The Gibbs energy change when one mole of compound forms from its elements in standard states.
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The standard Gibbs energy of formation, symbolized delta Gf standard, is the change in Gibbs energy when one mole of a compound forms from its elements in their standard states at 1 bar and 298 K. Elements in their standard states have a value of zero by convention. Standard free energies of formation allow the standard Gibbs energy change of a reaction to be calculated as the sum over products minus the sum over reactants, just as for enthalpies. A negative value means the compound is thermodynamically stable with respect to its elements, while a positive value means it tends to decompose spontaneously under standard conditions. These tables are routinely used in equilibrium and electrochemical calculations.
Key ideas: Treat these values like formation enthalpies: sum products minus reactants to obtain delta G degree for a reaction. Elements in their standard states are zero. A strongly negative value for a compound, like for water, suggests stability against decomposition into elements.
Common mistakes and connections: Remember that stability toward elements does not imply stability toward other reactions, such as combustion or disproportionation. Also these values apply at 298 K only. They link to equilibrium constants through delta G degree equals minus RT ln K.
Standard Gibbs energies of formation. Negative values mean a compound forms spontaneously from its elements; NO and acetylene are unstable relative to the elements. The interactive lesson shows this as a live animation or chart.
The standard Gibbs energy of formation of liquid water is about -237 kJ per mole, showing that water is stable relative to hydrogen and oxygen.
The Gibbs energy change when one mole of compound forms from its elements in standard states.
Standard Free Energy of Formation is a intermediate topic in Thermodynamics & Energetics. It builds on the vocabulary of beginner topics in this area.
Study these earlier topics in Thermodynamics & Energetics first: Gibbs Free Energy, Gibbs Energy and Spontaneity, Temperature Dependence of Spontaneity.
The next topics in Thermodynamics & Energetics are Free Energy and Equilibrium and Reaction Quotient and Free Energy.
Yes. The interactive lesson includes a self-check question on Standard Free Energy of Formation, and the Thermodynamics & Energetics quiz draws on every topic in this area.
Standard Free Energy of Formation is topic 40 of 91 in Thermodynamics & Energetics.