Among the weak intermolecular forces of attraction for simple molecular structures, for the same electron cloud size, hydrogen bonding between molecules is considered to be stronger, followed by permanent dipole – dipole, and lastly Van Der Waals’.
Hydrogen bond affected by:
- Extent of H bonding (priority)
- H-X bond polarity
Extent of H bonding is the most important factor, as long as the molecule has greater extent of H bonding, it will have a stronger H bond, regardless of bond polarity.
- Determination of extent of H bond
Hydrogen bond (H bond) is the electrostatic attraction between a hydrogen (H) atom bonded to N,O or F atom and a lone pair (lp) of electrons belonging to N,O or F atom from another molecule.
1 hydrogen (H) + 1 lone pair (lp) –> 1 H bond
H2O: 2 H + 2 lp –> 2 H bond
NH3: 3 H + 1 lp –> 1 H bond
Note: the lp is considered to be the ‘limiting reagent’ in this case, so even though there are 3 H, they are in ‘excess’, so only 1 H bond can be formed.
HF: 1 H + 3 lp –> 1 H bond
Note: the H is considered to be the ‘limiting reagent’ in this case, so even though there are 3 lp belonging to F, they are in ‘excess’, so only 1 H bond can be formed.
H2O would thus have the strongest hydrogen bond as it has the largest extent of hydrogen bonding with 2 hydrogen bonds on average.
NH3 and HF has the same extent of hydrogen bonding but HF will have a stronger hydrogen bond because F is more electronegative than N, which results in a larger bond polarity.