What is entropy ? Explain its spontaneity.

Explanation

Entropy is the measurement of randomness.
Entropy of solid is less whereas in gaseous state entropy in maximum. Whenever heat is added to the system, it increases molecular motions causing increased randomness in the system. Heat also depends on the temperature at which heat is added to the system.
A system at higher temperature has greater randomness in it than one at lower temperature. Thus, temperature is the measure of average chaotic .motion of particles in the system.
Heat added to a system at lower temperature causes greater randomness than when the same quantity of heat is added to it at higher temperature. This suggests that the entropy change is inversely proportional to the temperature.
`DeltaS= (q_("rev"))/( T)`
`q_("rev")=` The heat gain or loss inversely by the system
The total entropy change for the system and surroundings of a spontaneous process is given by
`DeltaS_("total") = DeltaS_("sys") + DeltaS_("surr") gt 0`
For spontaneous process `DeltaS= +ve`
Entropy is a state property, we can calculate the change in entropy of a reversible process by :
`DeltaS_("sys") = (q_("sys.,rev") )/( T)`
We find that both for reversible and irreversible expansion for an ideal gas, under isothermal conditions, `DeltaU=0`, but `DeltaS_("total")=DeltaS_("sys")+ DeltaS_("surr")` is not zero for irreversible process.