Intermolecular Forces The physical properties of melting point, boiling point, vapor pressure, evaporation, viscosity, surface tension, and solubility are related to the strength of attractive forces between molecules. These attractive forces are called Intermolecular Forces. The amount of "stick togetherness" is important in the interpretation of the various properties listed above.

Three main types of intermolecular forces: 1.Dispersion Forces (London Forces and van der Wall’s) 2.Dipole-dipole interactions 3.Hydrogen bonds Intermolecular means between molecules (any mentioned above) Intramolecular means within the molecule (covalent bonds) Intermolecular forces << intramolecular forces Relative strength of Intermolecular Forces: dispersion forces < dipole-dipole interactions < hydrogen bonds

Dispersion Forces Dispersion forces are the weakest intermolecular force (one hundredth-one thousandth the strength of a covalent bond), Dispersion forces are very weak forces of attraction between molecules resulting from momentary dipoles occurring due to uneven electron distributions in neighboring molecules as they approach one another The more electrons that are present in the molecule, the stronger the dispersion forces will be. Dispersion forces are the only type of intermolecular force operating between non-polar molecules.

Size (Volume and Shape) determines the magnitude of the dispersion force. The bigger the size, the larger the dispersion force.

Dipole-dipole Interactions Stronger intermolecular forces than Dispersion forces that occur between molecules that have permanent net dipoles (polar molecules) The partial positive charge on one molecule is attracted to the partial negative charge on a neighboring molecule.

Hydrogen Bonds They occur between molecules that have a permanent net dipole resulting from hydrogen being covalently bonded to either fluorine, oxygen or nitrogen. For example, hydrogen bonds operate between water (H 2 O) molecules, ammonia (NH 3 ) molecules, hydrogen fluoride (HF) molecules, etc. The attraction between these molecules will be greater than the attraction between polar molecules that do not have hydrogen covalently bonded to either fluorine, oxygen or nitrogen. Hydrogen bonds are the strongest intermolecular force (about one-tenth the strength of a covalent bond).

Since melting or boiling result from a progressive weakening of the attractive forces between the covalent molecules, the stronger the intermolecular force is, the more energy is required to melt the solid or boil the liquid. SubstanceMolecular Mass (g/mol) Dipole Moment (Debye) Normal Boiling Point (K) Propane Dimethyl ether Chloromethane Acetaldehyde Acetonitrile

Solubility Solubility is a substance’s ability to dissolve in another substance. Polar molecules and ionic substances are usually soluble in polar substances. Non-polar molecules only dissolve in non- polar substances.

When salt is dissolved in water, the ions of the salt dissociate from each other and associate with the dipole of the water molecules. This process is called “solvation”.

Additional References -forces forces unit-7-presentation unit-7-presentation homework.com/Chemistry_Help/Molecular_G eometry/Polar_Or_Nonpolar.html homework.com/Chemistry_Help/Molecular_G eometry/Polar_Or_Nonpolar.html