Heat of Fusion
Definition and meaning of Heat of Fusion in chemistry.
Heat of fusion is the amount of thermal energy required to convert one mole (or unit mass) of a substance from solid to liquid at its melting point, at constant pressure, without any change in temperature.
In more detail
During melting, added energy goes into overcoming the intermolecular or ionic/metallic forces holding particles in a fixed crystal lattice, not into raising average kinetic energy, so the temperature plateaus until melting is complete. Heat of fusion is a type of latent heat and is commonly reported as an enthalpy change, ΔHfus, since melting occurs at constant pressure. Because enthalpy is a state function, freezing the same liquid releases an identical amount of heat (the heat of crystallization), just with the opposite sign. Substances with strong intermolecular forces, such as hydrogen-bonded water, tend to have unusually high heats of fusion compared to their molar mass.
Key facts
| Field | Physical Chemistry |
|---|---|
| Symbol | ΔH_fus |
| Typical units | kJ/mol or J/g |
| Water's heat of fusion | 6.01 kJ/mol (334 J/g) |
The heat of fusion of water is 6.01 kJ/mol (334 J/g), which is why melting a tray of ice absorbs a large amount of heat at a steady 0°C, the same principle that keeps insulated ice packs cold for hours.
Frequently asked questions
Is heat of fusion the same as latent heat of fusion?
Yes, the two terms are used interchangeably; both describe the enthalpy absorbed during melting at constant temperature and pressure.
Why doesn't the temperature rise while a solid is melting?
The heat supplied is used to break the intermolecular attractions that hold the solid's ordered structure together, not to increase the particles' average kinetic energy, so temperature stays constant until melting is complete.