What does a negative ΔG mean?

A negative ΔG means the reaction is thermodynamically spontaneous (exergonic) — it can proceed without external energy input under the given conditions. This does not say anything about reaction rate; a catalyst may still be needed.

What is the unit of ΔS in this calculator?

Entropy change is entered in J/(mol·K) by default, which is the standard SI unit. The calculator also accepts cal/(mol·K). Internally everything is converted to J before computing ΔG.

Why must temperature be in kelvin?

The formula ΔG = ΔH − TΔS requires an absolute temperature scale so that T = 0 corresponds to absolute zero. Always convert Celsius to kelvin by adding 273.15 before entering a value.

Yes. When ΔG = 0 the system is at thermodynamic equilibrium — the forward and reverse reactions proceed at equal rates and there is no net change in free energy.

What if ΔH and ΔS have opposite signs?

If ΔH 0, ΔG is always negative (always spontaneous). If ΔH > 0 and ΔS < 0, ΔG is always positive (never spontaneous). When both have the same sign, spontaneity depends on temperature.

Does this calculator work for biochemistry?

Yes. For standard biochemical conditions (pH 7, 25 °C = 298.15 K), enter T = 298.15. The standard Gibbs free energy ΔG° uses standard-state ΔH° and ΔS° values, which you can source from thermodynamic tables.

Gibbs Free Energy Calculator

Name: Gibbs Free Energy Calculator
Availability: InStock
Author: Nham Vu

Compute ΔG = ΔH − TΔS and instantly see whether a reaction is spontaneous, non-spontaneous, or at equilibrium.

Inputs

Enthalpy Change (ΔH)

Temperature (T) — in kelvin

°C + 273.15 = K | standard = 298.15 K

Entropy Change (ΔS)

Display ΔG in

Enter values and click Calculate to see ΔG

Summary

Compute ΔG = ΔH − TΔS and instantly see whether a reaction is spontaneous, non-spontaneous, or at equilibrium.

How it works

Enter the enthalpy change (ΔH) with its unit (kJ/mol, kcal/mol, or J/mol).
Enter the absolute temperature (T) in kelvin.
Enter the entropy change (ΔS) with its unit (J/(mol·K) or cal/(mol·K)).
Click Calculate — the tool applies ΔG = ΔH − TΔS and converts all inputs to consistent SI units first.
Read the ΔG value and the spontaneity verdict: negative ΔG = spontaneous, positive = non-spontaneous, zero = equilibrium.
Use the unit selector to view ΔG in kJ/mol, kcal/mol, or J/mol.

Use cases

Determine whether a chemical reaction will occur spontaneously at a given temperature.
Find the temperature crossover point where a reaction switches from spontaneous to non-spontaneous.
Check thermodynamics homework problems with a step-by-step breakdown.
Verify ΔG calculations for biochemical reactions (ATP hydrolysis, metabolic pathways).
Explore how entropy and enthalpy contributions change with temperature.
Support physical chemistry coursework and exam preparation.

Frequently Asked Questions

Last updated: 2026-06-10 · Reviewed by Nham Vu