Hydrogen bonding
Introduced| English | Meaning |
|---|---|
| hydrogen bonding/ˈhaɪdrədʒn ˈbɒndɪŋ/ | An attraction involving a hydrogen atom bonded to an electronegative atom and an electron-rich site on another atom. |
| electronegative/ɪˈlektrəʊŋɡətɪv/ | electronegative |
| lone pair/ləʊn peə/ | A pair of outer-shell electrons on one atom that is not shared in a covalent bond. |
| surface tension/ˈsɜːfɪs ˈtenʃn/ | The tendency of a liquid surface to resist stretching because of cohesive forces between its molecules. |
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.
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Hydrogen bonding is a particularly strong type of permanent dipole force.
Hydrogen bonding occurs when H is bonded to N, O or F.
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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:
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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.
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Every "special" property of water comes back to its many hydrogen bonds.
Water has a high boiling point because:
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The extra energy needed to break the many hydrogen bonds raises water's boiling point.
Ice floats on water because:
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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)