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Gibbs Free Energy and Thermodynamic Favorability

Gibbs free energy (G) determines whether a reaction is spontaneous under constant temperature and pressure.

Gibbs Free Energy Equation​

[ \Delta G = \Delta H - T\Delta S ]

  • ΔG: Gibbs free energy change
  • ΔH: Enthalpy change
  • ΔS: Entropy change
  • T: Temperature in Kelvin

Thermodynamic Favorability​

ΔHΔSΔGSpontaneity
-+-Always spontaneous
--?Spontaneous at low T
++?Spontaneous at high T
+-+Never spontaneous
  • ΔG < 0: reaction is spontaneous
  • ΔG = 0: reaction is at equilibrium
  • ΔG > 0: reaction is non-spontaneous

Relation to Equilibrium​

[ \Delta G = \Delta G^\circ + RT \ln Q ]

  • ΔG°: Standard free energy change
  • Q: Reaction quotient
  • At equilibrium, ΔG = 0 and Q = K

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