(a) Why is the heat of fusion (ΔH fus ) of a substance smaller than its heat of vaporization (ΔH

step 1 compare the heat of fusion versus the heat of vaporization. Heat of fusion is when a solid is he

(a) Why is the heat of fusion (ΔH fus ) of a substance...

(a) Why is the heat of fusion $\left(\Delta H_{\text { fus }}\right)$ of a substance smaller than its heat of vaporization $\left(\Delta H_{\mathrm{vap}}\right) ?$ (b) Why is the heat of sublimation $\left(\Delta H_{\text { subl }}\right)$ of a substance greater than its $\Delta H_{\text { vap }} ?$ (c) At a given temperature and pressure, how does the magnitude of the heat of vaporization of a substance compare with that of its heat of condensation?

(a) Why is the heat of fusion $\left(\Delta H_{\text { fus }}\right)$ of a substance smaller than its heat of vaporization $\left(\Delta H_{\mathrm{vap}}\right) ?$
(b) Why is the heat of sublimation $\left(\Delta H_{\text { subl }}\right)$ of a substance greater than its $\Delta H_{\text { vap }} ?$
(c) At a given temperature and pressure, how does the magnitude of the heat of vaporization of a substance compare with that of its heat of condensation?

Key Concepts

Phase Transition Enthalpy

This concept refers to the energy change associated with a change of state, such as fusion, vaporization, and sublimation. Each transition requires a specific amount of energy (latent heat) to overcome the molecular interactions that hold the molecules together in one phase, allowing them to move into another phase without a change in temperature.

Intermolecular Forces

Intermolecular forces are the attractions between molecules, such as hydrogen bonds, van der Waals forces, or dipole-dipole interactions. The strength and nature of these forces determine the amount of energy required to disrupt the structure of a substance during a phase change; melting only partially disrupts these forces, whereas boiling requires complete overcoming of them.

Latent Heat Comparison in Phase Transitions

The latent heat of fusion is lower than that of vaporization because the process of melting only partially disrupts the intermolecular forces found in a solid, while vaporization fully separates molecules into a gas. Similarly, sublimation, which transforms a solid directly into a gas, requires enough energy to account for both fusion and vaporization processes, resulting in a higher energy requirement.

Reversibility of Phase Transitions

In thermodynamics, reversible phase transitions indicate that the amount of energy absorbed in a forward process (e.g., vaporization) is equal in magnitude but opposite in sign to the energy released in the reverse process (e.g., condensation). This equality of energy magnitude at equilibrium conditions ensures that the heat required for vaporization is the same as the heat released during condensation.

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