1.5.3Physical Chemistry
Hydrogen Bonding — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 19 of 42
Part 1 of 4What a Hydrogen Bond Is
Hydrogen bonding explains many unusual properties of water, ammonia and hydrogen fluoride.
Definition
- A hydrogen bond is a strong type of permanent dipole-dipole attraction between a δ⁺ hydrogen atom and a lone pair on a nitrogen, oxygen or fluorine atom in another molecule.
Conditions
- The hydrogen must be bonded directly to N, O or F, which are small and very electronegative, so the hydrogen has a strong δ⁺ charge.
Lone pair
- The attraction is to a lone pair on a neighbouring N, O or F atom, with the X–H···Y arrangement lying in a straight line.
Strength
- Hydrogen bonds are much weaker than covalent bonds but stronger than other intermolecular forces.
Diagrams
- A hydrogen bond diagram shows the δ⁺ hydrogen, the δ⁻ atom with its lone pair, and a dotted line drawn between them.
Part 2 of 4Effects on Boiling Point
Effects on Boiling Point
Group trend
- Normally, boiling points of hydrides rise down a group as instantaneous dipole-induced dipole forces strengthen.
Anomalies
- H₂O, NH₃ and HF have much higher boiling points than their group neighbours because of hydrogen bonding.
Hydrogen fluoride
- HF boils at about 20 °C, whereas HCl boils at about -85 °C.
Energy
- Extra energy is needed to break the hydrogen bonds between molecules during boiling.
Ammonia
- Ammonia molecules can form hydrogen bonds with each other, but fewer per molecule than water does, so ammonia boils at a lower temperature.
| Substance | Approx. boiling point / °C | Hydrogen bonding? |
|---|---|---|
| H₂O | 100 | Yes |
| H₂S | -60 | No |
| HF | 20 | Yes |
| HCl | -85 | No |
Part 3 of 4Water and Ice
1
Each water molecule has two δ⁺ hydrogen atoms and two lone pairs on oxygen, so it can form up to four hydrogen bonds.
2
In ice, molecules are held in an open hexagonal arrangement in which each is hydrogen bonded to four neighbours.
3
This open structure holds molecules further apart than in the liquid, so ice is less dense than water and floats.
4
Hydrogen bonding also gives water a high surface tension and high melting and boiling points for its size.
5
Substances with O–H or N–H groups, such as ethanol, can form hydrogen bonds with water, which is why they dissolve.
Part 4 of 4Exam Checklist & Tips
1.5.3 – Key Points for the Exam
- Hydrogen bond A strong permanent dipole-dipole attraction between H bonded to N, O or F and a lone pair on another N, O or F atom.
- Lone pair A pair of outer electrons not used in bonding, which attracts the δ⁺ hydrogen.
- Anomalous boiling point A boiling point higher than expected for the molecule's size, caused by hydrogen bonding.
- Intermolecular force An attraction between separate molecules, much weaker than a covalent bond.
Common Mistakes
- Saying hydrogen bonding is a true bond within a molecule; it is an attraction between molecules.
- Drawing the hydrogen bond to any hydrogen atom, when the receiving atom needs a lone pair on N, O or F.
- Claiming that hydrogen bonds are broken when water boils instead of covalent O–H bonds.
- Forgetting to draw the lone pair and the δ⁺ and δ⁻ labels on hydrogen bond diagrams.
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