Which thermodynamic quantity represents the amount of useful energy available to do work?

Gibbs' Free Energy is a fundamental thermodynamic quantity that indicates the amount of useful energy available to do work at constant temperature and pressure. It is defined as the difference between the enthalpy of a system and the product of its temperature and entropy.

In essence, Gibbs' Free Energy provides insight into the spontaneity of a process; when the change in Gibbs' Free Energy is negative, it implies that the process can occur spontaneously and the system can perform work. This makes Gibbs' Free Energy particularly relevant in chemical reactions and phase changes, where it helps predict whether a reaction will proceed and to what extent, effectively highlighting the energy that can be harnessed for work.

Understanding Gibbs' Free Energy also illustrates why it is more useful than other thermodynamic quantities in assessing work potential. Heat Capacity, for instance, reflects a material’s ability to store heat but does not directly correlate with the capacity to perform work. Internal Energy accounts for the total energy contained within the system, including both useful and non-useful forms of energy. Latent Heat pertains to the energy required for phase changes at constant temperature, but it does not quantify the usable energy for doing external work. Thus, Gibbs' Free Energy is the most appropriate choice for representing the