ΔS°rxn = Σ S°(products) − Σ S°(reactants)
Entropy (S) is a thermodynamic measure of molecular disorder or the number of accessible microstates. The second law of thermodynamics states that the total entropy of the universe always increases for a spontaneous process: ΔS_universe = ΔS_system + ΔS_surroundings > 0. Standard molar entropy (S°) is measured at 298 K and 1 atm. Entropy generally increases when: solids → liquids → gases, temperature increases, volume increases, or the number of particles increases. For a reaction: ΔS°rxn = Σ S°(products) − Σ S°(reactants). The Gibbs energy combines enthalpy and entropy: ΔG = ΔH − TΔS. A reaction is spontaneous when ΔG < 0. Boltzmann's equation S = k·ln(W) connects entropy to the number of microstates (W), providing a statistical interpretation of this fundamental quantity.