$word_{count}$: 210

Entropy (S) measures disorder/randomness. $delta_S$ = $\frac{q_rev}{T}$ (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$\frac{V2}{V1}$ = nR ln$\frac{P1}{P2}$. 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.