Entropy is a measure of disorder in a system.
The entropy is: S = k ln (Number of available states) and has units J/K.
While any thermodynamic system an entropy value, it is simplest to determine the entropy change in a thermodynamic process.
Entropy change ΔS occurs whenever there is a transfer of heat.
ΔS = S_{f}  S_{i}  =  ∫ 

If heat transfer takes place at a fixed temperature, the entropy change is:
ΔS  = 
 (isothermal process) 