Reversible and Irreversible Processes

A process in which the changes can be retraced in the reverse direction (e.g., melting of ice, freezing of water, condensation of steam) is called a reversible process.

A process in which the changes cannot be retraced in the reverse direction (e.g., puncturing an inflated balloon, burning a candle) is called an irreversible process.

The thermodynamic quantity that combines internal energy with the pressure-volume product, defined as H = U + PV, is called enthalpy.

Kelvin-Planck Statement: It is not feasible for any heat engine to utilise all the heat it receives from a source and convert it entirely into useful work. In other words, achieving 100% efficiency in converting heat into work is impossible.

Clausius Statement: Without any external assistance, a machine cannot transfer heat from a colder reservoir to a hotter one. In essence, heat cannot spontaneously move from a cooler object to a warmer one without external intervention or work being done.

• The Second Law states that there exists a useful state variable called entropy S.
• If a system's change in entropy ΔS = ΔQ/T, the combined entropy of the system and environment remains constant if the process is reversible.
• The Second Law shows: if a physical process is irreversible, the combined entropy of the system and environment increases.
• Final entropy must be greater than initial entropy for an irreversible process: S_f > S_i
• For any process approaching reversibility: ΔS_total ≥ 0
• For an isolated system: ΔU_system ≥ 0 and ΔS_total ≥ 0
• The Second Law is also known as the Law of Increased Entropy.
• No process is possible whose sole result is the absorption of heat from a reservoir and conversion of that heat entirely into work (Kelvin-Planck statement).