Chemical Equilibrium

Equilibrium exhibits the state of a process at which the measurable properties of the system do not show any change with time. The term chemical equilibrium is used for the chemical reaction when the concentration of the reactants and the products does not change with time.

We consider the following reactions:

(i)     BaCl_2 + H_2SO_4 \to BaSO_4 \downarrow + 2HCl

ii)      2KClO_3 \to 2KCl + 3O_2 \uparrow

(iii)    2HI \leftrightharpoons H_2 + I_2(Closed vessel)

(iv)     N_2 + 3H_2 \rightleftharpoons 2NH_3(Haber’s process)

Since the reactions (i) and (ii) proceed in one direction and the reaction is completed with time hence these are known as irreversible reactions. On the other hand, reactions (iii) and (iv) in which the products recombine under a similar set of conditions, to give back the reactants in significant quantities are called reversible reactions. Such reactions are represented by the sign of reversibility (\rightleftharpoons)

The reactions (ii) and (iii) are carried out an heating, but (ii) is irreversible reaction hence it is also called thermal decomposition whereas reaction (iii) is reversible hence it is also called thermal dissociation.

Some more examples of Irreversible and Reversible reactions are:

Irreversible reactions:

2Na + 2H_2O \to 2NaOH + H_2 [un - reactive products]

NaCl + AgNO_3 AgCl \downarrow + NaNO_3  [precipitation reaction]

SnCl_2 + 2FeCl_3 \to SnCl_4 + 2FeCl+2 [Redox]

I_2 + 2Na_2S_2O_3 \to 2NaI + Na_2S_4O_6 [Redox]

  • Decomposition of CaCO_3 in open
  • Evaporation of H_2O in open.

Reversible Reactions

  1. Decomposition of CaCO_3 and evaporation of H_2O in closed container.

3Fe + 4H_2O \leftrightharpoons Fe_3O_4 + 4H_2 [Reactive products]

PCl_5 \rightleftharpoons PCl_3 + Cl_2 N_2 + 3H_2 \rightleftharpoons 2NH_3


It is evident that chemical equilibrium is possible in reversible reactions only. In fact, the reaction does not stop but proceeds in both directions with equal rate, therefore, it is said that equilibrium is dynamic. Hence the state of chemical equilibrium may be defined as, “The state of reversible reaction in which the concentrations of the reactants and products do not change with time is called chemical equilibrium”.

Characteristics of Chemical Equilibrium are listed below:

(i)                  It can be achieved from any side.

(ii)                It is dynamic in nature.

(iii)               The rate of forward reaction is equal to that of backward reaction.

(iv)               The ratio between the concentration of the reactants and the products is constant in the mixture.

(v)                It is affected by the change of measurable properties of the system such as concentration, temperature, pressure, mass, volume etc.

Related posts:

  1. Chemical Kinetics Some chemical reactions are instantaneous while some proceed slowly. It...
  2. Second order Reaction A reaction is said to be the second order if...
  3. Determination of order of Reaction There are at least four different methods to ascertain the...
  4. Rate constant or Specific Reaction rate According to collision theory, the rate of the reaction is...
  5. Factors Influencing Rate of Reaction Some important factors which influence the rate of reaction are...