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Gibbs free energy and spontaneity

Gibbs free energy ΔG\Delta G determines whether a reaction is spontaneous at constant temperature and pressure: if ΔG<0\Delta G < 0, the reaction proceeds on its own in that direction. It combines enthalpy (heat effect) and entropy (disorder) via temperature. Spontaneous does not necessarily mean fast.

ΔG=ΔH−TΔS\Delta G = \Delta H - T\Delta SGibbs free energy
ΔG<0\Delta G < 0spontaneous reaction

Symbols

ΔH\Delta Hreaction enthalpykJ/mol
ΔS\Delta Sentropy changekJ/(mol·K)
TTabsolute temperatureK

Example

ΔH=50\Delta H = 50 kJ/mol, ΔS=0.05\Delta S = 0.05 kJ/(mol·K), T=250T = 250 K:

ΔG=50−250⋅0.05=37.5\Delta G = 50 - 250\cdot 0.05 = 37.5 kJ/mol >0> 0 — not spontaneous.

A reaction with ΔH>0\Delta H > 0 can still become spontaneous at a high enough temperature, if ΔS>0\Delta S > 0.
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