Chemical Equilibrium: Equilibrium in Physical Process or Phase Equilibrium (For CBSE, ICSE, IAS, NET, NRA 2022)

Get top class preparation for competitive exams right from your home: get questions, notes, tests, video lectures and more- for all subjects of your exam.

Equilibrium in Physical Process or Phase Equilibrium

Liquid - Vapour Equilibrium

  • Let us take some quantity of a liquid in an empty container and close it.
  • Initially the vapour pressure above the liquid will be zero. The liquid will evaporate, and its vapour will fill the empty space above it.

  • The rate of evaporation is maximum in beginning.
  • As the vapours build up pressure increases, and the rate of evaporation slows down, and the reverse process of condensation begins (fig) .

Liquid - Vapour Equilibrium

Rate gradually increases with the increase in the vapour pressure and after some time the two rates (of evaporation and condensation) become equal and the following equilibrium is established.

The maximum value of vapour pressure at equilibrium is known as the saturated vapour pressure.

Solid - Vapour Equilibrium

  • Volatile solids sublime to form vapour and the situation similar to the liquid – vapour equilibrium occur.
  • When kept in a closed container at a constant temperature the following equilibrium is established.

For example, an equilibrium can, be established by keeping some solid iodine in a gas jar covered with a lid. (fig) .

Solid - Vapour Equilibrium

The purple colour vapours fill the jar and the following equilibrium is established

Solid – Liquid Equilibrium

A liquid below its freezing point freezes spontaneously

When heated above its melting point the solid melts spontaneously:

At melting point, the two phases are in equilibrium

Such equilibrium is characterized by its temperature i.e.. the melting point of the solid.

Solute – Solution Equilibrium

  • When sugar crystals are put in a saturated solution of sugar in water; it will appear that no change is taking place and sugar appears to remain undissolved.
  • Actually, the undissolved sugar does dissolve in the saturated sugar solution; and an equal amount of sugar separates out from the solution.
  • It means the solid sugar and the sugar solution form an equilibrium system.

Solute – Solution Equilibrium
  • In this the rate of dissolution of sugar becomes equal to the rate of crystallization.
  • This equilibrium is known as Solubility Equilibrium.

Phase- Phase Equilibrium

  • A phase is defined as a homogenous part of a system which has uniform composition and properties throughout.
  • A phase is not the same as physical state. A mixture of two solids, even when powdered finely is a two-phase system.
  • A dynamic exchange of the common component between two phases takes place. When the rates of exchange becomes equal the equilibrium is establish.

Developed by: