⚡ Gibbs Free Energy Calculator (ΔG = ΔH − TΔS)
Find ΔG from enthalpy, temperature and entropy, and see whether the reaction is spontaneous, or solve for any of the four.
Gibbs free energy ΔG
-70.2 kJ/mol
ΔG = ΔH − TΔS. Enter ΔH in kJ/mol, ΔS in J/(mol·K), T in kelvin. ΔG < 0 ⇒ spontaneous. 🔒 Computed in your browser.
How the gibbs free energy calculator (δg = δh − tδs) works
Gibbs free energy is ΔG = ΔH − TΔS, with ΔH in kJ/mol, ΔS in J/(mol·K) and T in kelvin (the tool handles the kJ/J unit conversion). Enter any three and it solves for the fourth. A negative ΔG means the reaction is spontaneous (product-favoured) at that temperature. The crossover temperature, where ΔG changes sign, is T = ΔH/ΔS; for example, with ΔH = +178 kJ/mol and ΔS = +160 J/(mol·K) the reaction becomes spontaneous above 178,000 ÷ 160 ≈ 1113 K.
The single most common thermodynamics gotcha is mixing kJ and J. Here ΔH is kJ/mol and ΔS is J/(mol·K), converted internally, so you don’t have to. ΔG < 0 spontaneous, ΔG > 0 non-spontaneous, ΔG = 0 at equilibrium.
Frequently asked questions
What is the Gibbs free energy equation?
ΔG = ΔH − TΔS: the change in free energy equals the enthalpy change minus the absolute temperature times the entropy change. It predicts whether a process is spontaneous.
When is a reaction spontaneous?
When ΔG is negative. ΔG > 0 is non-spontaneous (the reverse is favoured), and ΔG = 0 is at equilibrium. This tool states the verdict from your values.
Why do I use kJ for ΔH but J for ΔS?
By convention ΔH is tabulated in kJ/mol and ΔS in J/(mol·K). They must be in the same units before subtracting, the tool converts ΔS to kJ automatically, avoiding the classic 1000× error.
How do I find the temperature where a reaction becomes spontaneous?
Set ΔG = 0 and solve for T = ΔH/ΔS. Choose to solve for T with ΔG left at 0 to find the crossover temperature.
What does temperature do to spontaneity?
It scales the −TΔS term. If ΔH and ΔS have the same sign, spontaneity flips at T = ΔH/ΔS; if they have opposite signs, the reaction is spontaneous (or not) at all temperatures.
What do the four sign combinations of ΔH and ΔS mean?
ΔH−/ΔS+: spontaneous at all temperatures. ΔH+/ΔS−: never spontaneous. ΔH−/ΔS−: spontaneous only at low temperature. ΔH+/ΔS+: spontaneous only at high temperature. The −TΔS term settles the temperature-dependent cases.
How does ΔG relate to the equilibrium constant?
Through ΔG° = −RT ln K. A negative ΔG° means K > 1 (products favoured); ΔG° = 0 means K = 1. This tool computes ΔG from ΔH, T and ΔS; the K link uses that result.