**Characteristics of entropy ‘S’**

i) The term ‘S’ entropy is evolved from the formulation of II law of thermodynamics as a thermodynamic state function.

ii) Entropy change ‘ΔS’ of a system under a process is defined as the constant equal to the ratio of the heat change accompanying a process at constant temperature to the temperature of the system under process. The process should be reversible at that temperature.

∆S_{rev} = ∆q_{rev}/T (K)

Heat, q is not a state function , But for a reversible process Δq = (q_{2} –q_{1}) divided by temperature (T) of the process is a state function.

iii) A spontaneous process is accompanied by increase in the ‘disorder’ (or) ‘randomness’ of the molecules constituting the system. Entropy increases in all spontaneous processes. Hence entropy may be regarded as a measure of disorder (or) randomness of the molecules of the system.

iv) When a system undergoes a physical (or) a chemical process, there occurs a change in the entropy of the system and also in its surroundings. This total change in the entropy of the system and its surroundings is termed as the entropy change of the universe brought about by the process. For an isothermal process (T=constant), the entropy change of the universe during a reversible process is Zero.

The entropy of the universe increases in an irreversible process.

v) The energy of the universe remains constant although the entropy of the universe tends to a maximum.

vi) For a spontaneous process, at constant T, ΔS is positive (ΔS > 0). ΔS is positive (ΔS > 0). For an equilibrium process, ΔS is zero.

For a non spontaneous process, ΔS is negative or (ΔS < 0).