Gibbs Free Energy Calculator (ΔG = ΔH − TΔS)

Q: Does spontaneous mean fast?

No. Spontaneous means thermodynamically favourable. The rate depends on the activation energy; diamond turning into graphite is spontaneous but extremely slow.

Enter ΔH, ΔS and temperature to see if a reaction is spontaneous and its equilibrium constant.

By TheCalcTools Updated October 17, 2026 Formula checked and tested

Your numbers

Enthalpy change ΔH (kJ/mol)

Entropy change ΔS (J/(mol·K))

Temperature (°C)

Your results

ΔG (kJ/mol)

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Spontaneous at this temperature?

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Equilibrium constant K

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Crossover temperature ΔH/ΔS (°C)

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Gibbs Free Energy Calculator (ΔG = ΔH − TΔS) by TheCalcTools

How the Gibbs Free Energy Calculator (ΔG = ΔH − TΔS) works

Gibbs free energy combines enthalpy and entropy: ΔG = ΔH − TΔS. A negative ΔG means the reaction can proceed spontaneously at that temperature.

The standard ΔG° relates to the equilibrium constant by ΔG° = −RT ln K. When ΔH and ΔS have the same sign, the reaction switches between spontaneous and non-spontaneous at T = ΔH/ΔS.

The formula

ΔG = ΔH − T × ΔS; K = e^(−ΔG ÷ RT)

Worked example

Using the default values shown in the calculator:

Enthalpy change ΔH (kJ/mol)	-92.2
Entropy change ΔS (J/(mol·K))	-198.7
Temperature (°C)	25

Ammonia synthesis: ΔG = −92.2 − 298.15 × (−0.1987) = -32.96 kJ/mol: Yes (ΔG < 0).
K = 594200; above 191 °C the reaction is no longer spontaneous.

Result:

ΔG (kJ/mol)	-32.96
Spontaneous at this temperature?	Yes (ΔG < 0)
Equilibrium constant K	594200
Crossover temperature ΔH/ΔS (°C)	190.9

Frequently asked questions

Does spontaneous mean fast?