Heat of Crystallization
Definition and meaning of Heat of Crystallization in chemistry.
Heat of crystallization is the quantity of heat released when a substance changes from a liquid, melt, or solution into a solid crystalline state. It is an exothermic process, since forming an ordered crystal lattice releases the energy that held the particles in a more disordered, higher-energy liquid arrangement.
In more detail
Heat of crystallization is equal in magnitude but opposite in sign to the heat of fusion, which is the energy absorbed when that same solid melts, because crystallization is simply the reverse of melting. This relationship means crystallization is thermodynamically favorable below the freezing point, since at the freezing point itself the solid and liquid phases coexist in equilibrium with no net driving force in either direction; it underlies why supercooled liquids release a burst of heat once crystallization finally begins. Industrially, controlling heat of crystallization is essential in crystallizer design, sugar and salt refining, and pharmaceutical purification, since removing this heat efficiently determines crystal size and purity.
Key facts
| Field | Physical Chemistry |
|---|---|
| Sign | Negative (exothermic) enthalpy change |
| Water example | -6.01 kJ/mol (-334 J/g) at 0 °C |
| Relation | Equal magnitude, opposite sign to heat of fusion |
When liquid water freezes to ice at 0 degrees Celsius, it releases 6.01 kJ/mol (334 J/g) of heat, the heat of crystallization, which is numerically identical to water's molar heat of fusion but carries a negative (exothermic) sign.
Frequently asked questions
How does heat of crystallization relate to heat of fusion?
They have the same numerical magnitude but opposite signs, since crystallization (liquid to solid) is the exact reverse of fusion, or melting (solid to liquid).
Why do supersaturated sodium acetate hand warmers feel hot?
Flexing the metal disk triggers rapid crystallization of the supersaturated solution, and the heat of crystallization released during this exothermic process warms the pack.