In thermodynamic analysis of chemical reactions, the term free energy denotes either of two related concepts of importance expressing the total amount of energy which is used up or released during a chemical reaction. Both attempt to capture that part of the total energy of a system which is available for "useful work" and is hence not stored in "useless random thermal motion". As a system undergoes changes, its free energy will decrease.
When a system of molecules undergoes change, whether chemical reaction or changes in physical states such as phase changes, there are two tendencies driving the changes:
- Free Energy tends to decrease,
- Entropy tends to increase.
If represents the internal energy, the temperature, and the entropy, these two tendencies can be combined by stating that the expression
- , the Helmholtz free energy function (named after Hermann von Helmholtz)
tends to decrease. Strictly, this is only true in situations where the volume is constant, as in sealed containers. The change in Helmholtz free energy is equal to the maximum work accompanying the process of the system occurring at constant volume
At constant temperature
If the pressure is constant, as in open containers, the enthalpy (where represents the pressure and represents the volume) replaces the energy, and thus the quantity that must be minimized is
- , the Gibbs free energy function (named after Willard Gibbs)
Famous quotes containing the words work and/or content:
“When I consider thy heavens, the work of thy fingers, the moon and the stars, which thou hast ordained; what is man, that thou art mindful of him? and the son of man, that thou visitest him? For thou hast made him a little lower than the angels, and hast crowned him with glory and honour.”
—Bible: Hebrew Psalms, 8:2.
“Man was kreated a little lower than the angells and has bin gittin a little lower ever sinse.” (Josh Billings, His Sayings, ch. 28, 1865)
“I am as content to die for God’s eternal truth on the scaffold as in any other way.”
—John Brown (1800–1859)