Gibbs Free Energy and Thermodynamic Favorability

Gibbs Free Energy (ΔG°)

A thermodynamic quantity used to predict the thermodynamic favorability of a process; it describes the energy available in a system to do work.

Thermodynamically Favorable

A process that occurs without ongoing outside intervention (formerly called "spontaneous"); indicated by a negative ΔG°.

ΔG° < 0

The process is thermodynamically favorable (products are favored).

ΔG° > 0

The process is thermodynamically unfavorable (reactants are favored).

ΔG° = 0

The system is at equilibrium.

Standard Gibbs Free Energy Equation (Formation)

ΔG°_{rxn} = ΣΔG°f_(products) - ΣΔG°f_(reactants).

Gibbs-Helmholtz Equation

ΔG° = ΔH° - T/ΔS° (where T is in Kelvin).

Kinetic Control

A situation where a reaction is thermodynamically favorable ΔG < 0) but occurs extremely slowly due to a high activation energy (e.g., rusting iron, diamond turning to graphite).

-ΔH and +ΔS

Favorable at ALL temperatures (Enthalpy driven, Entropy driven). ΔG is always negative.

+ΔH and -ΔS

Favorable at NO temperatures (Never favorable). ΔG is always positive.

-ΔH and -ΔS

Favorable only at LOW temperatures (Enthalpy driven).

+ΔH and +ΔS

Favorable only at HIGH temperatures (Entropy driven).

Unit Trap (ΔH vs ΔS)

ΔH is usually given in kJ/mol, while ΔS is given in J/mol·K. You must convert ΔS to kJ (divide by 1000) before subtracting in the equation ΔG = ΔH - T/ΔS.

Elements in Standard State

The standard Gibbs free energy of formation (ΔG°f) for an element in its standard state (e.g., O₂ (g), N₂ (g) is zero.