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Entropy (S) measures disorder/randomness. delta_S = q_rev/T (definition). Second Law: for any spontaneous process, delta_S_universe > 0 (entropy of universe always increases). For reversible processes: delta_S_universe = 0. Third Law: entropy of a perfect crystal at 0 K is zero (one microstate). Unlike delta_Hf, standard molar entropy S_standard is always positive for any substance at T > 0 K. Entropy ordering: S(gas) >> S(liquid) > S(solid). Entropy increases during: melting, vaporisation, sublimation, dissolution, reactions producing more gas moles, heating, mixing, expansion. For reactions: delta_S = sum(S products) - sum(S reactants). Entropy of surroundings: delta_S_surr = -delta_H_system/T. For exothermic reactions, delta_S_surr > 0 (surroundings gain heat). For ideal gas isothermal expansion: delta_S = nR ln(V2/V1) = nR ln(P1/P2). Entropy is a state function — its change depends only on initial and final states, regardless of whether the actual process is reversible or irreversible. This is crucial for solving problems with irreversible processes.