About the Gibbs Free Energy Calculator
What is the Gibbs Free Energy Calculator?
The Gibbs free energy change tells you whether a reaction can proceed on its own at a given temperature: negative ΔG means spontaneous.
Enter the enthalpy and entropy changes and the temperature. The calculator also gives the equilibrium constant and the temperature at which spontaneity switches.
How to use the Gibbs Free Energy Calculator
- Enthalpy Change ΔH (kJ/mol) (sample: -92.2)
- Entropy Change ΔS (J/mol·K) (sample: -198.7)
- Temperature (°C) (sample: 25)
- Press Calculate ΔG (results also refresh as you type). The result card shows the Gibbs Free Energy Change (ΔG), along with Spontaneous at this temperature?, Equilibrium constant K and Temperature where ΔG changes sign.
Formula used
ΔG = ΔH − T × ΔS
- K = e^(−ΔG ÷ RT)
- Crossover temperature = ΔH ÷ ΔS
Spontaneous does not mean fast – kinetics decide the rate.
Worked example
These are the sample values already filled in the calculator above, and the result it returns for them:
| Input | Sample value |
|---|---|
| Enthalpy Change ΔH (kJ/mol) | -92.2 |
| Entropy Change ΔS (J/mol·K) | -198.7 |
| Temperature (°C) | 25 |
| Result | Value |
|---|---|
| Gibbs Free Energy Change (ΔG) | -32.958 kJ/mol |
| Spontaneous at this temperature? | Yes (ΔG < 0) |
| Equilibrium constant K | 5,94,183.840256 |
| Temperature where ΔG changes sign | 190.9°C |
Tips for using the Gibbs Free Energy Calculator
- Exothermic reactions that decrease entropy (like ammonia synthesis) are spontaneous only below the crossover temperature.
- Keep units consistent: ΔH in kJ/mol and ΔS in J/(mol·K) – the calculator converts.