In an isolated system, there is no exchange of energy or matter between the system and surrounding. For a spontaneous process in an isolated system, the change in entropy is positive, i.e. S > 0.

Most of the spontaneous chemical reactions are exothermic. A number of endothermic reaction are spontaneous e.g melting of ice (an endothermic process) is a spontaneous reaction.

The two factors which are responsible for the spontaneity of process are

(i) Tendency to acquire minimum energy

(ii) Tendency to acquire maximum randomness.