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Gibbs Free Energy

General

Gibbs Free Energy (G)

A thermodynamic quantity that predicts whether a chemical reaction will occur spontaneously at constant temperature and pressure, based on changes in enthalpy and entropy.

Definition

Gibbs free energy (G) is a thermodynamic quantity that combines a system's enthalpy and entropy to predict whether a chemical reaction will occur spontaneously at constant temperature and pressure. Chemists almost always work with the change in Gibbs free energy (ΔG) between reactants and products, rather than the absolute value of G itself, since it is the change that determines reaction direction.

When ΔG is negative, the reaction is thermodynamically spontaneous and will proceed forward without continuous outside energy input, releasing usable energy in the process. When ΔG is positive, the reverse reaction is favored instead, and the forward reaction requires an external energy source to proceed. A ΔG of exactly zero indicates the system is already at equilibrium.

Gibbs free energy is directly connected to the equilibrium constant of a reaction — a very negative ΔG corresponds to a large Keq, meaning the reaction strongly favors products at equilibrium. The Gibbs Free Energy Calculator and Equilibrium Constant Calculator both let you move between ΔG, Keq, enthalpy, and entropy depending on which values you have.

Formula

ΔG = ΔH - TΔS

Where:

  • ΔG = change in Gibbs free energy (kJ/mol or J/mol)
  • ΔH = change in enthalpy (heat content) of the reaction
  • T = absolute temperature in Kelvin
  • ΔS = change in entropy (disorder) of the reaction

Gibbs free energy also relates to the equilibrium constant by:

ΔG° = -RT × ln(Keq)

Worked Example

A reaction has ΔH = -92 kJ/mol and ΔS = -198 J/mol·K at a temperature of 298 K.

First convert ΔS to kJ: -198 J/mol·K = -0.198 kJ/mol·K

ΔG = ΔH - TΔS = -92 - (298 × -0.198) = -92 - (-59.0) = -92 + 59.0 = -33 kJ/mol

Since ΔG is negative, this reaction is spontaneous at 298 K. Confirm the arithmetic with the Gibbs Free Energy Calculator.

Key Things to Know

  • Sign of ΔG determines spontaneity: A negative ΔG means the reaction proceeds forward on its own, while a positive ΔG means the reverse direction is favored under those conditions.
  • Temperature can flip the outcome: Because ΔG depends on the term TΔS, a reaction that is non-spontaneous at low temperature can become spontaneous at higher temperature if ΔS is positive, or vice versa.
  • It links directly to the equilibrium constant: A strongly negative ΔG corresponds to a Keq much greater than 1, meaning the reaction favors products heavily at equilibrium.
  • Spontaneity does not mean fast: A reaction can have a favorable (negative) ΔG yet still proceed very slowly in practice if its activation energy is high, since thermodynamics and kinetics answer different questions.
  • Standard conditions matter: ΔG° values assume standard conditions (1 atm pressure, 1 M concentrations); actual ΔG under non-standard conditions requires an additional correction term involving the reaction quotient.

Related Calculators

Gibbs Free Energy Calculator →Calculate the Gibbs free energy change ΔG from enthalpy ΔH, entropy ΔS, and temperature T using ΔG = ΔH − TΔS. Determine if a reaction is spontaneous at a given temperature.
Equilibrium Constant Calculator →Calculate the equilibrium constant Kc from equilibrium concentrations of reactants and products for a reversible reaction. Also compute ΔG° from Kc at 25°C.

Related Terms

Equilibrium ConstantEquilibrium Constant (Keq)Activation EnergyActivation Energy (Ea)

Frequently Asked Questions

A negative Gibbs free energy change (delta G less than 0) means the reaction is spontaneous under the given conditions and will proceed on its own without continuous external energy input. A positive delta G means the reaction is non-spontaneous and requires energy input to proceed as written.
Gibbs free energy change is calculated using the formula delta G = delta H minus T times delta S, combining the enthalpy change, temperature in Kelvin, and entropy change of the reaction. The Gibbs Free Energy Calculator computes this directly when you provide enthalpy, entropy, and temperature values.
Gibbs free energy and the equilibrium constant are linked through the equation delta G equals negative RT times the natural log of Keq, meaning a large positive Keq (favoring products) corresponds to a strongly negative delta G. The Equilibrium Constant Calculator can use this relationship to convert between the two values.
Yes, a reaction with positive (unfavorable) enthalpy can still be spontaneous if it has a sufficiently large positive entropy change and the temperature is high enough, since the T times delta S term can outweigh delta H. This is common in reactions where a solid or liquid converts into a gas, increasing disorder significantly.
Gibbs free energy determines whether a reaction is thermodynamically favorable overall, while activation energy determines how fast that reaction proceeds by describing the energy barrier reactants must overcome. A reaction can be thermodynamically spontaneous (negative delta G) yet still proceed extremely slowly if its activation energy is very high.