Does the law of Dulong and Petit (Exercise 3.43) apply to network or molecular solids?

(a) To check network solids, compare the molar heat capacities of graphite, diamond, and SiO₂. Use your findings to determine if the atoms in a network solid behave as independently as atoms in an ionic crystal. Justify your conclusion.

(b) To check molecular solids, compare the molar heat capacities of benzoic acid, urea, and glycine. Use your findings to determine whether the atoms in molecules vibrate completely independently or whether they move to some extent as an entire molecule. Use the data to justify your conclusion.

Exercise 3.43

The molar heat capacity of a substance is the heat required to raise the temperature by 1 °C per mole of molecules or formula units of the substance. For a given substance, the more varieties of movement—such as vibrations—that the atoms in a lattice undergo, the greater the heat capacity. In 1819 the chemists Pierre Dulong and Thérèse Petit claimed (in modern language) that the molar heat capacity of a crystalline solid per atom is the same for all crystals.