Phase Equilibria or Phase Equilibrium | Science Tutor

Phase Equilibria or Phase Equilibrium

Equilibrium existing between tow or more different phases is heterogeneous in nature and behavior of such an equilibrium can be studied by one rule called “Phase Rule”. This rule was deduced by Willard Gibbs in 1876. Mathematically,

F + P = C + 2

Where F is number of degree of freedom, P is number of phases and C is number of the components.

This value does not involve any assumptions and is valid provided the equilibrium between number of phases is affected only by temperature, pressure and concentration and not by any other force such as electric or magnetic etc.

Various Terms:

Phase: It is any homogeneous, physically distinct and mechanically separable part of a system which is boundary surfaces e.g. ice, water and water vapours constitute three phases; each is homogeneous and physically distinct and there well-defined boundaries between ice and water, water and water vapours and ice and vapour.

Thus, a phase must fulfill following three condition,

1) It should be physically homogeneous

2) It should be separated from other phases of the system at equilibrium by surfaces of contact.

3) Equilibrium between different phases of the system should be dynamic in nature.

Components:

The number of components of a system at equilibrium is the smallest number of independently variable constituents by means of which composition in the form of a chemical equation; negative and zero signs or values being permitted.

Let us consider the water system. It has three phases: ice, liquid water and water vapour in equilibrium. The composition of each of three phases can be expressed in terms of the component H₂O. Hence it is a single component system.

Similarly, sulphur system has four phases: Sulphur Rhombic, Sulphur monoclinic, liquid sulphur and sulphur vapour and the composition of each phase can be expressed in terms of one component, ie. sulphur.

Now let us consider the system represented by equilibrium:

CaCO_3(s) ↔ CaO_(s) + CO_2(g)

In this system, there are three phases, CaCO₃, CaO_(s) and CO_2(g), which are in chemical equilibrium with one another. But Composition of each phase can be represented by any two constituents. Thus, it is a two component system. As shown below:

1) If CaO and CO₂ are taken, composition of various phases can be expressed as follows:

CaCO₃ = CaO + CO₂

CaO = CaO + OCO₂

CO₂ = OCaO + CO₂

2) If CaCO₃ and CO₂ are taken:

CaCO₃ = CaCO₃ + OCO₂

CaO = CaCO₃ + CO₂

CO₂ = OCaO₃ + CO₂

3) If CaO and CO₂ are taken

CaCO₃ = CaO + CO₂

CaO = CaO + OCO 2₂

Thus, smallest number by which composition of each phase can be expressed is two, thus it is a Two – Component System.

It is important to note that for

1. Non reactive system, the number of components is equal to number of species present.

2. Reactive systems number of components can be calculated by using the following relation

C = S – E – R

Where S is number of chemical species present, E is the number of independent chemical reactions equilibria between constituents and R is number of restrictions due to electrical neutrality.

In the above example, S = 3, E = 1 and R = 0

hence C = 3-1-0 = 2.

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