The heat of fusion (ΔHfus )is the energy required to change a substance from a solid to a liquid; the heat of vaporization (ΔHvap) 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 ΔHsub, 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 ΔHfus.
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 ΔHvap.
Melting and vaporization take place at constant temperatures, as any added heat is will be used as the energy for the phase transformation itself.
Practice Questions
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Key Points
• ΔHfus represents the heat of fusion; transformation of a solid to a liquid, units in Joules per mol, J/mol.
• ΔHvap represents the heat of vaporization; transformation of a liquid to a gas, units in J/mol.
• ΔHfus + ΔHvap = ΔHsub
