Gibbs Free Energy Calculator

Gibbs free energy (G) is a thermodynamic potential that combines a system's enthalpy and entropy at a given temperature, G=H−TS, its change (ΔG) directly determines whether a process is spontaneous at constant temperature and pressure.

ΔG = ΔH − TΔS, where ΔH is the enthalpy change, T is the absolute temperature in Kelvin, and ΔS is the entropy change. This calculator uses ΔH and ΔG in kJ and ΔS in J/K, automatically converting units.

If ΔG is negative, the process is spontaneous (thermodynamically favorable) at that temperature. If ΔG is positive, it is non-spontaneous (requires external energy input to proceed). If ΔG is exactly zero, the system is at equilibrium.

ΔG=0 marks equilibrium, the system has no thermodynamic tendency to move in either direction. Phase transitions like boiling and melting occur exactly at the temperature where ΔG=0 for that phase change, which is why this calculator's boiling-point example gives exactly zero.

Yes, if the entropy increase (ΔS) is large enough and the temperature is high enough, the −TΔS term can outweigh a positive ΔH, making ΔG negative overall, this is called an entropy-driven process, common in reactions that produce gas from solids or liquids.

Because ΔG=ΔH−TΔS, temperature directly scales the entropy contribution, a process that is non-spontaneous at low temperature can become spontaneous at higher temperature (or vice versa) if ΔS has the right sign, exactly why some reactions only proceed above or below a specific threshold temperature.

Using the standard formation values for liquid water (ΔH≈−285.8 kJ/mol, ΔS≈−163.2 J/(mol·K)) at 298.15 K, this calculator gives ΔG≈−237.14 kJ/mol, closely matching the real published standard Gibbs free energy of formation of water (−237.13 kJ/mol).

This is simply a historical convention in chemistry and thermodynamics, entropy values are typically much smaller in magnitude when expressed per Kelvin, so J/K keeps the numbers in a convenient range, while enthalpy changes are usually large enough that kJ is more convenient. This calculator handles the unit conversion between them automatically.

No, Gibbs free energy (G=H−TS) applies to processes at constant pressure, while Helmholtz free energy (A=U−TS) applies to processes at constant volume, both predict spontaneity but under different physical constraints.

Enter your enthalpy and entropy changes with their actual signs (negative for exothermic reactions or entropy decreases), the calculator computes ΔG directly and reports it with the same sign convention.