Why is the Heat of Vaporization for both Na and NaCl is higher than the Heat of Fusion? Why does Sodium has higher Hvap, whereas sodium chloride has the higher heat of fusion?

Question

H_fus(kJ/kg) H_vap(kJ/kg)

Na 113 4236

NaCl 517 3517

Answer 1

The values of heat of vaporization for both Na and NaCl are higher than their values of heat of fusion, because in the phase change from liquid to gas, particles must overcome all their inter-particle attractions to move freely, whereas in the phase change from solid to liquid, many of inter-particle attractions still weakly attracted and remain operative.

Sodium is a metal element. The bonding is metallic that the valence electrons are collectively shared, and delocalized. These electrons are uniformly distributed throughout the structure. In the transition from liquid to gas, all the particles must break all these inter-particle attractions, that requires a lot of energy. Therefore, Na has a higher heat of vaporization. Since the valence electrons are delocalized, it takes less energy than NaCl to melt Na, which has a lower heat of fusion.

NaCl is a ionic solid. The bond between Na⁺ and Cl^- ions is ionic that held together by strong electrostatic attraction between them. The valence electrons in NaCl are confined rather than being delocalized. NaCl would take more energy than Na to melt and to break the ionic bond in NaCl. In molten NaCl all the ionic bonds are broken and Na⁺ and Cl^- ions can move, so it takes less energy than Na to vaporize.