The term "free energy" refers to a thermodynamic quantity that is produced through a process of subtraction between the internal energy in a system and the product of its entropy and absolute temperature. In chemistry, this is also known as the multiplication of entropy and absolute temperature subtracted from the enthalpy in a system.
In thermodynamics, free energy refers to the portion of any first-law energy that can perform thermodynamic work, such as the work that is mediated through thermal energy. Through the course of such work, free energy is subject to irreversible and unstoppable loss. Because first-law energy is always conserved, there is evidence that free energy is expendable, and second-law energy can perform within a finite amount of time.
The quantity called "free energy" was developed as a more accurate and advanced representation of the outdated term "affinity," which was previously used by chemists to describe the force that causes chemical reactions. During the 18th and 19th centuries, researchers began to gravitate toward the theory of heat, which states that heat is a form of energy that has relation to vibratory motion. This was meant to supplant the previous understanding that heat was fluid. This also began to replace the four elements theory, by which heat was regarded to be the lightest of the elements.