Characteristics of Equilibrium State

The state of chemical equilibrium is reached in a reversible reaction when

  1. the temperature of the system attains a constant value.
  2. the pressure of the system attains a constant value.
  3. the concentrations of all the reactants and products attain constant values.

The state of equilibrium has following characteristics properties:

Chemical Equilibrium is dynamic in nature

The chemical equilibrium is the result of two equal but opposite processes occurring in the forward and reverse directions and there is no net change occurring in the system.

Equilibrium can be attained from either side

The same state of equilibrium (characterized by its equilibrium constant) can be reached whether the reaction is started from the reactants or products side. For example, the same equilibrium is established whether you start the reaction with N2O4 or NO2.

N2O4 (g) ⇌ 2NO2 (g)

Equilibrium can be attained only in a closed system

Equilibrium can be attained only if no substance among, reactants or products, is allowed to escape i.e. the system is a closed one. Any system consisting of gaseous phase or volatile liquids must be kept in a closed container. For example,

N2 (g) + 3H2 (g) ⇌ 2NH3 (g)

A system consisting of only non-volatile liquid and solid phases can be kept even in an open container because such substances have no tendency to escape. For example,

FeCl3 (aq) + 3 NH4SCN (aq) ⇌ Fe(SCN)3 (s) + 3NH4Cl (aq)

A catalyst can not change the equilibrium state

Addition of a catalyst speeds up the forward and reverse reactions by same extent and help in attaining the equilibrium faster. However, the equilibrium concentrations of reactants and products are not affected in any manner.