The term describing energy available to do work defined by ΔG = ΔH − TΔS is

• A. Enthalpy
• B. Entropy
• C. Internal energy
• D. Gibbs free energy

Answer: (D)

Gibbs free energy is the energy available to do useful work at constant pressure and temperature. The relation G = H − TS shows how usable energy comes from the total heatContent (enthalpy, H) minus a term that accounts for the energy tied up in maintaining order (temperature times the change in entropy, TS). When you look at small changes, ΔG = ΔH − TΔS, so if the overall change makes ΔG negative, the process can proceed on its own and can do non-expansion work; if ΔG is positive, work must be put in to drive it. Enthalpy describes heat content, internal energy is the total energy stored in the system, and entropy measures disorder. None of those alone quantify the energy available to do work under these fixed conditions, which is why Gibbs free energy is the correct term.