Calculate ΔG from ΔH and ΔS, or find the temperature at which a reaction becomes spontaneous.
Enter ΔH (enthalpy change), ΔS (entropy change), and temperature to calculate ΔG. Or solve for any other variable. When solving for T, the calculator finds the temperature at which ΔG = 0 — the crossover point between spontaneous and non-spontaneous.
Watch your units: ΔH is commonly in kJ/mol while ΔS is in J/(mol·K). The calculator handles conversion automatically, but make sure you select the correct units from the dropdowns.
The Gibbs free energy equation ΔG = ΔH − TΔS determines whether a process is thermodynamically spontaneous at a given temperature.
Interpreting ΔG: If ΔG < 0, the reaction is spontaneous (thermodynamically favorable). If ΔG > 0, the reaction is non-spontaneous. If ΔG = 0, the system is at equilibrium.
Worked example: The combustion of methane has ΔH = −890.3 kJ/mol and ΔS = −242.2 J/(mol·K). At 25°C (298.15 K): ΔG = −890.3 − (298.15 × −0.2422) = −890.3 + 72.2 = −818.1 kJ/mol. Strongly spontaneous.
Temperature dependence: For reactions where ΔH and ΔS have the same sign, there's a crossover temperature T = ΔH/ΔS where spontaneity switches. Exothermic reactions with negative ΔS (like combustion) are spontaneous at low T but not at very high T. Endothermic reactions with positive ΔS (like melting ice) become spontaneous above the crossover temperature.
Relationship to K: ΔG° = −RT ln(K), connecting thermodynamics to the equilibrium constant.