Concept:Thermodynamics distinguishes between total energy changes and the energy available for useful work.
Explanation:The total energy change of a system is not entirely available to do useful work.
Some energy is always lost as disorder or entropy, reducing the usable portion.
Gibb's free energy,
G, represents the energy capable of doing useful work at constant temperature and pressure.
Enthalpy change,
ΔH, is the total heat absorbed or released, not necessarily useful work.
Entropy,
S, measures disorder, not available work.
Therefore, the useful work portion of total energy is Gibb's free energy.
Answer:C. Gibb's free energy.