<P> It is equal to the increased internal energy of the vapor phase compared with the liquid phase , plus the work done against ambient pressure . The increase in the internal energy can be viewed as the energy required to overcome the intermolecular interactions in the liquid ( or solid , in the case of sublimation ) . 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 . On the other hand , the molecules in liquid water are held together by relatively strong hydrogen bonds , and its enthalpy of vaporization , 40.65 kJ / mol , is more than five times the energy required to heat the same quantity of water from 0 ° C to 100 ° C ( c = 75.3 J K mol ) . Care must be taken , however , when using enthalpies of vaporization to measure the strength of intermolecular forces , as these forces may persist to an extent in the gas phase ( as is the case with hydrogen fluoride ) , and so the calculated value of the bond strength will be too low . This is particularly true of metals , which often form covalently bonded molecules in the gas phase : in these cases , the enthalpy of atomization must be used to obtain a true value of the bond energy . </P>
<P> It is equal to the increased internal energy of the vapor phase compared with the liquid phase , plus the work done against ambient pressure . The increase in the internal energy can be viewed as the energy required to overcome the intermolecular interactions in the liquid ( or solid , in the case of sublimation ) . 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 . On the other hand , the molecules in liquid water are held together by relatively strong hydrogen bonds , and its enthalpy of vaporization , 40.65 kJ / mol , is more than five times the energy required to heat the same quantity of water from 0 ° C to 100 ° C ( c = 75.3 J K mol ) . Care must be taken , however , when using enthalpies of vaporization to measure the strength of intermolecular forces , as these forces may persist to an extent in the gas phase ( as is the case with hydrogen fluoride ) , and so the calculated value of the bond strength will be too low . This is particularly true of metals , which often form covalently bonded molecules in the gas phase : in these cases , the enthalpy of atomization must be used to obtain a true value of the bond energy . </P>
latent heat of vaporization for water in j / kg
latent heat of vaporization for water in j / kg
40.65 kJ / mol
40.65 kJ / mol