Gibbs Free Energy Calculator
Calculate ΔG from ΔH and ΔS, find the temperature where a reaction becomes spontaneous, and convert between ΔG° and the equilibrium constant K.
What this calculator does
Use ΔG = ΔH − TΔS to decide whether a reaction is spontaneous at a given temperature, find the transition temperature, or convert ΔG° to K and back.
When you need it
Gibbs free energy tells you whether a reaction can go on its own at a given temperature, which matters for metabolism, battery chemistry and designing industrial reactions.
Formulas
- ΔG = ΔH − TΔS
- ΔG° = −RT ln K
- T (when ΔG = 0) = ΔH ÷ ΔS
How to use it
- Pick what to calculate: ΔG, K from ΔG°, or ΔG° from K.
- Enter ΔH in kJ/mol, ΔS in J/(mol·K) and the temperature in kelvin.
- Read ΔG, whether the reaction is spontaneous and the transition temperature.
Worked example
For N2 + 3 H2 → 2 NH3, ΔH = −92.2 kJ and ΔS = −198.7 J/K. Is it spontaneous at 298 K?
- Convert ΔS: −198.7 J/K = −0.1987 kJ/K
- TΔS = 298 K × (−0.1987 kJ/K) = −59.2 kJ
- ΔG = −92.2 kJ − (−59.2 kJ) = −33.0 kJ
- Transition temperature = −92.2 ÷ −0.1987 = 464 K
Answer
ΔG = −33.0 kJ, so the reaction is spontaneous at 298 K. It stops being spontaneous above about 464 K.
Common mistakes
- Leaving ΔS in J/K while ΔH is in kJ.
- Using °C for T.
- Reading a negative ΔG as a fast reaction: ΔG says nothing about the rate.
Related tools
Frequently asked questions
What does a negative ΔG mean?
The reaction is spontaneous at that temperature and pressure, meaning it can proceed without a continuous input of energy.
How do ΔG° and K relate?
ΔG° = −RT ln K. A negative ΔG° means K is greater than 1.
When is a reaction spontaneous at all temperatures?
When ΔH is negative and ΔS is positive.