Intermolecular Forces: They are weak interactions between dipoles of different molecules. There
are three types. Intermolecular forces are responsible for physical properties like melting points.
Covalent bonds determine its reactions.
• Bond Enthalpy: For lowest strength to highest strength- London forces -> permanent dipoledipole interactions -> hydrogen bonds -> covalent bonds.
• Atoms only have London Forces. All molecules have London Forces.
• If have to talk about many molecules simplify to H bond > permanent dipole > London forces etc.
• First of three intermolecular forces…
• London Forces: It is the attractive forces between induced dipoles in molecules. Aka induced
dipole- dipole interactions or Van der Walls’ forces. They exist between all molecules.
• How London Forces Arise:
– There is an uneven distribution of electrons.
– This creates an instantaneous dipole.
– This causes induced dipoles in neighbouring molecules, which then attract one another.
– Induced dipoles are only temporary as in the next instant of time the induced dipoles may
disappear.
• Strength of London Forces:
– As the number of electrons increases (e.g. down a group).
– The larger the instantaneous and induced dipole, the greater the induced dipole- dipole
interaction between molecules.
– Therefore more energy needed to overcome London forces.
– Increases boiling point e.g. down a group.
• So P4 has weaker London Forces than S8.
• Explain why solid sulphur has no other intermolecular forces. Only one type of atom. No
permanent dipoles as non- polar.
• The second of the three intermolecular forces…
• Permanent Dipole- Dipole Interactions: It is the attractive forces between the permanent dipoles.
Intermolecular force only acts between atoms of opposite delta charges- of polar molecules. PH3
is an example of permanent dipole- dipole interactions.
• Anything containing highly electronegative atoms such as Cl or P, most like polar.
• Boiling Point:
– A polar molecule hasstronger permanent dipole- dipole interactions as well as London Forces.
– So extra energy is needed to break additional permanent dipole- dipole interactions between
the polar molecules.
– Increases boiling point.
• Simple Molecular: Substance made up of a definite number of atoms are simple molecular
(covalent). Weak intermolecular forces between molecules. Covalent bonds between atoms
which do not break when changing state.
• Electrical Conductivity: No mobile charged particles and can’t conduct.
• Melting and Boiling Point:
– Other bonds are stronger than London Forces.
– Weak London forces between molecules.
– Less energy needed to overcome London Forces than other bond.
– So simple molecular substances have low melting points.
• Solubility: Like dissolves like.
• Non-Polar Simple Molecular + Non-Polar Solvent: When non- polar simple molecular added to a
non- polar solvent, intermolecular forces form between the molecules in lattice and molecules in
solvent. The interactions weaken the intermolecular forces in the simple molecular lattice so the
compound dissolves.
• Non-Polar Simple Molecular + Polar Solvent: When added to a polar solvent, little interaction
between molecules and solvent. The intermolecular bonding within the polar solvent too strong
to be broken.
• If insoluble, forms layers in test tube.
• Polar Simple Molecular Substances: Polar simple molecular may dissolve in polar solvents as the
polar solvent and polar simple molecular attract. Process is similar to dissolving of an ionic
compound.
• But the solubility depends on the strength of the dipole. Some compounds have both polar and
non- polar bonds and dissolve in both polar and non- polar solvents.
• Biological Molecules: Some biological molecules have hydrophilic part that is polarelectronegative atom. The hydrophobic part non-polar= carbon chain.