Entropy and its physical concept

The extent of disorder or randomness in a system may be expressed by a property known as entropy. Entropy may be termed as the property of a system which measures the degree of disorder or randomness in the system. It is generally expressed by the symbol, S. Entropy like internal energy and enthalpy, is a state function and change in entropy therefore, depends only on the initial and final states of the system. The change in entropy during the process when a system undergoes a change from one state to another is represented by DS. Thus, DS = S_{(final state)} - S_{(initial state)} and for chemical reactions,DS = S_(products) - S_(reactants). For a reversible process at equilibrium, the change in entropy may be expressed as:

DS = q_rev / T

where q_rev represents the heat supplied at temperature T in a reversible process.

Like pressure, volume, temperature and internal energy, we have another thermodynamic variable of a system, named entropy. In a given state of equilibrium, the system has a definite value of entropy. If the system has a temperature T (in absolute scale) and a small amount of heat DQ is given to it, we define the change in the entropy of the system as

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).

Physical meaning of Entropy

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.