Define the following terms: (a) molar heat of vaporization, (b) molar heat of fusion, (c) molar heat of sublimation. What are their units?
The Correct Answer and Explanation is:
Here are the definitions and units of the terms you requested:
(a) Molar Heat of Vaporization:
The molar heat of vaporization refers to the amount of heat required to vaporize one mole of a substance at its boiling point, under constant pressure. It represents the energy needed to break the intermolecular forces that hold the liquid molecules together, allowing them to escape into the gas phase.
- Units: The units of molar heat of vaporization are typically Joules per mole (J/mol) or kilojoules per mole (kJ/mol).
(b) Molar Heat of Fusion:
The molar heat of fusion is the amount of heat required to melt one mole of a solid substance at its melting point, at constant pressure. It represents the energy needed to overcome the intermolecular forces in the solid phase so that the substance can transition into the liquid phase.
- Units: The units of molar heat of fusion are also typically Joules per mole (J/mol) or kilojoules per mole (kJ/mol).
(c) Molar Heat of Sublimation:
The molar heat of sublimation is the amount of heat required to change one mole of a solid directly into a gas, bypassing the liquid phase, at its sublimation point. This is the total energy needed for the transition from solid to gas, accounting for both the heat of fusion and the heat of vaporization combined.
- Units: The units for molar heat of sublimation are typically Joules per mole (J/mol) or kilojoules per mole (kJ/mol), similar to the other terms.
Explanation:
Each of these terms relates to phase transitions of substances. The heat of vaporization deals with the transition from liquid to gas, the heat of fusion deals with the transition from solid to liquid, and the heat of sublimation involves the transition directly from solid to gas.
- The molar heat of vaporization is typically higher than the heat of fusion because it requires more energy to break the forces between molecules in a liquid than to separate them in a solid.
- The molar heat of fusion is usually lower than the heat of vaporization but can vary depending on the material.
- The molar heat of sublimation can be considered as the sum of the heat of fusion and the heat of vaporization for a substance, because it involves both processes.
In all cases, the higher the molar heat, the more energy is needed for that phase change.
