Dipole–dipole forces are an essential part of IB Chemistry Topic 4 (Bonding). These forces explain why polar molecules behave differently from nonpolar ones and why some substances have higher boiling points than expected. Although dipole–dipole forces are not as strong as hydrogen bonds, they are significantly stronger than London dispersion forces and play a major role in determining physical properties.
What Are Dipole–Dipole Forces?
Dipole–dipole forces are attractive forces between the positive end of one polar molecule and the negative end of another polar molecule.
They occur only in molecules that have:
- Permanent dipoles
- Uneven electron distribution
- Polar covalent bonds with asymmetrical shape
The permanent dipole means one side of the molecule is slightly negative (δ–), while the other is slightly positive (δ+). These opposite partial charges attract neighboring molecules.
How Dipole–Dipole Forces Are Formed
Dipole–dipole attractions result from:
- Differences in electronegativity
- Asymmetrical molecular geometry
- Permanent separation of charge
Example: HCl
- H carries δ+
- Cl carries δ–
Cl is more electronegative, pulling electron density toward itself.
When many HCl molecules come together, the δ+ hydrogen of one molecule is attracted to the δ– chlorine of another.
This attraction, though weaker than hydrogen bonding, significantly affects physical properties.
Requirements for Dipole–Dipole Forces
A molecule must be polar to have dipole–dipole forces.
This requires:
- Polar bonds (different electronegativities)
- Asymmetrical shape so dipoles do not cancel out
Examples of polar molecules:
- HCl
- CH₃Cl
- SO₂
- PCl₃
- H₂S
Examples of nonpolar despite polar bonds (because dipoles cancel):
- CO₂
- CCl₄
- BF₃
These nonpolar molecules only experience London dispersion forces.
Dipole–Dipole Forces vs London Dispersion Forces
London dispersion forces are temporary and weak.
Dipole–dipole forces are permanent and stronger.
Comparing them:
- Dipole–dipole forces increase boiling point more than LDFs for molecules of similar mass.
- Polar molecules interact more strongly due to persistent charge separation.
- Larger nonpolar molecules may still have strong LDFs, sometimes surpassing dipole–dipole strength.
A typical IB example:
- Propanone (polar) has higher boiling point than propane (nonpolar), even though they have similar molar masses.
Effects of Dipole–Dipole Forces on Physical Properties
1. Boiling and melting points
Polar substances have higher boiling points because more energy is needed to separate molecules.
Example:
- HCl boils at –85°C
- F₂ (similar mass) boils at –188°C
The difference is due to dipole–dipole attraction in HCl.
2. Solubility
Polar molecules dissolve well in polar solvents.
“Like dissolves like” applies:
- Polar → dissolves in water or alcohols
- Nonpolar → dissolves in hexane or oil
Dipole–dipole interactions attract solute molecules into polar solvents.
3. Volatility
Polar molecules are less volatile due to stronger IMF attractions.
4. Surface tension and viscosity
More polarity = stronger intermolecular attraction = higher viscosity and surface tension.
Dipole–Dipole Forces in Liquids and Solids
In liquids, dipole–dipole forces:
- Hold molecules together
- Increase density
- Decrease vapor pressure
In solids, these forces help lattice-like structures form in polar molecular solids.
Dipole–Dipole vs Hydrogen Bonding
Hydrogen bonding is a special, stronger form of dipole–dipole force.
Hydrogen bonding requires:
- H bonded to N, O, or F
- Lone pairs on neighboring molecules
Dipole–dipole forces do not need such specific atoms.
Strength comparison:
Hydrogen bonding > Dipole–dipole > London dispersion
Common IB Misunderstandings
“All molecules with polar bonds show dipole–dipole forces.”
Not true—molecular shape must be asymmetrical.
“Dipole–dipole forces are stronger than hydrogen bonds.”
False—hydrogen bonding is much stronger.
“Only charged molecules attract each other.”
Dipole–dipole attractions occur between neutral molecules.
“Larger mass always means stronger IMFs.”
Molecule polarity can outweigh molar mass.
FAQs
Are dipole–dipole forces permanent?
Yes, they involve permanent dipoles, unlike temporary LDFs.
Can a molecule have both dipole–dipole and dispersion forces?
Yes—polar molecules always have both types.
Why do polar molecules have higher boiling points?
Stronger attractions require more energy to separate molecules.
Conclusion
Dipole–dipole forces are attractions between polar molecules caused by permanent partial charges. Though weaker than hydrogen bonds, they significantly affect boiling points, solubility, and volatility. Mastering dipole–dipole forces helps IB Chemistry students explain molecular behavior and compare physical properties across different substances.
