DS = qrev / Twhere qrev represents the heat supplied at temperature T in a reversible process.
In general, the temperature of the system may change during a process. If the process is reversible, the change in entropy is defined as
In an adiabatic reversible process, no heat is given out to the system. The entropy of the system remains constant in such a process.
Entropy is related to the disorder in the system. Thus, if all the molecules in a given sample of a gas are made to move in the same direction with the same velocity, the entropy will be smaller than that in the actual situation in which the molecules move randomly in all directions.An interesting fact about entropy is that, it is not a conserved quantity. More interesting is the fact that entropy can be created but cannot be destroyed. The second law of thermodynamics may be stated in terms of entropy as follows:
It is not possible to have a process in which the entropy of an isolated system decreases.
Units of entropy
Since entropy change is expressed as heat divided by temperature, the units of entropy are calories per
degree (expressed as entropy units eu) or joules per kelvin, JK-1 (expressed as EU).
The physical meaning of entropy is that entropy is a measure of degree of disorder (or randomness) of a system. The relation between entropy and disorder provides a suitable explanation for entropy change in various processes. The greater the disorder in a system, the higher is the entropy. Obviously, for a given substance the solid state is the state of lowest entropy (most ordered state), the gaseous state is the state of highest entropy and the liquid state, is intermediate between the two. In the case of mixing of two gases when the stopcock is opened, the gases mix to achieve more randomness or disorder. In this case, there is no exchange of matter or energy between the system and the surroundings. The change occurs from ordered state (less entropy) to disordered state (higher entropy). Thus the change in entropy is positive.