How to Calculate Gibbs Free Energy
What is Gibbs Free Energy?
Gibbs free energy (G) determines whether a chemical reaction can occur spontaneously at constant temperature and pressure. ΔG = ΔH − TΔS: negative ΔG means spontaneous; positive means non-spontaneous.
Step-by-Step Guide
- 1ΔG = ΔH − TΔS
- 2ΔH = enthalpy change (bond energies)
- 3T = temperature in Kelvin
- 4ΔS = entropy change (disorder)
- 5ΔG < 0: spontaneous; ΔG > 0: non-spontaneous; ΔG = 0: equilibrium
Worked Examples
Input
ΔH=−200kJ · T=298K · ΔS=+150J/K
Result
ΔG = −200−(298×0.15) = −244.7 kJ → spontaneous
Both enthalpy and entropy favour spontaneity
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