Compute ΔG = ΔH − TΔS and check spontaneity
A Gibbs free energy calculator computes the change in Gibbs free energy (ΔG) of a chemical reaction from its enthalpy change, temperature, and entropy change. The sign of ΔG tells you whether a reaction is spontaneous (thermodynamically favourable) under the given conditions, making it one of the most important quantities in chemical thermodynamics.
The Gibbs free energy change is calculated with:
A related form links ΔG° to the equilibrium constant: ΔG° = −RT ln(K).
For H₂ + ½O₂ → H₂O at 298 K, ΔH = −285.8 kJ/mol and ΔS = −163.2 J/mol·K. ΔG = −285.8 − (298 × −0.1632) = −285.8 + 48.6 = −237.2 kJ/mol. Because ΔG is negative, the reaction is spontaneous.
If ΔH = +50 kJ/mol and ΔS = +120 J/mol·K, then at 298 K, ΔG = 50 − (298 × 0.120) = 50 − 35.8 = +14.2 kJ/mol (non-spontaneous). Raising the temperature to 500 K gives ΔG = 50 − 60 = −10 kJ/mol (spontaneous) — showing how temperature can flip spontaneity.
A negative ΔG means the reaction is spontaneous (thermodynamically favourable) and can proceed in the forward direction without external energy input at the given temperature.
The formula is ΔG = ΔH − TΔS, where ΔH is enthalpy change, T is absolute temperature in kelvin, and ΔS is entropy change. Keep ΔH and ΔS in consistent units (convert ΔS from J to kJ).
The term TΔS requires an absolute temperature scale so that the product is physically meaningful. Convert Celsius to kelvin by adding 273.15.
Yes. When ΔH and ΔS have the same sign, the TΔS term can outweigh ΔH at certain temperatures, flipping the sign of ΔG and changing whether the reaction is spontaneous.
ΔG is the free energy change under actual conditions, while ΔG° is measured under standard conditions (1 bar, 1 M, usually 298 K). They are related to the equilibrium constant by ΔG° = −RT ln(K).