Entropy is a thermodynamic state quantity which is a measure of randomness or disorder of the molecules of the system.
Entropy is represented by the symbol 'S'. The entropy is a state function. It depends only on the initial and final states of the system. It is expresed in terms of calories per degree, i.e; cal K^-1
Change in entropy = S final - S initial.
When S final > S initial, the change in entropy is positive.
For a chemical reaction, S = Sproducts - Sreactants
Spontaneity in terms of Entropy Change.
For a spontaneous process in an isolated system, the change in entropy is positive. If the system is not isolated, the entropy change of both the system and the surroundings are taken into consideration. The total entropy change (S total) is the sum of the change in entropy of the system (S system) and the change in entropy of the surroundings (S surroundings).
S total = S system + S surroundings
For a spontaneous process, S total must be positive. During spontaneous process, the entropy of the system goes on increasing and becomes a maximum till no more increase in entropy of the system is possible. The system attains an equilibrium. i.e; Change in entropy = 0 (at equilibrium for an isolated system)
If S total is negative, the direct process is non-spontaneous and the reverse process is spontaneous.