Gibbs Free Energy Calculator

Calculate Gibbs free energy change using delta G = delta H - T*delta S. Determine if reactions are spontaneous, non-spontaneous, or at equilibrium at any temperature.

⚡ Thermodynamics📐 ΔG = ΔH - TΔS🧪 Chemistry
ΔH (kJ/mol)
T (K)
ΔS (J/mol·K)
Please enter valid values.

Formula & Reference

VariableSymbolFormulaUnits
Gibbs Free Energy CalculatorSee formulaΔG = ΔH - TΔSkJ/mol

Step-by-Step Examples

Example 1
Combustion of CH₄

ΔH=-890.3 kJ/mol, ΔS=-242.8 J/mol·K, T=298 K.

  • ΔG = -890.3 - (298×-242.8/1000)
  • ΔG = -890.3 - (-72.35) = -817.9 kJ/mol
✓ Spontaneous: ΔG = -817.9 kJ/mol
Example 2
Melting of Ice

ΔH=+6.01 kJ/mol, ΔS=+22.0 J/mol·K at 298 K.

  • ΔG = 6.01 - 298×0.0220 = 6.01 - 6.56 = -0.55 kJ/mol
  • Spontaneous at 298 K (ice melts above 273 K)
✓ Spontaneous at 25°C: ΔG = -0.55 kJ/mol
Example 3
Find Crossover Temperature

ΔH=+40.7 kJ/mol (vaporization), ΔS=+109 J/mol·K. Find T where ΔG=0.

  • ΔG = 0 when T = ΔH/ΔS = 40700/109 = 373 K
  • 373 K = 100°C (boiling point of water)
✓ ΔG = 0 at 373 K (100°C) — transition temperature

Real-World Applications

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Electrochemistry
ΔG = -nFE°. Spontaneous reactions have negative ΔG and positive cell potential.
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Reaction Feasibility
Chemists predict if a reaction will proceed using ΔG before attempting synthesis.
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Process Engineering
Industrial reactions must have ΔG < 0 or require energy input, affecting process economics.
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Geochemistry
Mineral stability and weathering reactions follow ΔG < 0 for thermodynamic spontaneity.

Common Mistakes to Avoid

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ΔS must be in J not kJ

Common error: ΔS is in J/mol·K but ΔH is in kJ/mol. Divide ΔS by 1000 when subtracting: TΔS in kJ.

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Spontaneous does not mean fast

Thermodynamically spontaneous (ΔG < 0) means the reaction is favorable, not that it is fast. Kinetics (activation energy) controls rate.

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Temperature in Kelvin

Always use Kelvin for T. 25°C = 298.15 K. Celsius gives wrong ΔG values.

Frequently Asked Questions

What does ΔG < 0 mean? ▾
The reaction is thermodynamically spontaneous (favorable). Products are more stable than reactants. Does not guarantee fast reaction.
What is the relationship between ΔG and K? ▾
ΔG° = -RT ln K. At 298 K: ΔG° = -5.71 × log K (kJ/mol). Large negative ΔG° means large K (products favored at equilibrium).
What happens when ΔG = 0? ▾
System is at equilibrium. For phase transitions, this gives the exact transition temperature (melting point, boiling point).
How does temperature affect spontaneity? ▾
Depends on signs of ΔH and ΔS. If ΔH < 0 and ΔS > 0: always spontaneous. If ΔH > 0 and ΔS < 0: never spontaneous. Mixed signs: spontaneous above or below crossover T.
What is standard Gibbs energy? ▾
ΔG° is at standard state (1 atm, 25°C, 1 M concentrations). ΔG = ΔG° + RT ln Q for non-standard conditions.

Related Chemistry Calculators

🔥 Enthalpy⚗️ Entropy⚖️ Equilibrium Constant⚡ Nernst Equation🔥 Hess Law

Formula Explorer connections

Interpretation: ΔG balances enthalpy against the temperature-weighted entropy change. Negative ΔG is thermodynamically favorable, zero marks equilibrium, and positive ΔG is unfavorable under the stated conditions. Assumption: ΔH and ΔS must use compatible units and temperature must be kelvin; spontaneity does not imply a fast reaction.

Enthalpy →Entropy →Equilibrium Constant →Chemistry Formula Explorer →