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Entropy and Second Law of Thermodynamics

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First law gives us an insight into a new property called internal energy. Second law leads to a new property cal entropy.

Entropy is a property which sets the direction of irreversible processes in a closed system. According to the entropy postulate - If an irreversible process occurs in a closed system, the entropy S of the system always increases, it never decreases.

Consider a hot bowl of soup in a room. Heat will flow from the bowl to the room till the temperature is equalized. It is an irreversible process. Heat can never flow from the room back to the soup bowl and heat up the soup again! This flow takes place till equilibrium is attained. The entropy increases in this process.

Change in entropy: 
The change in entropy S_f - S_i of a system during a process that takes the system from an initial state i to a final state f as

S = S_f - S_i = dQ/T

where:
Q - Energy transferred as heat to or from the system during the process.

T - Temperature of the system in Kelvin.

Entropy as a state function:
Entropy like pressure, energy and temperature is a property of the state of a system and is independent of how that state is reached.

Second law of thermodynamics:

 S >= 0

In words, if a process occurs in a closed system, the entropy of the system increases for irreversible processes and remains constant for reversible processes.

Thus if entropy decreases in one part of the system, then it increases by an equal or larger amount in another part of the system. Thus, the total change in entropy is either zero or greater than zero.