Ethanol and Molar Heat of Vaporization

Question

The molar heat of vaporization of ethanol (38.56 kJ/mol) is larger than methanol's (35.2 kJ/mol). Provide an explanation for this difference.

Answer 1

Looking at the structure of the two molecules, we see they are identical with the exception that ethanol has one additional carbon atom (with associated hydrogens).

CH3OH = methanol

CH3CH2OH = ethanol

So, they will both exhibit hydrogen bonding as the main intermolecular force holding molecules together. However, ethanol will have slightly greater dispersion forces because of the additional carbon atom, thus it will take more heat energy to vaporize ethanol than it will to vaporize methanol.