Hydrogen bonding
| English | Chinese | Pinyin |
|---|---|---|
| hydrogen bonding | 氢键 | qīng jiàn |
| electronegative | 电负性 | diàn fù xìng |
| lone pair | 孤对电子 | gū duì diàn zi |
| surface tension | 表面张力 | biǎo miàn zhāng lì |
The strongest weak force
- Hydrogen bonding 氢键 is a strong, special case of permanent dipole forces.
- It needs hydrogen bonded to a very electronegative 电负性 atom.
- It explains the strange behaviour of water.
Hydrogen bonding is the strongest of the intermolecular forces covered here.
Hydrogen bonding is a particularly strong type of permanent dipole force.
Hydrogen bonding occurs when H is bonded to N, O or F.
It explains water's high boiling point.
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 孤对电子 on an N, O or F atom nearby.
- Look for N–H and O–H groups (as in ammonia and water).
Hydrogen bonding between water molecules: an H is attracted to a lone pair on a neighbour's O
Why hydrogen bonds make water special
Step through it. One weak-but-strong force — the hydrogen bond — explains water's high boiling point, why ice floats, and why it dissolves so much.
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 表面张力.
- Ice is less dense than water — hydrogen bonds hold an open structure, so ice floats.
Match each special property of water to the reason for it.
Every "special" property of water comes back to its many hydrogen bonds.
Water has a high boiling point because:
The extra energy needed to break the many hydrogen bonds raises water's boiling point.
Ice floats on water because:
In ice the hydrogen bonds space the molecules out, making it less dense than liquid water, so it floats.
The power of hydrogen bonding
Worked example. Water boils at 100 °C while H₂S (no hydrogen bonding) boils at −60 °C — even though H₂S is heavier. Hydrogen bonds make the difference.
You've got it
- 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)