Gibbs free energy calculator
A reaction is spontaneous when delta G is negative. Entropy decides whether raising the temperature helps.
kJ/mol
J/(mol K)
Gibbs free energy change
-32.958kJ/mol
- Equilibrium constant K
- 594 184
- Temperature where delta G changes sign
- 190.87 degrees C
Show the working
- Enthalpy change = -92.2
- Entropy change = -198.7
- Temperature = 25
- tk = 25 + 273.15 = 298.15
- Gibbs free energy change = -92.2 - 298.15 * -198.7 / 1000 = -32.9576
- crossover = -198.7 != 0 ? -92.2 * 1000 / -198.7 : 0 = 464.016
- Temperature where delta G changes sign = 464.016 - 273.15 = 190.866
- Equilibrium constant K = exp(--32.9576 * 1000 / (Rgas * 298.15)) = 594 184
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- delta G = delta H - T delta S
- delta G = -R T ln K, so K = exp(-delta G / R T)
- crossover temperature = delta H / delta S
Enthalpy is conventionally in kJ/mol and entropy in J/(mol K); the factor of a thousand between them is the classic slip.
The default values are the Haber process for ammonia synthesis.