The enthalpy of vaporization is the energy, in the form of heat, that a substance needs to change from a liquid to a gas at a constant temperature and pressure. Enthalpy of vaporization, designated as "delta H vap," is measured in kilojoules per mole of substance. The amount of heat needed to change 2.2 pounds of water into steam is 2,257 kilojoules, or 2,2257 kJ/mol.
Tables denote various enthalpies of vaporization that scientists use to calculate how much heat is needed to change a liquid to a gas. Oxygen has an enthalpy of vaporization of 6.82, methane's is 8.18, chlorine's is 20.41, and carbon tetrachloride's is 30.00 kJ/mol. Heavier substances require greater heat, such as aluminum at 284 kJ/mol and lead at 178 kJ/mol. When the boiling points of substances are higher, the enthalpies of vaporization are also higher.
Enthalpy denotes energy added or taken away from atoms and molecules. When matter changes states, heat is added or taken away from the chemical. Enthalpies of vaporization denote heat added to mass to change from a liquid to a gas. Conversely, the enthalpy of condensation is the heat released by substances when they condense from a gas to a liquid.
When materials take in heat to change states of matter, the physical change is endothermic. Exothermic reactions occur when chemicals release heat into the surrounding environment.