Why is the latent heat of vaporization higher than the latent heat of fusion of a substance
The Correct Answer and Explanation is:
The latent heat of vaporization is higher than the latent heat of fusion for a substance because of the different energy requirements for changing the state of a substance from solid to liquid (fusion) and from liquid to gas (vaporization). These two phase transitions involve different amounts of energy due to the distinct nature of the molecular changes that occur during each process.
- Molecular Structure and Interactions:
In the case of fusion (solid to liquid), the molecules are already close together in the solid state, and only enough energy is required to overcome the forces that keep the molecules in a fixed position. These forces are primarily intermolecular forces like hydrogen bonds or van der Waals forces. While energy is needed to break some of these bonds, the molecules remain in close proximity even after melting, allowing them to move freely within the liquid state. However, during vaporization (liquid to gas), the molecules are more spread out, and a much larger amount of energy is required to overcome the attractive intermolecular forces entirely. In the gas state, the molecules move freely, and these forces must be completely overcome to allow the molecules to escape into the air. The energy needed to overcome these forces in the liquid phase is much higher than that needed to break some of the bonds during fusion. - Energy Distribution and Kinetic Energy:
In the liquid phase, molecules still retain significant kinetic energy, but to change into a gas, the molecules need to gain enough energy to move far apart from each other and break all the remaining intermolecular bonds. This requires not only a significant input of heat but also an increase in the system’s energy to overcome the latent energy that keeps molecules bound together in the liquid phase. - Overall Energy Change:
Therefore, since vaporization requires breaking all the intermolecular attractions between molecules, it requires a larger amount of energy compared to fusion, where only partial breaking of bonds occurs.
In summary, the latent heat of vaporization is higher than the latent heat of fusion because transforming a liquid to a gas requires a greater amount of energy to fully overcome the intermolecular forces than simply transitioning from solid to liquid, where these forces are only partially broken.
