct and mechanically separable portion of a system, which is separated from other parts of the system by definite bounding surfaces.

Example: 1. A gaseous mixture, which is thoroughly miscible in all proportion, consists of a
single phase.

2} COMPONENT·
It is the_smallest number of independent variable constituents taking part in the state of equilibrium, by means of which the composition of each phase can be expressed in the form of chemical equation. The components do not represent the number of constituents or chemical individual present in the system.

Examples: 1. In water system the phases present are ice, water and water vapour. The
composition of each phase can be expressed by a single component, H20. Hence it is a
component system.

3} DEGREE OF FREEDOM:
It is defined as the smallest number of independent variables such as pressure, temperature and concentration that must be specified in order to define completely the state of a system.

Example: 7. For a system consisting of water vapour phase only, to describe the system completely, the values of both, the temperature and pressure, must be stated. Hence , the system is bivariant or degree of freedom is two.