Gibbs Free Energy Calculator

How Gibbs Free Energy Is Calculated

Gibbs Free Energy (G) predicts whether a chemical reaction occurs spontaneously at constant temperature and pressure — the most common conditions in chemistry and biology.

Gibbs Free Energy Formula: ΔG = ΔH − TΔS

Where:

• ΔG = change in Gibbs free energy (kJ/mol)
• ΔH = enthalpy change (kJ/mol), negative means exothermic
• T = temperature in Kelvin (K = °C + 273.15)
• ΔS = entropy change (kJ/mol·K)

Spontaneity Rules:

• ΔG < 0: spontaneous (reaction proceeds forward)
• ΔG > 0: non-spontaneous (requires energy input)
• ΔG = 0: equilibrium (no net change)

Worked Example: Melting Ice at 25°C:

• ΔH = +6.01 kJ/mol (endothermic, absorbs heat)
• ΔS = +22.0 J/mol·K = +0.0220 kJ/mol·K
• T = 25 + 273.15 = 298.15 K
• ΔG = 6.01 − (298.15 × 0.0220) = 6.01 − 6.56 = −0.55 kJ/mol

ΔG < 0 confirms ice melts spontaneously at 25°C. At exactly 0°C (273K): ΔG = 6.01 − (273 × 0.0220) = 6.01 − 6.01 = 0 (equilibrium — ice and water coexist).

Standard Conditions vs Real Conditions: ΔG = ΔG° + RT × ln(Q)

where Q is the reaction quotient, R = 8.314 J/mol·K.

At Equilibrium: ΔG = 0, so ΔG° = −RT × ln(K), allowing calculation of the equilibrium constant K.