Thermodynamic potentials can be derived from the energy balance equation applied to a thermodynamic system. This article is focused mainly on classical thermodynamics which primarily studies systems in thermodynamic equilibrium. Thermal radiation is initially defined for a cavity in thermodynamic equilibrium. In classical thermodynamics, processes are initiated by interventions called thermodynamic operations. There is an important distinction between thermal and thermodynamic equilibrium. Rider argued that all plasma systems at thermodynamic equilibrium were fundamentally limited. These are inhomogeneous systems that do not meet the strict definition of thermodynamic equilibrium. Bailyn also distinguishes the two macroscopic approaches as the mechanical and the thermodynamic. Then the assumption of thermodynamic equilibrium is to be abandoned. It implies the existence of a quantity called the entropy of a thermodynamic system. The question of the link between information entropy and thermodynamic entropy is a debated topic. The result is strong thermodynamic and kinetic aromatic stabilization. The ISM is usually far from thermodynamic equilibrium. The reaction is usually an irreversible process due to thermodynamic and kinetic reasons. An empirical temperature is a numerical scale for the hotness of a thermodynamic system.