Molar enthalpy of vaporization is the amount of energy needed to convert 1 mole of a liquid substance to the gaseous phase at constant temperature and pressure conditions. It is a function of the strength of intermolecular forces, i.e., how strongly are molecules bound together. The higher the value of intermolecular forces, the more force will be required to break them apart. This has a direct bearing on enthalpy of vaporization.
For the given chemicals, the order of enthalpy of vaporization is as well follows:
CH3OH > HCl > C2H6
The values for these chemicals are 38.74 kJ/mol, 16.2 kJ/mol, and 14.7 kJ/mol, respectively.
CH3OH (methanol) has the highest enthalpy of vaporization since the intermolecular forces for this chemical are a combination of dispersion force and hydrogen bonding. In case of HCl, the intermolecular forces include the ordinary dipole-dipole attraction and dispersion forces. Finally, for C2H6 (ethane), only dispersion forces are present. These differences in the intermolecular forces result in different enthalpy of vaporizations.
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