⚡Gibbs Free Energy (ΔG=ΔH-TΔS)
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.
- 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
ΔH=−200kJ · T=298K · ΔS=+150J/K=ΔG = −200−(298×0.15) = −244.7 kJ → spontaneousBoth enthalpy and entropy favour spontaneity
Temperature-dependent spontaneity
When ΔH and TΔS have opposite signs, temperature determines spontaneity. Endothermic reactions with positive ΔS become spontaneous at high T; exothermic reactions with negative ΔS become spontaneous at low T.
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Fun Fact
Josiah Willard Gibbs developed this function in 1873–1876 — arguably the most significant contribution to chemistry in the 19th century, enabling prediction of chemical equilibria.
References
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