Heat of Condensation
Definition and meaning of Heat of Condensation in chemistry.
Heat of condensation is the amount of energy released when a given quantity of a substance changes from vapor to liquid at constant temperature and pressure. It is numerically equal to the heat of vaporization for that substance but opposite in sign, since condensation is an exothermic process.
In more detail
As vapor molecules condense, they slow down and move close enough for intermolecular attractive forces (such as hydrogen bonding or van der Waals forces) to hold them together in the liquid state. This reorganization releases the same amount of energy that was originally absorbed to vaporize the liquid, given off to the surroundings as heat. Because this energy transfer occurs at a fixed temperature (the boiling point), it is a form of latent heat rather than sensible heat. Heat of condensation explains phenomena like the intense heat released when steam contacts skin and the warming effect of cloud formation in the atmosphere.
Key facts
| Field | Physical Chemistry |
|---|---|
| Relation | ΔH_condensation = −ΔH_vaporization |
| Water (100°C, 1 atm) | −40.7 kJ/mol (−2257 J/g) |
| Sign convention | Exothermic (negative ΔH) |
When 1 mole (18.0 g) of steam at 100°C and 1 atm condenses to liquid water at the same temperature, it releases about 40.7 kJ of heat, identical in magnitude to water's molar heat of vaporization.
Frequently asked questions
Why is heat of condensation negative while heat of vaporization is positive?
They describe the same phase change in opposite directions on the same substance. Vaporization absorbs energy to break intermolecular attractions (endothermic, +ΔH), while condensation releases that identical amount of energy as those attractions reform (exothermic, −ΔH).
Why do steam burns hurt more than burns from boiling water?
Steam must first release its heat of condensation (about 2257 J/g for water) upon contacting skin, on top of the heat given off as the resulting hot water then cools, delivering far more energy than liquid water at the same temperature.