Why alcohol is volatile although it has hydrogen bonds?

Question

We are always told that hydrogen bonds increase the boiling point of a liquid and as a result they decrease the liquid's volatility.

I want to know the reason why, alcohol with hydrogen bonds is still volatile ?

Answer 1

In general what you have been told is true. Liquids are 'held together' by intermolecular forces (such as due to dipole-dipole and induced dipole-induced dipole (dispersion forces) ) and additionally some have hydrogen bonds. All molecules have dispersion energy between them.

Thermal energy continuously disrupts these forces so that some molecules can escape the liquid thus all liquids have a vapour pressure. What is important is the total of these interactions relative to the average thermal energy RT of the liquid. ($RT=2.49$ kJ/mol at $300$ K). Using the Boltzmann distribution of energies the chance of a molecule reaching, by random collisions, an energy sufficient to exceed a intermolecular interaction of, say $10RT$, is only $\exp(-10) \approx 4.5\cdot10^{-5}$; very small indeed.

The average intermolecular energies are relatively weak, only a few times larger than RT i.e. a few times typical thermal energy for small molecules, such as methane, but can be several times this for larger molecules. By comparison chemical bond energies are hundreds of times RT so normally molecules do not decompose before they can boil. Hydrogen bonds have an interaction energy of $\approx 4 RT$ to $\approx 15 RT$ depending on individual types of molecules. Thus if the molecules in a liquid have additional interaction energy due to hydrogen bonding then the vapour pressure will be lower at a given temperature and the boiling point higher. This means that, by random motion, the chance of exceeding this energy becomes exponentially smaller as the intermolecular energy increases.

Comparing three molecules with almost the same molecular weight ($86, 88, 88$ respectively) shows the clear effect of hydrogen bonding.

n-hexane; b.p. $69$ C, ethyl propyl ether; $64$ C, $1-$pentanol; $137$ C

Finally hydrogen bonding in water is somewhat a special case as there are multiple hydrogen bonds although they are only moderately strong; a water dimer has a single H bond of $\approx 22.4$ kJ/mol $\approx 9 RT$.

Answer 2

I'm going to compare ethanol to water (I think its the obvious choice).

Volatility depends on inter-molecular forces, for example hydrogen-bonds.

The hydrogen bonds ethanol can develop are fewer than in water. It has a alkyl-residue which can only interact via van der Waals interactions.

You could say the hydrogen-bond-density is a lot lower than in water.