Today, the Second Law, as applied to chemical systems, is firmly associated with the concept of entropy as expressed in the 1865 statement of Clausius and given mathematically by equation (2.41). Similarly, Helmholtz Free Energy F= U− TS (where U is internal energy) is useful in situations where volume and temperature T are constant, but it is not a fundamental quantity either. This makes Gibbs Free Energy extremely valuable for chemists, but it should not obscure the fact that entropy is the fundamental physical quantity and that Gibbs Free Energy is a derived quantity of no independent physical importance. Tables of Gibbs Free Energies of common substances are readily available, and such a table is all one needs to determine how entropy changes when an event occurs under conditions of constant pressure and temperature. Adding these two entropy changes together yields a total entropy change of Δ H/ T−Δ S=(1/ T)(Δ H− TΔ S)=(1/ T)Δ G, where Δ G is known as the Gibbs Free Energy. Under conditions of constant pressure and temperature, the former is the energy released by the system into its environment (traditionally represented by −Δ H) times 1/ T, and the latter is Δ S. To calculate the entropy change when an event occurs, the resulting entropy change of the system's surroundings must be added to the entropy change undergone by the system itself.

Lozada, in Encyclopedia of Energy, 2004 5.4 Entropy and Free Energy

Change in entropy