Hydrogen bonding
Hydrogen bonding
- Hydrogen bonding is a strong, special case of permanent dipole forces.
- It needs hydrogen bonded to a very electronegative atom.
- It explains the strange behaviour of water.
Practice
Hydrogen bonding is the strongest of the intermolecular forces covered here.
Hydrogen bonding is a particularly strong type of permanent dipole force.
When it forms
- It forms when hydrogen is bonded to nitrogen, oxygen or fluorine (N, O, F).
- That $\delta+$ hydrogen is attracted to a lone pair on an N, O or F atom nearby.
- Look for N–H and O–H groups (as in ammonia and water).
Practice
Hydrogen bonding forms when hydrogen is bonded to:
H must be bonded to a very electronegative atom (N, O or F) and be attracted to a lone pair on N/O/F nearby.
Why water is special
- High melting and boiling points — many hydrogen bonds must be broken.
- High surface tension.
- Ice is less dense than water — hydrogen bonds hold an open structure, so ice floats.
Practice
Water has a high boiling point because:
The extra energy needed to break the many hydrogen bonds raises water's boiling point.
Practice
Ice floats on water because:
In ice the hydrogen bonds space the molecules out, making it less dense than liquid water, so it floats.
You've got it
Key idea
- hydrogen bonding needs H bonded to N, O or F, attracted to a lone pair on N/O/F
- look for N–H and O–H groups
- it gives water a high boiling point, high surface tension, and makes ice float (open structure)