Entropy (S) measures randomness/disorder. delta_S = q_rev/T. Second Law: the entropy of the universe increases for any spontaneous process (delta_S_total > 0). Entropy increases in: melting, vaporisation, sublimation, dissolution, reactions that increase gas moles, heating, mixing, expansion. Standard molar entropy S_standard is always positive (> 0) for any substance at T > 0 K (unlike delta_Hf which can be zero for elements). Gases have the highest S, then liquids, then solids. For a reaction: delta_S = sum(S products) - sum(S reactants). The Third Law: S = 0 for a perfect crystalline substance at absolute zero (0 K). This provides an absolute reference for entropy.