Phase Rule BY Dr.KEDAR MOHAPATRA GITA ,BBSR INTRODUCTION 1. It is a statement given by J. Willard Gibbs which explains the behaviour of a heterogeneous system at equilibrium under the changes in the variables such as temperature, pressure and concentration. 2. It is only applicable to macroscopic(large) systems. 3. It can be applied to both physical and chemical equilibrium systems. 4. It predicts the appearance and disappearance of various substances present in the equilibrium under different sets of variable. Before discussing the phase rule and its applications to various equilibria, it is desirable to define and explain the various terms contained in it. Explanation of different terms: SYSTEM It is a part of the universe which on under thermodynamic study. Types of system It is of two types : (1) Homogeneous system (2) Heterogeneous system Homogeneous system: The systems in which the physical and chemical properties are same throughout are called Homogeneous systems. Example-all pure substances, salt solutions, acid solutions, etc. Heterogeneous system: The systems in which the physical and chemical properties are not same throughout are called heterogeneous systems. Example-oil in water,benzene in water,decomposition of CaCO3 solid,etc. Phase of a System It is any homogeneous and physically distinct segment of a system which remains bounded by a surface and is mechanically separable from other part of the system. It is represented by ‘P’ 1. Pure Substance: It forms a single phase.i.e P=1 2. Mixture of gases: It forms a single phase. i.e P=1 3. Miscible liquids: It forms a single phase. i.e P=1
4. Immiscible Liquids: It forms a multiphase system. 5. Aquous solution of salts: Unsaturated solution forms a single phase system. Saturated solution forms multi phase system. 6. Mixture of solids : It forms a multi-phase system Examples: For a homogeneous system 1. Two miscible liquids (alcohol & water) will form one liquid phase only. 2. A gaseous mixture, being thoroughly miscible in all proportions will form one phase only. 3. A solution of a substance in a solvent consists of one phase only. For eg., Solution of glucose in water. For a heterogeneous system 1. Fe(s)+ H2O (g) ↔ FeO (s)+ H2(g), It consists of two solid phases, Fe(s)& FeO (s)and one gaseous phase consists of H2O (g) & H2(g) Thus, three phases exist in equilibrium. 2. CaCO3(s) ↔ CaO(s)+ CO2(g) It consists of two solid phases, CaCO3(s)& CaO(s)and one gaseous phase, CO2(g) Thus, three phases exist in equlibirium. 4. Two immiscible liquids (benzene & water) will form two separate liquid phases. 5. Water consists of three phases at the freezing point. Ice (s)↔Water (l)↔Water vapour (g) Phase Reaction. There are certain physical or chemical reactions which are accompanied by the appearance or disappearance of a phase. Such reactions are known as phase reactions.
Phase Reaction in one Component system For Examples: 1.Change of solid to liquid(melting) 2. Change of Liquid to solid (Freezing) 3. Change of Liquid to gas(vapourisation) 4. Change of Solid to gas(sublimation) 5. Change of gas to liquid(condensation) 6. Change of gas to solid(de-sublimation ) 7. Change of one solid to another solid (transition) Phase Reaction in Two Component system Eutectic Phase Reaction: It involves the transformation of a single liquid phase into two other solid phases on cooling and vice versa, which is expressed as Cooling Single Liquid Phase(L) ↔ Solid-1 +Solid-2 Heating Eutectoid Phase reaction: It involves the transformation of a Solid phase into two other solid phases on cooling and vice versa. which is expressed as Cooling Phase Solid-1 ↔ Solid-2 +Solid-3 Heating Peretectic Phase reaction: It involves the transformation of a Solid & Liquid phase into single solid phase on cooling and vice versa. which is expressed as Phase Solid-1 +Liquid Phase Cooling ↔ Solid-2
Heating Peretectoid Phase reaction: It involves the transformation of two different solid phases into a single solid phase on cooling and vice versa. which is expressed as Cooling Solid-1 +Solid-2 ↔ Solid-3 Heating Conditions for Equilibrium between phases The following conditions must be satisfied for the existence of equilibrium between various phases in a multi phase system. 1.Thermal Equilibrium: All the phases must be at the same temperature otherwise there will be flow of heat from one phase to another. 2.Mechanical Equilibrium: All the phases must be under the same pressure otherwise the volume of one phase will increase at the expense of another. 3.Chemical Equilibrium: Chemical potential of a component must be same in all the phases.[µ=(-dG/dt)p] Phase Diagram: The diagram indicating the relationship between the phases at equilibrium under a given sets of condition is known as phase diagram. Example: The water equilibrium system exist at critical temperature (3740 C) and critical pressure(218 atm). So the diagram of water system can be represented under variable temperature and variable pressure Phase Diagram: Component of a system It may be defined as the least number of independent chemical constituents necessary to express the composition of each phase present in the system at equilibrium, directly or indirectly in the form of a chemical equation. It is always represented by C.