This expression has commonly been interpreted to mean that work is extracted from the internal energy "U" while "TS" represents energy not available to perform work. However, this is incorrect. For instance, in an isothermal expansion of an ideal gas, the internal energy change is Δ"U" = 0 and the expansion work "w" = -"T" Δ"S" is derived exclusively from the "TS" term supposedly not available to perform work. But it is noteworthy that the derivative form of the free energy: "dA = -SdT - PdV" (for Helmholtz free energy) does indeed indicate that a spontaneous change in a non-reactive system's free energy (NOT the internal energy) comprises the available energy to do work (compression in this case) "-PdV" and the unavailable energy -SdT. Similar expression can be written for the Gibbs free energy change.