A Short Note on Phase Equilibrium

Phase equilibrium is a topic in Chemistry that focuses on studying equilibrium between gas, solid, and liquid. Equilibrium is the state in chemistry in which no change takes place in the chemical potential of components. In chemistry, a phase is an area where there is a uniform intermolecular interaction. The phase rule is a law in thermos dynamics. It was proposed by J. Gibbs. The phase rule is associated with variables involved in thermodynamic equilibrium. The Phase Rule governs the systems which are described and affected by such variables as temperature, volume and pressure, together represented as pVT. Allotropes exist in two distinct phases. The answer to what is phase rule has been discussed in the article.

Phase Equilibrium

In a phase equilibrium, a substance and its many phases have a common surface with minimum variation quantitatively. In the study of phase equilibrium, there is no energy loss when any particle in the system goes through a change from one phase to the other. For example, Liquid and saturated vapor have a phase equilibrium. The rule as put forward by Gibbs states that in a component where there are components represented as, only k + 2 components are co-existing at a point. When two systems are having two components, then three phases are at equilibrium with distinct temperatures. The component concentration and the pressure are given in this case. Phase transitions occur at a certain temperature (melting or boiling point) depending on the pressure changes. The Clapeyron equation is used to find out the phase equilibrium when infinitesimal pressure change results in a temperature change. 

Phase Rule

The expression of the phase rule, abiding by the rules in thermodynamic equilibrium is F = C – P + 2, where F is freedom degrees, P is phases and C is components. According to the Phase Rule, in a system where there is only a single component, the number of variables is two and any pressure or temperature within reasonable limits can be attained. For a system having one component but having three phases, there are no variables or degrees of freedom. Pressure and temperature in this case are fixed. The point at which they are fixed is known as the triple point. 

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Thermodynamic Equilibrium

The phase equilibrium is a part of thermodynamic equilibrium. The latter is an axiomatic concept. It is concerned with the internal state of the thermodynamic system. In this theory, there are no macroscopic flows of energy or matter. A thermodynamic system has a spatial temperature that is uniform. Both phase equilibrium and thermodynamic equilibrium at large depend on the Second Law of Thermodynamics. 

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Allotropes

Allotropy is a curious phenomenon that is a part of phase equilibrium. When the temperature and pressure meet certain conditions, certain metals exist in many phases in a crystalline form. Iron is the best-known allotrope to do so since it has many allotropic transformations. This type of transformation is called a solid-state phase transition. Here, the temperature remains constant throughout all its phases. 

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Conclusion

In Chemistry, phases serve as distinct categorisation of matter into solids, liquids, and gases. The phases have stability which is determined by their chemical potential. A chemical potential phase is a form that is the most stable and has a minimum alteration in its chemical potential at any pressure or temperature value. The Phase Equilibrium criterion is that pressure and temperature remain the same, free energy is least and the chemical potentials of all components in the phase must be equal.