Phase Equilibrium

What is phase?

A chemically homogeneous part of a system that is physically distinct and mechanically separable is called a phase

Phase of pure substance

Homogenous. A phase may or may not be continuous

Phase of mixture of gasses

Homogenous

phase of miscible liquids

Homogenous

phase of non-miscible liquids

heterogenous

phase of aqueous solution

Homogenous

Phase of mixture of solids

Heterogenous

component of a system

The least number of independent chemical constituents in terms of which the composition of every phase can be expressed by means of a chemical reaction.

Nitrogen (N2)

Oxygen (O2)

Components = No. of gases present = 2

Phase = 1 = homogenous gaseous mixtures

Rhombic sulfur

Monoclinic Sulfur

Liquid sulfur

Sulfur vapor

Components: 1

Phase: 4

Chemical properties: Same

Physical properties: Different

NaCl Solution

Components = No. of chemical constituents = 2

Phase = 1 (homogenous solution)

NaCl Solution

Solid NaCl

Components: 2

Phase: 2

Water

Ice

Water vapor

Components: 1

Phase: 3

Chemical properties: Same

Physical properties: Different

Rhombic sulfur

Monoclinic Sulfur

Sulfur vapor

Components: 1

Phase: 3

Chemical properties: Same

Physical properties: Different

CaCO3 (s)

Cao (s)

Co2 (s)

Components: 2

Phase: 3

Chemical properties: Different

Physical properties: Different

Degrees of freedom

The least number of variable factors (concentration, pressure and temperature) which must be specified so that the remaining variables are fixed automatically and the system is completely defined

nonvarient

A system with F = 0 is known as nonvariant or having no degree of freedom.

Univarient

A system with F = 1 is known as univariant or having one degree of freedom.

bivarient

A system with F = 2 is known as bivariant or having two degrees of freedom.

phase rule

By means of a diagram it is possible to predict qualitatively the effect of changing pressure, temperature and concentration on a heterogeneous system in equilibrium.

What is Gibb’s phase rule free from?

Gibb’s Phase Rule is free from flaws and limitations

Degrees of freedom o fWater vapor confined to a particular volume

F = 1 - 1 + 2 = 2 (C=1, P=1)

A system comprising a liquid water, in equilibrium with its vapor (DOF)

F = 1 - 2 + 2 = 1

Here, C=1, P=2

Let’s cool liquid water and its vapor until a third phase (ice) separates out (DOF)

F = 1 - 3 + 2 = 0

Here, C=1, P=3