Entropy and the second law of thermodynamics

The second law of thermodynamics establishes that the total entropy of an isolated system can never decrease. Real processes are irreversible, so entropy always increases until the system reaches thermodynamic equilibrium, where entropy is at its maximum.

Key Definition Entropy is a measure of the disorder of a system; it describes the tendency of localized energy to spread out and the growing unavailability of that energy to do useful work.

Because entropy is a state function, it depends only on the system's current configuration, not on the path or history by which that state was reached.

Isolated system

• Entropy increases for any irreversible (real) process and can never decrease.
• Entropy remains constant only for an idealized, perfectly reversible process.

Closed system

• Entropy can decrease when energy is transferred out of the system as heat.
• This local decrease is always offset by an equal or greater entropy increase in the surroundings, so the total entropy of the system plus its surroundings still increases or stays constant.

The behavior of entropy therefore depends on the type of system. The total entropy of any isolated combination of system plus surroundings never decreases. On the AP exam, entropy questions are qualitative — connect every answer to the physical mechanism of energy spreading out or to the distinction between isolated and closed systems.