2.2.7Additional lessons
Molecular Polarity — IB Chemistry HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 31 of 172
Part 1 of 5Net Dipole
Molecular polarity depends on both the polarity of the bonds and the shape.
Bond dipole
- Each polar bond has a dipole with a size and a direction.
Vector sum
- The molecular dipole is the sum of all the bond dipoles taking direction into account.
Polar molecule
- If the dipoles do not cancel, the molecule has a net dipole and is polar.
Non-polar molecule
- If the dipoles cancel completely, the molecule has no net dipole even with polar bonds.
Part 2 of 5Symmetrical Molecules
Symmetrical Molecules
Carbon dioxide
- The two C=O dipoles point in opposite directions in a linear molecule and cancel.
Boron trifluoride
- Three B–F dipoles at 120° cancel, so BF₃ is non-polar.
Tetrachloromethane
- Four C–Cl dipoles in a tetrahedron cancel, so CCl₄ is non-polar.
Diatomic elements
- Molecules such as O₂ have identical atoms and no dipole at all.
Part 3 of 5Unsymmetrical Molecules
Unsymmetrical Molecules
Water
- The bent shape means the two O–H dipoles add to a net dipole, so water is polar.
Ammonia
- The pyramidal shape leaves a net dipole pointing towards the nitrogen.
Chloromethane
- CH₃Cl has one C–Cl bond that is much more polar than the C–H bonds, giving a net dipole.
Isomers
- cis-1,2-dichloroethene is polar but the trans isomer is non-polar because its dipoles cancel.
| Molecule | Shape | Polar? |
|---|---|---|
| CO₂ | Linear | No |
| H₂O | Bent | Yes |
| CCl₄ | Tetrahedral | No |
| NH₃ | Trigonal pyramidal | Yes |
Part 4 of 5Consequences
1
Polar molecules dissolve well in polar solvents such as water, following the rule that like dissolves like.
2
Non-polar molecules dissolve in non-polar solvents such as hexane.
3
Polar molecules attract each other through dipole–dipole forces and usually have higher boiling points than non-polar molecules of similar size.
4
A thin stream of a polar liquid is deflected by a charged rod, but a non-polar liquid is not.
Part 5 of 5Exam Checklist & Tips
2.2.7 – Key Points for the Exam
- Molecular dipole The overall separation of charge in a molecule.
- Polar molecule A molecule with a net dipole.
- Symmetry A balanced arrangement of identical bonds that cancels dipoles.
- Like dissolves like Substances with similar intermolecular forces tend to mix.
Common Mistakes
- Assuming that any molecule with polar bonds is polar.
- Forgetting to consider lone pairs when finding the shape.
- Saying CO₂ is non-polar because C=O bonds are non-polar.
- Claiming that polar molecules are always soluble in water.
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