Gibbs Free Energy Calculator
Gibbs free energy calculator (ΔG = ΔH − TΔS): find ΔG and whether a reaction is spontaneous, or the crossover temperature.
ΔG = ΔH − T · ΔS
How to use this calculator
- Choose the quantity you want under “Solve for”.
- Type the known values and pick their units: any combination works.
- The result updates as you type. Open “Show the steps” to see the substitution.
The Gibbs free energy change ΔG = ΔH − TΔS decides whether a reaction can proceed on its own at constant temperature and pressure: it is spontaneous when ΔG is negative.
Set ΔG = 0 and solve for T to find the temperature where spontaneity switches. Mind the units: ΔH is usually in kJ/mol and ΔS in J/(mol·K); the calculator converts them for you.
Formula
ΔG = ΔH − T · ΔS
| Symbol | Quantity | Units |
|---|---|---|
| ΔH | Enthalpy change | kJ/mol, J/mol, kcal/mol, cal/mol |
| T | Temperature | K, °C, °F, °R |
| ΔS | Entropy change | J/(mol·K), kJ/(mol·K), cal/(mol·K) |
| ΔG | Gibbs free energy change | kJ/mol, J/mol, kcal/mol, cal/mol |
Worked example
Given: ΔH = -92.2 kJ/mol · T = 298.15 K · ΔS = -198.7 J/(mol·K)
ΔG = ΔH − T · ΔS
Result: ΔG = -32.9576 kJ/mol
Frequently asked questions
When is a reaction spontaneous at all temperatures?
When ΔH < 0 and ΔS > 0. If both are negative it is spontaneous only below T = ΔH/ΔS.
Does a negative ΔG mean the reaction is fast?
No. ΔG says whether it can happen, not how fast. Kinetics depend on the activation energy.