Hydrogen bonding is one of the most important intermolecular forces in IB Chemistry Topic 4 (Bonding). It explains why water has such unusual properties, why alcohols dissolve in water, and why DNA strands stay together. Hydrogen bonding is stronger than other intermolecular forces and influences physical properties such as boiling point, melting point, viscosity, and solubility. Understanding hydrogen bonding is essential for interpreting trends and solving comparison questions in IB exams.
What Is Hydrogen Bonding?
Hydrogen bonding is a strong intermolecular force that occurs when hydrogen is bonded directly to nitrogen (N), oxygen (O), or fluorine (F) and interacts with a lone pair on a nearby N, O, or F atom.
These three atoms—N, O, and F—are:
- very electronegative
- small in size
- capable of creating highly polar bonds
Because hydrogen has only one electron, when it bonds to N, O, or F, the electron density is pulled away, making the hydrogen extremely δ+. This strong polarity creates an unusually powerful attraction to lone pairs on neighboring molecules.
Requirements for Hydrogen Bonding
A molecule can hydrogen bond only if BOTH conditions are met:
- Hydrogen must be bonded to N, O, or F.
Examples: O–H, N–H, F–H - The molecule must have (or be near) a lone pair on N, O, or F.
These lone pairs form the hydrogen bond.
Common hydrogen-bonding molecules:
- Water (H₂O)
- Ammonia (NH₃)
- Alcohols (ROH)
- Carboxylic acids (RCOOH)
- DNA bases
Hydrogen bonding is intermolecular—not the same as covalent bonding.
Why Hydrogen Bonds Are Strong
Hydrogen bonding is stronger than dipole–dipole forces because:
- N, O, and F create very large electronegativity differences.
- Hydrogen’s small size allows close approach between molecules.
- The partial charges are unusually strong.
Although hydrogen bonds are still weaker than covalent bonds, they are the strongest type of intermolecular force.
Effects of Hydrogen Bonding on Physical Properties
Hydrogen bonding dramatically affects how substances behave.
1. Higher boiling points
Substances with hydrogen bonding require more energy to separate molecules.
Examples:
- Water (H₂O) boils at 100°C
- Hydrogen sulfide (H₂S), which cannot hydrogen bond, boils at –60°C
The huge difference is due to hydrogen bonding.
2. Higher melting points
Hydrogen bonds create structured molecular networks, raising melting points.
Ice has a lattice structure held together by hydrogen bonds, requiring significant energy to break.
3. Higher viscosity and surface tension
Liquids with hydrogen bonds flow more slowly and have stronger surface effects.
Example: glycerol is very viscous due to extensive hydrogen bonding.
4. Solubility in water
Polar molecules with hydrogen bonding dissolve easily in water because they can form hydrogen bonds with water molecules.
This explains why:
- Alcohols are water-soluble
- Carboxylic acids dissolve well
- Many biological molecules function in aqueous environments
Hydrogen Bonding in Biological Systems
Hydrogen bonding is essential in biology, providing structure and functionality.
1. DNA base pairing
A–T and G–C pairs are held together by hydrogen bonds, stabilizing the double helix.
2. Protein structure
Hydrogen bonds form:
- α-helices
- β-pleated sheets
These secondary structures depend heavily on hydrogen-bond interactions.
3. Water as a biological solvent
Hydrogen bonding allows water to:
- Dissolve polar molecules
- Stabilize biochemical reactions
- Maintain temperature stability
Life depends on hydrogen bonding.
Hydrogen Bonding vs Other Intermolecular Forces
Strength comparison:
Hydrogen bonding > Dipole–dipole > London dispersion
Hydrogen bonding is the strongest IMF because of:
- Extreme polarity of N–H, O–H, and F–H bonds
- Strong electrostatic attraction
Hydrogen bonding gives molecules properties far beyond what their size suggests.
Common IB Misunderstandings
“Any molecule with hydrogen can hydrogen bond.”
False. Hydrogen must be bonded to N, O, or F.
“Hydrogen bonds occur within molecules.”
Usually false—hydrogen bonds are between molecules.
(Some molecules like proteins show intramolecular H-bonding, but this is not the default.)
“Hydrogen bonding is a covalent bond.”
It is not. It is a strong intermolecular attraction.
“Oxygen always forms hydrogen bonds.”
Only when it is bonded to hydrogen (O–H).
FAQs
Why does water have such a high boiling point?
Because each water molecule can form multiple hydrogen bonds, creating a strong network.
Can hydrogen bonding occur in alcohols?
Yes. The O–H group in alcohols forms hydrogen bonds with water and with other alcohol molecules.
Do all polar molecules hydrogen bond?
No. Only molecules with N–H, O–H, or F–H bonds can hydrogen bond.
Conclusion
Hydrogen bonding is a strong intermolecular force involving highly polar bonds between hydrogen and N, O, or F. It dramatically affects boiling points, solubility, viscosity, and biological structure. Understanding hydrogen bonding is essential for explaining physical trends and connecting chemistry to real-world systems.
