Heat of Vaporization
Definition and meaning of Heat of Vaporization in chemistry.
Heat of vaporization is the amount of energy required to convert a given quantity of a liquid into vapor at constant temperature and pressure, typically measured at the substance's normal boiling point.
In more detail
This energy overcomes the intermolecular attractive forces holding molecules together in the liquid, so the added heat goes into separating molecules rather than raising temperature during the phase change. Substances with stronger intermolecular forces, such as hydrogen bonding, have higher heats of vaporization. It can be expressed as a molar quantity (kJ/mol) or per unit mass (J/g), and its value decreases as temperature rises, reaching zero at the critical point where liquid and vapor become indistinguishable.
Key facts
| Symbol | ΔHvap |
|---|---|
| Typical units | kJ/mol or J/g |
| Water (H2O), 100°C | 40.7 kJ/mol (2260 J/g) |
| Field | Physical Chemistry |
Water has a molar heat of vaporization of 40.7 kJ/mol (about 2260 J/g) at 100°C and 1 atm, an unusually high value among common liquids because of extensive hydrogen bonding between water molecules.
Frequently asked questions
How is heat of vaporization related to boiling point?
They are related but distinct: boiling point is the temperature at which a liquid's vapor pressure equals the surrounding pressure, while heat of vaporization is the energy needed to complete the liquid-to-gas transition at that temperature.
Why does sweating cool the skin?
Evaporating sweat draws its heat of vaporization from the skin, producing a cooling effect even though the sweat is well below its boiling point.