The Basics[]
Permanent Dipolar Forces are Intermolecular forces between polar bonds.
They are stronger than Van der Waals forces but weaker than Hydrogen Bonds.
You can recognise a polar bond by the difference in electronegativity between the atoms in a covalent bond.
If a molecule has polar bonds but it symmetrical then it has no δ+ or δ- end and so is a non-polar molecule such as CCl4. There can be no permanent dipole forces.
Exam Hint[]
- You should use the words "Electrostatic force of attraction" in your description of these forces.
- You don't need to learn a table of electronegativities but you should know the trends. (See electronegativity).
- This allows you to compare the relative strength of polarity and hence the relative strength of permanent dipolar forces.
- You should know that if that if two molecules have the same very polar bond then the one with bigger Van der Waals forces will have the higher boiling point.
eg. Chlorohexane and Chloromethane both have polar C-Cl bonds. But the hexyl group adds more Van der Waals than the Methyl group and sos Chlorohexane has the higher boiling point.
