Gibbs Free Energy Calculator

Calculate ΔG from ΔH and ΔS, from equilibrium constant K, or from cell potential E. Determine spontaneity and find the temperature threshold.

kJ/mol
K
J/(mol·K)
ΔG (kJ/mol)
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Spontaneity —
T at ΔG=0 (K) —
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kJ/mol
K
J/(mol·K)
ΔG (kJ/mol)
—
Spontaneity —
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kJ/mol
K
J/(mol·K)
ΔG° standard (kJ/mol)
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ΔG at Q (kJ/mol) —
ΔG° from K (kJ/mol) —
Spontaneity (standard) —
T at ΔG=0 (K) —
ATP equivalents (÷30.5) —

How to Use This Calculator

  1. Enter ΔH (kJ/mol), T (K), and ΔS (J/(mol·K)).
  2. The calculator returns ΔG and spontaneity classification.
  3. Use tabs to calculate ΔG from equilibrium constant K or from cell potential.

Formula

ΔG = ΔH − TΔS  (kJ/mol, convert ΔS/1000)

ΔG° = −RT ln K  (R = 8.314 J/(mol·K))

ΔG = −nFE  (F = 96485 C/mol)

Example

Example: ΔH = −286 kJ/mol, T = 298.15 K, ΔS = −163 J/(mol·K). ΔG = −286 − 298.15 × (−0.163) = −286 + 48.6 = −237.4 kJ/mol (spontaneous).

Frequently Asked Questions

  • Gibbs free energy (G) combines enthalpy and entropy into a single thermodynamic potential. ΔG = ΔH − TΔS. A negative ΔG means the process is spontaneous at that temperature; positive means non-spontaneous.
  • When ΔG < 0. If ΔH < 0 and ΔS > 0 (exothermic and disorder-increasing), the reaction is spontaneous at all temperatures. If ΔH > 0 and ΔS < 0, it is never spontaneous.
  • ΔG° = −RT ln K, where R = 8.314 J/(mol·K) and K is the dimensionless equilibrium constant. If K > 1, ΔG° < 0 (products favored). If K < 1, ΔG° > 0 (reactants favored).
  • ΔG = −nFE, where n is the number of electrons transferred, F = 96485 C/mol (Faraday constant), and E is the standard cell potential in volts. A positive E means negative ΔG (spontaneous).
  • At equilibrium temperature T = ΔH / ΔS. Above or below this temperature (depending on the signs of ΔH and ΔS), the reaction spontaneity changes.

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Sources & References (5) ▾
  1. Gibbs Free Energy — ACS Education — American Chemical Society
  2. OpenStax Chemistry 2e, Chapter 16 — Thermodynamics — OpenStax
  3. IUPAC Quantities Units and Symbols in Physical Chemistry (Green Book) — IUPAC
  4. NIST-JANAF Thermochemical Tables — NIST
  5. Atkins, P. & de Paula, J. — Physical Chemistry, 10th Ed., Ch. 3 — Oxford University Press