On the other hand, the molecules in liquid water are held together by relatively strong hydrogen bonds, and its enthalpy of vaporization, 40.8 kJ/mol, is more than five times the energy required to heat the same quantity of water from 0 вЊ to 100 вЊ (cp = 75.3 J Kв€’1 molв€’1). Hence helium has a particularly low enthalpy of vaporization, 0.0845 kJ/mol, as the van der Waals forces between helium atoms are particularly weak. The enthalpy of vaporization can be viewed as the energy required to overcome the intermolecular interactions in the liquid (or solid, in the case of sublimation). The enthalpy of condensation (or heat of condensation) is numerically exactly equal to the enthalpy of vaporization, but has the opposite sign: enthalpy changes of vaporization are always positive (heat is absorbed by the substance), whereas enthalpy changes of condensation are always negative (heat is released by the substance). Values are usually quoted in kJ/mol, although kJ/kg, kcal/mol, cal/g and Btu/lb are also possible, among others. It is measured at the normal boiling point of the substance, although tabulated values are usually corrected to 298 K: the correction is small, and is often smaller than the uncertainty in the measured value. The enthalpy of vaporization (О”H vap), also known as the heat of vaporization or heat of evaporation, is the energy required to transform a given quantity of a substance into a gas. This article is part of the ChemHelp Tutoring Wiki

Entropy of vaporization