Intermolecular Forces
Van der Waals
Van der Waals forces: Attractions between instantaneous and induced dipoles on
neighbouring molecules
VdWs exist between all simple molecules and atoms in the solid and liquid state. There are
no forces of attraction between molecules in a gas
The more electrons present in a molecule = more VdWs
In non-polar molecules, only VdWs exist e.g. Cl2, Br2
As alkane chain length increases BP increases as no. of electrons increases thus no. of VdWs
increases, also works with RMM
Permanent Dipole-Dipole Attractions.
-When a simple covalent molecule is POLAR, it is said to have a permanent dipole
Permanent Dipole: Attractions that occur between the positive end δ+, of the permanent
dipole on a molecule with the negative end δ-, of the permanent dipole on a neighbouring
molecule.
Hydrogen Bonding
Bond formed between a δ+ hydrogen atom and a δ- N,O or F of another molecule. Hydrogen
bond is formed due to the attraction between the lone pair of electrons on the δ- atom and
the δ+ hydrogen.
Explanation of Properties
Melting Point or BP: a large amount of energy is needed to break the ……. Between the
molecules
Viscosity: The greater the ……. Between the molecules, the greater the viscosity
Miscibility: Liquids often mix due their ability to form the same intermolecular forces
Solubility: Solids are soluble in a solvent because they have similar bonds between their
molecules.
Hydrides of groups IV, V, VI and VII
Boiling points of H20, HF and NH3 are higher than expected due to hydrogen bonds. CH4
doesn’t form hydrogen bonds as it is non-polar
Ice
-Hydrogen bonds in ice are more ordered/fixed and the water molecules in ice are further
apart, leading to a more open structure and lower density. Ice then can float on water.
Van der Waals
Van der Waals forces: Attractions between instantaneous and induced dipoles on
neighbouring molecules
VdWs exist between all simple molecules and atoms in the solid and liquid state. There are
no forces of attraction between molecules in a gas
The more electrons present in a molecule = more VdWs
In non-polar molecules, only VdWs exist e.g. Cl2, Br2
As alkane chain length increases BP increases as no. of electrons increases thus no. of VdWs
increases, also works with RMM
Permanent Dipole-Dipole Attractions.
-When a simple covalent molecule is POLAR, it is said to have a permanent dipole
Permanent Dipole: Attractions that occur between the positive end δ+, of the permanent
dipole on a molecule with the negative end δ-, of the permanent dipole on a neighbouring
molecule.
Hydrogen Bonding
Bond formed between a δ+ hydrogen atom and a δ- N,O or F of another molecule. Hydrogen
bond is formed due to the attraction between the lone pair of electrons on the δ- atom and
the δ+ hydrogen.
Explanation of Properties
Melting Point or BP: a large amount of energy is needed to break the ……. Between the
molecules
Viscosity: The greater the ……. Between the molecules, the greater the viscosity
Miscibility: Liquids often mix due their ability to form the same intermolecular forces
Solubility: Solids are soluble in a solvent because they have similar bonds between their
molecules.
Hydrides of groups IV, V, VI and VII
Boiling points of H20, HF and NH3 are higher than expected due to hydrogen bonds. CH4
doesn’t form hydrogen bonds as it is non-polar
Ice
-Hydrogen bonds in ice are more ordered/fixed and the water molecules in ice are further
apart, leading to a more open structure and lower density. Ice then can float on water.
