Gibbs Free Energy Calculator
Gibbs free energy from enthalpy and entropy — whether a reaction is spontaneous, and the temperature at which that changes.
ΔG = ΔH − TΔS.
How the gibbs free energy calculator works
ΔG = ΔH − TΔS. Negative ΔG means the reaction is spontaneous as written; positive means the reverse is.
The interesting case is when ΔH and ΔS have the same sign, because then temperature decides. The crossover is at T = ΔH/ΔS — melting ice is exactly this, spontaneous above 273 K and not below.
Formula: ΔG = ΔH − TΔS; T_crossover = ΔH/ΔS
Worked examples
| Inputs | ΔG | Note |
|---|---|---|
| Ammonia synthesis at 25 °C | -32.9576 kJ/mol | ΔG −33 kJ/mol — spontaneous, but only below 191 °C |
| The same reaction at 400 °C | 41.5549 kJ/mol | no longer spontaneous |
| Melting ice at 25 °C | -0.5493 kJ/mol | spontaneous — entropy wins |
FAQFrequently asked questions
What does a negative ΔG mean?
The reaction is spontaneous in the direction written — it can proceed without external work. It says nothing about how fast.
Why does temperature sometimes flip the answer?
Because the TΔS term grows with temperature. When ΔH and ΔS have the same sign, one term dominates at low temperature and the other at high.
What is the crossover temperature?
ΔH/ΔS — where ΔG passes through zero and spontaneity reverses. For ammonia synthesis it is about 464 K.
Does spontaneous mean fast?
No, and this is the most common confusion. Diamond turning to graphite is spontaneous and takes geological time. Kinetics decides speed; thermodynamics decides direction.
How does ΔG relate to the equilibrium constant?
ΔG° = −RT ln K. A ΔG of −5.7 kJ/mol at 25 °C corresponds to K of about 10.
Where these figures come from
- IUPAC — Standard atomic weights (2021 conventional values) — the molar-mass table
- NIST — CODATA 2018 fundamental physical constants — Avogadro constant, gas constant, speed of light
- NIST Chemistry WebBook — thermochemical data
- National Institute of Standards and Technology — the US measurement and reference-data authority
Last checked: September 2026. Atomic masses are the IUPAC conventional values; constants are CODATA 2018; equations are the standard textbook forms.