2.2.1Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols
Intermolecular Forces – Introduction — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 4 parts Lesson 17 of 53 Unit quiz
Part 1 of 4Forces Inside and Between Molecules
Covalent bonds hold atoms together inside a molecule, but separate molecules also attract one another through much weaker forces.
Forces Inside and Between Molecules
Intramolecular
- Covalent bonds within a molecule are strong, and they stay intact when a simple covalent substance melts or boils.
Intermolecular
- Attractions between neighbouring molecules are weaker,
- and these are the forces overcome when a simple molecular substance changes state.
Three types
- Induced dipole-dipole forces, permanent dipole-dipole forces and hydrogen bonds make up the intermolecular forces you must know.
Relative size
- A hydrogen bond is roughly a tenth of the strength of a covalent bond, so boiling does not break molecules apart.
Part 2 of 4Induced Dipole-Dipole (London) Forces
These forces act between all atoms and molecules, polar or non-polar.
1
The electron cloud in a molecule is constantly moving, so at any instant it can be slightly more concentrated on one side.
2
This uneven distribution creates a temporary dipole, with one end briefly δ⁻ and the other end briefly δ⁺.
3
The temporary dipole distorts the electron cloud of a neighbouring molecule, inducing an opposite dipole in it.
4
The δ⁺ end of one molecule and the δ⁻ end of its neighbour then attract each other weakly.
5
Because the electrons keep moving, the dipoles appear and vanish continually, but the net effect is a constant weak attraction.
Part 3 of 4Permanent Dipoles and Hydrogen Bonds
Polar molecule
- A molecule with polar bonds that do not cancel by symmetry has a permanent δ⁺ end and a permanent δ⁻ end.
Permanent dipole-dipole
- The δ⁺ end of one polar molecule is attracted to the δ⁻ end of the next,
- giving stronger attraction than London forces in small molecules.
Hydrogen bond
- A special, stronger permanent dipole attraction between H bonded to N, O or F and a lone pair on N, O or F of another molecule.
Drawing it
- Show a hydrogen bond as a straight dashed line running from the lone pair to the δ⁺ hydrogen, with the angle about the H atom near 180°.
Strength order
- For molecules of comparable size, hydrogen bonds are strongest, then permanent dipole-dipole forces, then London forces.
| Force | Acts between | Relative strength |
|---|---|---|
| Induced dipole-dipole | All molecules and atoms | Weakest, grows with electron number |
| Permanent dipole-dipole | Polar molecules | Intermediate |
| Hydrogen bond | Molecules with N-H, O-H or H-F | Strongest intermolecular force |
Part 4 of 4Exam Checklist & Tips
2.2.1 – Key Points for the Exam
- Intermolecular force An attraction between separate molecules, much weaker than a covalent bond.
- Temporary dipole A short-lived charge separation caused by the movement of electrons in a molecule.
- Permanent dipole A lasting charge separation that exists because of differences in electronegativity within polar bonds.
- Hydrogen bond An attraction between a δ⁺ hydrogen on N, O or F and a lone pair on a nearby N, O or F atom.
- Polar molecule A molecule in which bond dipoles do not cancel, leaving one end δ⁺ and the other δ⁻.
Common Mistakes
- Saying covalent bonds break when a simple molecular substance boils;
- only the intermolecular forces are overcome.
- Drawing a hydrogen bond to the nucleus of hydrogen instead of from the lone pair to the δ⁺ hydrogen, or using a solid line.
- Claiming London forces exist only in non-polar molecules, when in fact every molecule has them.
- Describing hydrogen bonding as a full covalent bond rather than the strongest of the weak intermolecular attractions.
Last updated:
\n