The heat of fusion (ΔH_fus )is the energy required to change a substance from a solid to a liquid; the heat of vaporization (ΔH_vap) is the energy required to change a substance from a liquid to a gas. Both values are expressed in J/mol; the addition of the two values gives ΔH_sub, or the energy required to transform a solid to a gas.

The heat of vaporization of a liquid is the energy it takes for the substance to transform into a gas. As energy is added to a system, the bonds in the liquid gain energy and reach a point where they can break and undergo a phase change, signified by ΔH_fus.

The heat of fusion of a solid is the energy it takes for the substance to transform into a liquid. As energy is added to a system, the bonds in the solid gain energy and reach a point where they can break and undergo a phase change, signified by ΔH_vap.

Melting and vaporization take place at constant temperatures, as any added heat is will be used as the energy for the phase transformation itself.

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Key Points

• ΔH_fus represents the heat of fusion; transformation of a solid to a liquid, units in Joules per mol, J/mol.

• ΔH_vap represents the heat of vaporization; transformation of a liquid to a gas, units in J/mol.

• ΔH_fus + ΔH_vap = ΔH_sub