Intermediate Topic 365 of 1102 Thermodynamics & Energetics
Gibbs Free Energy
G equals H minus TS; a negative change signals a spontaneous process at constant T and P.
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In-depth explanation of Gibbs Free Energy
Gibbs free energy, symbolized G, is defined as G equals H minus T times S and combines the enthalpy and entropy of a system into a single function. At constant temperature and pressure, a negative change in G signals a spontaneous process, a zero change indicates equilibrium, and a positive change indicates a nonspontaneous process that requires external work. The criterion applies to the system itself, while the second law still concerns the total entropy change of the universe. Gibbs energy is the most useful criterion for chemists because most reactions occur at constant pressure and ordinary temperature. Its value tells us whether a reaction can proceed, although not how fast it will do so.
Going deeper: Gibbs Free Energy
Key ideas: Compute delta G from delta H minus T delta S, or from formation data, or from the relation with K or with cell potential. A negative value means spontaneous, but only at the stated temperature and pressure and with the stated concentrations.
Common mistakes and connections: Do not interpret a negative delta G as meaning that the reaction will happen quickly. Also keep standard and non-standard conditions apart: delta G degree is for standard conditions, while actual spontaneity depends on Q. Gibbs energy links to equilibrium, to electrochemistry and to biochemical energetics.
Visualization of Gibbs Free Energy
G = H - TS combines energy and disorder. The plot shows delta G against temperature for a given enthalpy and entropy change. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Ice melts above 0 degrees Celsius because at that temperature the term T times delta S becomes large enough to make delta G negative.
Frequently asked questions about Gibbs Free Energy
What is Gibbs Free Energy?
G equals H minus TS; a negative change signals a spontaneous process at constant T and P.
Is Gibbs Free Energy a beginner, intermediate or advanced topic?
Gibbs Free Energy is a intermediate topic in Thermodynamics & Energetics. It builds on the vocabulary of beginner topics in this area.
What should I learn before Gibbs Free Energy?
Study these earlier topics in Thermodynamics & Energetics first: Standard Molar Entropy, Entropy Changes in Reactions, Entropy of Phase Change.
What comes after Gibbs Free Energy?
The next topics in Thermodynamics & Energetics are Gibbs Energy and Spontaneity and Temperature Dependence of Spontaneity.
Can I practise Gibbs Free Energy with quizzes?
Yes. The interactive lesson includes a self-check question on Gibbs Free Energy, and the Thermodynamics & Energetics quiz draws on every topic in this area.
Where Gibbs Free Energy fits in Thermodynamics & Energetics
Gibbs Free Energy is topic 37 of 91 in Thermodynamics & Energetics.