Chemical Equilibrium: Homogeneous and Heterogeneous System (For CBSE, ICSE, IAS, NET, NRA 2022)

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Homogeneous and Heterogeneous System

  • Homogeneous system is one which has one phase.
  • It has same chemical composition and uniform properties throughout.
  • It is formed by particles of molecular size only.
  • Pure solids, liquids, gases and solutions are the examples of homogeneous system.
  • Any system consisting of two or more phases is called heterogeneous system.
  • A mixture of solids or immiscible liquids constitutes a heterogeneous system.

Homogeneous and Heterogeneous Equilibrium System

  • Chemical equilibrium can be homogeneous or heterogeneous in nature.
  • It is homogeneous if both the reactants and products are present only in one phase gas or liquid.
  • It is heterogeneous if present in more than one phase. In the following sections we shall study such systems.

Homogeneous Chemical Equilibrium System

Gas – Phase Homogeneous Systems

These systems contain only gaseous reactants and products.

Examples of this type of equilibrium are:



Liquid – Phase Homogeneous Systems

These are the systems in which both the reactants and products are present in only in liquid phase.

For example,



Heterogeneous Chemical Equilibrium

The systems in which reactants and products are present in more than one phase belong to this type.

For example:

Law of Mass Action

The rate at which a substance reacts is proportional to its active mass (molar concentration) and the overall rate of the reaction is proportional to the product of active masses (molar concentrations) of all the reacting substances.

Consider a reaction


K = constant of proportionality and is called the specific rate constant of the reaction

Consider another reaction

Can be written as

According to the law of mass action state

For the reaction

For a more general reaction

On applying the law of mass action to the reversible reaction

The rate of the forward reaction is

The rate of the reverse or backward reaction, is

and are the rate constants of the forward and backward reactions reversibly.

At equilibrium

Rearranging these, we get

equilibrium constant of the reaction and also called as concentration equilibrium constant and the symbol given is .

Developed by: