Phase Rule

Phase Rule

                   The phase rule is an important tool in the study of heterogeneous equilibrium. It relates the condition which mistake specified to describe the state of a system equilibrium. A substance can exist in various states depending on the external conditions. For example, water can exist in the form of a solid, liquid, and vapor. A change in the state of a substance is called phase transition. Equilibrium is a system in which phase transitions occur in addition to a chemical process is called phase equilibrium.

Explanation of the Terms Involved in Phase Equilibrium or Terminology:-

1.      Phase (p): 

                            A phase is defined as the homogeneous parts of heterogeneous system. which can be separated from each other by simple mechanical method by means of a definite boundary. A phase is a physically distinct and mechanically separable part of a system.

Example:

i.                    Water exists in three forms –ice, water and vapors so it is a three phase system and represented as-

 

Each phase is separate by a phase boundary known as interface.

ii.                  A gas a gaseous mixture is a single phase because there is no interface between one gas and another. For example; air ect.

iii.                Let us consider the decomposition of CaCO₃ as

Here are three phases. Two phases are of CaCO₃ (s) and CaO(s) and third phase is of CO₂ (g). All phases are separated by interface.

iv.                Every solid in a system in an individual phase and each is separated from others by a definite bounding surface.

 

2.      Components(c):

                                      The components of a system can be described in terms of the species present in the system. The component of a system are defined as the minimum number of independent variable constituents by means of which the composition of any phase of the system can be expressed either directly or in the form of a chemical equation. For example,

i.                    Water exists in the three phases as ;

 Each phase can be represented by H₂O. Thus, the number of components is one.

ii.                  The sulphur system consists of four phases i.e, rhombic, monoclinic, liquid, and vapour. The chemical composition of each phase can be expressed in terms of S. Hence it is a one component system.

iii.                In case of chemically reactive system where various chemical reactions take place between the species, the number of components is calculated by the relation

     C=S-R

                  Where S is the number of chemical species present in the system and R is the number of independent chemical reactions.

Decomposition of CaCO₃.

There are three phases, three species and one reaction. Here S=3 , R=1

So ,  C=S-R= 3-1=2

Hence the number of components is two.

iv.                When ions are also present in the system then the condition of electroneutrality is also considered. Hence the number of components may be calculated by the modified relation as C=S-(R+1)

3.      Degree of Freedom:

                                 The degree of freedom or variance is defined as the smallest number of independent variables such as pressure, temperature and concentration that must be specified in order to describe complete the state of the system. System having degrees of freedom three ,two, one , zero are known  as trivariant, bivariant, univariant (or monovariant), and nonvariant system, respectively.

For example;

i.                    For ice-water-vapor system, F=0. In the system, the three phases co-exist at the freezing point of water. As the freezing temperature of water has a fexed value the vapor pressure of water also has a definite value. The system has two variables (T and p) , both of which are already fixed. The system is completely defined automatically and there is no need to specify any variable. It has no degree of freedom.  

                                  i.e, F=0

ii.                  For saturated NaCl solution, F=1. The saturated solution of sodium chloride in equilibrium with solid NaCl and water vapor i.e.

is completely defined if we specify temperature only. The other two variables, i.e, composition of NaCl sample and vapor pressure have a definite value at a fixed temperature. So, the system has one degree of freedom.

4.      Triple Point:

                         Triple point is the point at which three phases co-exist in equilibrium. The degree of freedom of one component system is zero.

 

5.      Eutectic Mixture and Eutectic Point:

                                                           Eutectic mixture is a mixture of two or more components without chemically reacted in solution state having lowest freezing or melting point among all possible ratio of mixing that components at particular temperature, such type of system where, eutectic mixture is formed is called the eutectic system and the lowest melting point of that eutectic mixture is called the eutectic point.

6.      True Equilibrium:

                                A system is said to be in state of true equilibrium under a given set of conditions if the same state can be realized by approach from either direction by following any possible procedure.

7.      Metastable Equilibrium:

                                                A system is said to be in a state of metastable equilibrium under a given set of condition if a state can be realized only from one direction by a careful change of conditions.