Q.30.4 kJ is required to melt one mole of sodium chloride. The entropy change during melting is 28.4 JK$^{-1}$mol$^{-1}$. Calculate the melting point of sodium chloride.

Everyday spontaneous processes -- ice melting, water evaporating -- share a common pattern: molecular randomness (entropy) increases as rigidly-ordered solid molecules gain freedom of movement in the liquid, and freer still in the vapour phase. Because entropy (unlike $U$ or $H$) has a genuine absolute-zero reference point, ABSOLUTE entropies can be tabulated: the standard entropy $S^0$ is a substance's absolute entropy at 298 K, 1 bar, and (per the third law) is always positive above 0 K. This lets a reaction's standard entropy change be computed directly: $\Delta S_r^0=\sum S_{products}^0-\sum S_{reactants}^0$ -- the exact same sum-of-products-minus-reactants pattern used for $\Delta H_r^0$ from formation enthalpies. …