Gibbs Free Energy Calculator
Enter the enthalpy change (ΔH), entropy change (ΔS), and absolute temperature to calculate the Gibbs free-energy change (ΔG) for a process.
Last updated: September 26, 2026
Frequently asked questions
What does Gibbs free energy tell you?
The change in Gibbs free energy (ΔG) indicates whether a process is thermodynamically favorable at constant temperature and pressure. A negative ΔG means the process can occur spontaneously as written; a positive ΔG means it cannot, under the stated conditions.
What is the formula?
ΔG = ΔH − TΔS, where ΔH is the enthalpy change, T is the absolute temperature in kelvin, and ΔS is the entropy change. ΔS is typically given in J/(mol·K), so it must be converted to kJ if ΔH is in kJ/mol before subtracting.
Does a negative ΔG mean the reaction will happen quickly?
No. ΔG describes thermodynamic favorability, not reaction rate. A reaction with negative ΔG can still be very slow if it has a high activation energy or lacks a catalyst - kinetics and thermodynamics are separate questions.
What's the difference between ΔG and ΔG°?
ΔG° specifically refers to the free-energy change under standard-state conditions (usually 1 bar pressure, 1 M concentrations, 298.15 K). ΔG is the more general free-energy change under whatever conditions you specify, which this calculator computes directly from your ΔH, ΔS, and T.
Why does temperature affect whether ΔG is positive or negative?
Because ΔG = ΔH − TΔS, temperature scales the entropy term. A reaction with unfavorable ΔH but favorable ΔS (or vice versa) can switch from unfavorable to favorable, or the reverse, at some crossover temperature.
How is this related to the Equilibrium Constant Calculator?
The standard Gibbs free-energy change ΔG° relates directly to the equilibrium constant K through ΔG° = −RT ln K. The Equilibrium Constant Calculator can convert between the two.