Gibbs Free Energy Calculator
Enter a reaction's enthalpy change, temperature, and entropy change to get the change in Gibbs free energy (ΔG) in joules per mole — and see at a glance whether the reaction is spontaneous.
Use SI units
Enthalpy in J/mol, temperature in kelvin, and entropy in J/(mol·K) give ΔG in J/mol — add 273.15 to a Celsius value to get kelvin first.
What is Gibbs free energy?
The energy available to do work
This Gibbs free energy calculator finds the change in Gibbs free energy, ΔG, the quantity that predicts whether a chemical reaction can happen on its own at a given temperature. It combines the enthalpy change (the heat exchanged), the absolute temperature, and the entropy change (the change in disorder) into a single value in joules per mole. The sign of ΔG is the key: negative means the reaction is spontaneous, positive means it is not.
Enter ΔH, the temperature in kelvin, and ΔS to get the change in Gibbs free energy in joules per mole instantly.
The change in Gibbs free energy is the enthalpy change minus the temperature multiplied by the entropy change.
ΔG = ΔH − T × ΔSThe entropy term, T × ΔS, links spontaneity to temperature: because it scales with T, raising the temperature can flip a reaction from non-spontaneous to spontaneous, or the reverse. Keep ΔH in joules per mole, T in kelvin, and ΔS in joules per mole per kelvin and ΔG comes back in joules per mole. Both ΔH and ΔS can be negative.
Suppose a reaction has ΔH = −100,000 J/mol and ΔS = 50 J/(mol·K) at 298.15 K (25 °C).
Multiply temperature by entropy
298.15 × 50 = 14,907.5 — the entropy term T × ΔS, in J/mol.
Subtract it from the enthalpy change
−100,000 − 14,907.5 = −114,907.5 J/mol — the change in Gibbs free energy.
Read the sign
ΔG is negative, so the reaction is spontaneous at this temperature: it can proceed without any outside energy input.
The sign of ΔG answers one question: will the reaction run on its own at this temperature? A negative ΔG (like the −114,907.5 J/mol above) means the reaction is spontaneous and releases free energy — it can proceed without help. A positive ΔG means the reaction is non-spontaneous and needs an energy input to occur. When ΔG = 0, the reaction sits at equilibrium with no net change in either direction. The magnitude matters too: a large negative value points to a reaction that runs strongly toward products, while a value close to zero signals a reaction that is finely balanced and easily pushed either way by a small change in conditions. Because the entropy term T × ΔS grows with temperature, a reaction that is non-spontaneous when cold can become spontaneous once it is hot enough — which is exactly why heating or cooling can start or stop a reaction.
The formula is exact, but a couple of practical points are worth keeping in mind for accuracy.
Constant temperature, consistent units
This calculator uses the standard relationship ΔG = ΔH − T·ΔS, which assumes a single temperature held constant and treats ΔH and ΔS as constant over the range of interest — a good approximation across modest temperature spans. Temperature is an absolute temperature in kelvin and cannot be negative. Keep your units consistent — J/mol for ΔH, kelvin for T, and J/(mol·K) for ΔS — or convert any kJ/mol value to J/mol before you enter it, otherwise ΔG will be off by a factor of a thousand.