A thermodynamic state function, which is classically defined as a change, ΔS, in a system involving reversible heat (q_rev) in a process and absolute temperature (T), such that ΔS_sys = q_rev/T. A reversible heat process implies an equilibrium process, and thus the change in entropy allows the determination of the favored direction of a process or reaction. If the total entropy change is positive from the initial state to the final state, the process spontaneously occurs. Where an increase in entropy, ΔS, is positive, this indicates a greater degree of motion or randomness of atoms, molecules, or ions in a system.
See enthalpy, Gibbs energy, state function