free energy

* * *

Measure of the total combined energies within a system, derived from heats of transformation, disorder, and other forms of internal energy (e.g., electrostatic charges).

A system will change spontaneously to achieve a lower total free energy. Thus, free energy is the driving force toward equilibrium conditions. The change in free energy between an initial and a final state is useful in evaluating certain thermodynamic processes and can be used to judge whether transformations will occur spontaneously. There are two forms of free energy, with different definitions and applications: the Helmholtz (see Hermann von Helmholtz) free energy, sometimes called the work function, and the Gibbs (see J. Willard Gibbs) free energy.

* * *

      in thermodynamics, energy-like property or state function of a system in thermodynamic equilibrium. Free energy has the dimensions of energy, and its value is determined by the state of the system and not by its history. Free energy is used to determine how systems change and how much work they can produce. It is expressed in two forms: the Helmholtz free energy F, sometimes called the work function, and the Gibbs free energy G. If U is the internal energy of a system, PV the pressure-volume product, and TS the temperature- entropy product (T being the temperature above absolute zero), then F = U − TS and G = U + PV − TS. The latter equation can also be written in the form G = H – TS, where H = U + PV is the enthalpy. Free energy is an extensive property, meaning that its magnitude depends on the amount of a substance in a given thermodynamic state.

      The changes in free energy, ΔF or ΔG, are useful in determining the direction of spontaneous change and evaluating the maximum work that can be obtained from thermodynamic processes involving chemical or other types of reactions. In a reversible process the maximum useful work that can be obtained from a system under constant temperature and constant volume is equal to the (negative) change in the Helmholtz free energy, −ΔF = −ΔU + TΔS, and the maximum useful work under constant temperature and constant pressure (other than work done against the atmosphere) is equal to the (negative) change in the Gibbs free energy, −ΔG = −ΔH + TΔS. In each case, the TΔS entropy term represents the heat absorbed by the system from a heat reservoir at temperature T under conditions where the system does maximum work. By conservation of energy (energy, conservation of), the total work done also includes the decrease in internal energy U or enthalpy H as the case may be. For example, the energy for the maximum electrical work done by a battery as it discharges comes both from the decrease in its internal energy due to chemical reactions and from the heat TΔS it absorbs in order to keep its temperature constant, which is the ideal maximum heat that can be absorbed. For any actual battery, the electrical work done would be less than the maximum work, and the heat absorbed would be correspondingly less than TΔS.

      Changes in free energy can be used to judge whether changes of state can occur spontaneously. Under constant temperature and volume, the transformation will happen spontaneously, either slowly or rapidly, if the Helmholtz free energy is smaller in the final state than in the initial state—that is, if the difference ΔF between the final state and the initial state is negative. Under constant temperature and pressure, the transformation of state will occur spontaneously if the change in the Gibbs free energy, ΔG, is negative.

      Phase transitions provide instructive examples, as when ice melts to form water at 0.01 °C (T = 273.16 K), with the solid and liquid phases in equilibrium. Then ΔH = 79.71 calories per gram is the latent heat of fusion, and by definition

ΔS = ΔH/T = 0.292 calories per gram∙K
is the entropy change. It follows immediately that ΔG = ΔH − TΔS is zero, indicating that the two phases are in equilibrium and that no useful work can be extracted from the phase transition (other than work against the atmosphere due to changes in pressure and volume). Furthermore, ΔG is negative for T > 273.16 K, indicating that the direction of spontaneous change is from ice to water, and ΔG is positive for T < 273.16 K, where the reverse reaction of freezing takes place.
 

* * *


Universalium. 2010.

Look at other dictionaries:

  • Free energy — may refer to:In science: * Thermodynamic free energy, the energy in a physical system that can be converted to do work, in particular: ** Helmholtz free energy, the energy that can be converted into work at a constant temperature and volume… …   Wikipedia

  • free energy — Gibbs free energy (G) the thermodynamic function G = H − TS, where H is enthalpy, T absolute temperature, and S entropy. For chemical reactions occurring at a constant temperature and pressure, the free energy change ΔG =… …   Medical dictionary

  • free energy — (= Gibbs free energy, G) A thermodynamic term used to describe the energy that may be extracted from a system at constant temperature and pressure. In biological systems the most important relationship is: DG = RTln(Keq), where Keq is an… …   Dictionary of molecular biology

  • free energy — noun a) The difference between the internal energy of a system and the product of its entropy and absolute temperature b) The Gibbs free energy …   Wiktionary

  • free energy — a thermodynamic quantity that is a function of both the internal energy and entropy (or randomness) of a system; at equilibrium the free energy is at a minimum …   Mechanics glossary

  • free energy — Helmholco energija statusas T sritis Standartizacija ir metrologija apibrėžtis Laisvoji energija arba izochorinis potencialas – termodinaminės sistemos parametras, kai sistemos tūris ir temperatūra nekinta. atitikmenys: angl. free energy;… …   Penkiakalbis aiškinamasis metrologijos terminų žodynas

  • free energy — Helmholco energija statusas T sritis chemija apibrėžtis Termodinaminės sistemos parametras, kai sistemos tūris ir temperatūra nekinta. atitikmenys: angl. free energy; Helmholtz energy; isochoric potential rus. изохорный потенциал; свободная… …   Chemijos terminų aiškinamasis žodynas

  • free energy — laisvoji energija statusas T sritis fizika atitikmenys: angl. free energy vok. freie Energie, f rus. свободная энергия, f pranc. énergie libre, f …   Fizikos terminų žodynas

  • free energy — noun (physics) a thermodynamic quantity equivalent to the capacity of a physical system to do work; the units of energy are joules or ergs energy can take a wide variety of forms • Syn: ↑energy • Derivationally related forms: ↑energize (for: ↑ …   Useful english dictionary

  • free energy — /fri ˈɛnədʒi/ (say free enuhjee) noun that portion of the energy of a system which is the maximum available for doing work …   Australian English dictionary

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.