Thermodynamics — Energy in Chemical Reactions

Key variables:

ΔH — enthalpy change (heat at constant pressure)

ΔS — entropy change (disorder)

ΔG — Gibbs free energy change (spontaneity)

Gibbs Free Energy:

ΔG = ΔH − TΔS

ΔG < 0 → spontaneous (exergonic)

ΔG > 0 → nonspontaneous (endergonic)

ΔG = 0 → equilibrium

Spontaneity summary:

ΔH < 0, ΔS > 0 → always spontaneous

ΔH > 0, ΔS < 0 → never spontaneous

ΔH < 0, ΔS < 0 → spontaneous at low temperature

ΔH > 0, ΔS > 0 → spontaneous at high temperature

Equilibrium constant:

ΔG° = −RT ln K

K > 1 → products favored; ΔG° < 0

K < 1 → reactants favored; ΔG° > 0

Le Chatelier's Principle: System at equilibrium shifts to minimize stress. Add reactant → shifts right; increase pressure → shifts to fewer moles of gas; increase temp → shifts toward endothermic side.

Reference:

Wikipedia: Thermodynamics

https://en.wikipedia.org/wiki/Thermodynamics