Discusses the concepts of mechanical, thermal, and thermodynamic equilibrium and their applications. Covers the molecular basis for internal energy, entropy, thermodynamic equilibrium, and reversibility. Enables the reader to model irreversibility and determine the net loss in performance of a thermal system compared to an idealized system and approach an ideal one. Demonstrates entropy as a path independent property by use of reversible heat engines and reversible heat pumps interacting with a process between two states, the environment and the reservoir. Covers the role of reversibility from a thermodynamics standpoint and relates it to other areas, such as gas dynamics, combustion, propulsion, power plant engineering, and engines.