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# Exergonic process

An exergonic process is one in which there is a positive flow of energy from the system to the surroundings. This is in contrast with an endergonic process. Constant pressure, constant temperature reactions are exergonic if and only if the Gibbs free energy change is negative (∆G  All physical and chemical systems in the universe follow the second law of thermodynamics and proceed in a downhill, i.e., exergonic, direction. Thus, left to itself, any physical or chemical system will proceed, according to the second law of thermodynamics, in a direction that tends to lower the free energy of the system, and thus to expend energy in the form of work. These reactions occur spontaneously. A chemical reaction is also exergonic when spontaneous. Thus in this type of reactions the Gibbs free energy decreases. The entropy is included in any change of the Gibbs free energy. This differs from a exothermic reaction or a endothermic reaction where the entropy is not included. The Gibbs free energy is calculated with the Gibbs–Helmholtz equation: \Delta G = \Delta H- T \cdot \Delta S where: T = temperature in kelvins (K) ΔG = change in the Gibbs free energy ΔS = change in entropy (at 298 K) as ΔS = Σ{S( Product)} − Σ{S( Reagent)} ΔH = change in enthalpy (at 298 K) as ΔH = Σ{H(Product)} − Σ{H(Reagent)} A chemical reaction progresses only spontaneously when the Gibbs free energy decreases, in that case the ΔG is negative. In exergonic reactions the ΔG is negative and in endergonic reactions the ΔG is positive: \Delta_\mathrm{R} G < 0 exergon \Delta_\mathrm{R} G > 0 endergon where: \Delta_\mathrm{R} G equals the change in the Gibbs free energy after completion of a chemical reaction.

## References

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This article based upon the http://en.wikipedia.org/wiki/Exergonic_process, the free encyclopaedia Wikipedia and is licensed under the GNU Free Documentation License.
Further informations available on the list of authors and history: http://en.wikipedia.org/w/index.php?title=Exergonic_process&action=history
presented by: Ingo Malchow, Mirower Bogen 22, 17235 Neustrelitz, Germany