Summarizing Free Energy

Key Concepts and Summary

Gibbs free energy (G) is a state function defined with regard to system quantities only and may be used to predict the spontaneity of a process. A negative value for ΔG indicates a spontaneous process; a positive ΔG indicates a nonspontaneous process; and a ΔG of zero indicates that the system is at equilibrium. A number of approaches to the computation of free energy changes are possible.

Key Equations

• ΔG = ΔH − TΔS
• ΔG = ΔG° + RT ln Q
• ΔG° = −RT ln K

Glossary

Gibbs free energy change (G)

thermodynamic property defined in terms of system enthalpy and entropy; all spontaneous processes involve a decrease in G

standard free energy change (ΔG°)

change in free energy for a process occurring under standard conditions (1 bar pressure for gases, 1 M concentration for solutions)

standard free energy of formation \(\left(\text{Δ}{G}_{\text{f}}^{°}\right)\)

change in free energy accompanying the formation of one mole of substance from its elements in their standard states