Gibbs Free Energy Calculator
Gibbs free energy change, equilibrium constant and crossover temperature from enthalpy and entropy changes.
Results
What this tool does
Gibbs free energy is the accounting that decides which way a reaction runs: enthalpy is the heat it releases, entropy times temperature is the disorder it buys, and the balance between them is what nature actually minimises. The temperature term is why some reactions reverse when heated — the default values are ammonia synthesis, which is favourable at room temperature and stops being so above about 466 K.
Formula
dG = dH - T dS ; K = e^(-dG/(R T))
Variables
| Symbol | Meaning | Unit |
|---|---|---|
dh | Enthalpy change | kJ/mol |
ds | Entropy change | J/(mol·K) |
tt | Temperature | K |
DG | Gibbs free energy | kJ/mol |
KK | Equilibrium constant | — |
TE | Crossover temperature | K |
TH | Entropy term | kJ/mol |
Worked example
- Enthalpy change-92.4 kJ/mol
- Entropy change-198.3 J/(mol·K)
- Temperature298.15 K
- Gibbs free energy-33.2769 kJ/mol
- Equilibrium constant675,853.813182
- Crossover temperature465.96 K
- Entropy term-59.1231 kJ/mol
Limitations
- Mixing units is the most common source of error. Convert every input to the units shown next to each field before calculating.
- The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.
Frequently asked questions
Does a negative delta G mean the reaction will actually happen?
It means it can, not that it will. Thermodynamics tells you where the system would end up if it got there; kinetics tells you how long that takes. Diamond turning into graphite has a negative delta G at room temperature and takes longer than the age of the universe, because the activation barrier is enormous. The crossover temperature shown here is where the sign of delta G flips.