Reversible and Irreversible Reactions

Chemical reactions can be classified as Reversible and Irreversible reactions.

Reversible Reactions

A reaction is said to be reversible if under certain conditions of temperature and pressure, the forward and reverse reactions occur simultaneously. For example,

CH₃COOH (l) + C₂H₅OH (l) ⇌ CH₃COOC₂H₅ (l) + H₂O (l)

When ethyl acetate and water are formed in the forward reaction, the reverse reaction also starts in which ethanol and acetic acid are formed. After some time the concentrations of all the reactants and products become constant. This happens when the rates of forward and reverse reactions become equal; and all the properties of the system become constant. It is said that the system has attained state of equilibration.

At the time of equilibrium, forward and reverse reactions are taking place and it is in a state of dynamic equilibrium because no change is taking place.

A reversible reaction is said to be in the equilibrium state when the forward and backward reaction occur simultaneously at the same rate in a closed system and the concentrations of reactants and products do not change with time.

A + B ⇌ C + D

Irreversible Reactions

Most of the reactions occur only in one direction. They are called irreversible reactions. For example when carbon is burnt in air to form carbon dioxide the reaction goes only in
one direction.

C (s) + O₂ (g) → CO₂ (g)