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The standard Gibbs free energy of formation of a compound is the change of Gibbs free energy that accompanies the formation of 1 mole of that substance from its component elements, in their standard states (the most stable form of the element at 25 °C and 100 kPa).
Historically, the term 'free energy' has been used for either quantity. In physics, free energy most often refers to the Helmholtz free energy, denoted by A (or F), while in chemistry, free energy most often refers to the Gibbs free energy. The values of the two free energies are usually quite similar and the intended free energy function is ...
The Gibbs–Helmholtz equation is a thermodynamic equation used to calculate changes in the Gibbs free energy of a system as a function of temperature. It was originally presented in an 1882 paper entitled "Die Thermodynamik chemischer Vorgänge" by Hermann von Helmholtz.
The change of Gibbs free energy (ΔG) in an exergonic reaction (that takes place at constant pressure and temperature) is negative because energy is lost (2). In chemical thermodynamics, an exergonic reaction is a chemical reaction where the change in the free energy is negative (there is a net release of free energy). [1]
The green curve is the total (Gibbs if this is at constant pressure) free energy as a function of radius. Shown is the free energy barrier, , and radius at the top of the barrier, . This total free energy is a sum of two terms. The first is a bulk term, which is plotted in red.
The activation energy for the reaction is typically larger than the overall energy of the exergonic reaction (1). Endergonic reactions are nonspontaneous. The progress of the reaction is shown by the line. The change of Gibbs free energy (ΔG) during an endergonic reaction is a positive value because energy is gained (2).
For example, the Gibbs free energy change is used when considering processes that occur under constant pressure and temperature conditions, whereas the Helmholtz free energy change is used when considering processes that occur under constant volume and temperature conditions. The value and even the sign of both free energy changes can depend ...
The Gibbs free energy change = determines whether mixing at constant (absolute) temperature and pressure is a spontaneous process.This quantity combines two physical effects—the enthalpy of mixing, which is a measure of the energy change, and the entropy of mixing considered here.