Question

What is entropy? What are its units?

Answer 1

Hint: In thermodynamics, entropy is generally referred to as a measure of spontaneous changes that occur in a system. In simpler words, it is the tendency of the universe towards disorder.

Complete step by step answer:
Entropy is defined as the measure of randomness or disorder of a thermodynamic system. It is a thermodynamic function represented by ‘S’.
Certain characteristics of entropy are -
Value of entropy is dependent upon the amount of substance present in the system. Hence, we call it to be an extensive property.
Changes in the value of entropy depend on changes in quantities such as temperature, volume, pressure and number of moles of substance present in the system. Hence, it is said to be a state function.
The total change in entropy of an isolated system and change in entropy of the surrounding is always equal. Sum of two is called the entropy change of the universe. Mathematically this can be represented as - \[\Delta {{\text{S}}_{{\text{universe}}}} = \Delta {{\text{S}}_{{\text{system}}}} + \Delta {{\text{S}}_{{\text{surroundings}}}}\].
For a cyclic process, change in entropy is 0.
For a reversible process \[\Delta {{\text{S}}_{{\text{universe}}}} = 0\]and hence \[\Delta {{\text{S}}_{{\text{system}}}} = {\text{ - }}\Delta {{\text{S}}_{{\text{surroundings}}}}\].
For an irreversible process, \[\Delta {{\text{S}}_{{\text{universe}}}} > 0\] i.e. entropy of the universe increases in an irreversible process.

Additional Information:
Thermodynamics does not take microscopic details into consideration. Entropy describes behaviour of a system in terms of temperature, pressure and heat capacity. This takes into consideration the state of equilibrium of the systems.

Note: Since, it is difficult and practically impossible to determine the actual total entropy content of a given system, it is described as the change in entropy accompanying change in the state of the system. Change in entropy is represented by \[\Delta {\rm{S}}\]. However, under standard conditions, it is \[\Delta {\text{S}}^\circ \]. Its SI unit is J/Kmol and CGS unit is cal/Kmol.