Gibbs Free Energy Calculator

Determine reaction spontaneity. Leave one field blank to solve for it.

ΔG = ΔH - TΔS

Worked Examples: ΔG = ΔH − TΔS

Example 1: Compute ΔG at 298 K

Given ΔH = −92.2 kJ/mol and ΔS = −198 J·mol−1·K−1 (e.g., water formation).

  1. Convert ΔS to kJ: −198 J/K = −0.198 kJ/K.
  2. ΔG = ΔH − TΔS = (−92.2) − 298×(−0.198) = −92.2 + 59.0 = −33.2 kJ/mol (spontaneous).

Example 2: Temperature for spontaneity

Given ΔH = +40.0 kJ/mol, ΔS = +125 J·mol−1·K−1. Find T where ΔG = 0.

  1. ΔG = 0 ⇒ T = ΔH/ΔS. Convert ΔS → 0.125 kJ·mol−1·K−1.
  2. T = 40.0 / 0.125 = 320 K. Above this, reaction becomes spontaneous (ΔG < 0).

Example 3: Solve for ΔS

Given ΔG = −12.0 kJ/mol, ΔH = −25.0 kJ/mol at T = 350 K.

  1. Rearrange: ΔS = (ΔH − ΔG)/T = (−25.0 − (−12.0)) / 350 = −13.0/350 = −0.03714 kJ·mol−1·K−1.
  2. Convert to J units: ×1000 → −37.1 J·mol−1·K−1.