Gibbs Free Energy Calculator
ΔG from ΔH, ΔS and temperature — spontaneity, the crossover temperature where ΔG = 0, the equilibrium constant K, and ΔG under non-standard conditions from the reaction quotient.
How it works
ΔG° = ΔH° − TΔS°; ΔG = ΔG° + RT·ln Q; K = exp(−ΔG°/RT); T_cross = ΔH°/ΔS°
Gibbs free energy balances the drive toward lower energy (ΔH) against the drive toward disorder (ΔS) at a given temperature. A negative ΔG means the forward reaction is spontaneous under those conditions — not necessarily fast. When ΔH and ΔS have the same sign the answer flips at the crossover temperature: exothermic reactions that lose entropy (like ammonia synthesis) favour low temperature, endothermic ones that gain entropy favour heat. The reaction quotient term shows how far from equilibrium a real mixture is.
Worked example
Haber process N₂ + 3H₂ → 2NH₃: ΔH = −92.2 kJ, ΔS = −198.7 J/K. At 298 K ΔG° = −33.0 kJ (K = 6 × 10⁵); above 464 K it becomes non-spontaneous — hence catalysts and pressure rather than heat alone.
Frequently asked questions
Why is a spontaneous reaction sometimes not happening?
ΔG says nothing about rate; diamond → graphite is spontaneous with an enormous activation barrier.
Units mismatch?
ΔH is usually tabulated in kJ and ΔS in J — the calculator converts, but check your own working: 1 kJ = 1,000 J.
Related calculators
For study and planning — check units and conditions against your data source before using a result in the lab.