14.0 (chem 20 intermolecular forces review)

12/14/25

what characteristics of a substance can be explained by its intermolecular forces?

• Boiling and melting points

• Solubility

• Viscosity

• Surface tension

• Evaporation rate

what are the three types of intermolecular forces

London Dispersion Forces, Diplo-Dipole, Hydrogen Bonding

LDF overview

occurs in: all molecules

relative strength: weakest

main cause: temporary electron shift creating dipoles

dipole-dipole overview

occurs in: polar molecules

relative strength: medium

main cause: permanent partial changes (having dipoles)

hydrogen bonding overview

occurs in: molecules with a Hydrogen bonded to a FON atom

relative strength: strongest

main cause: very strong dipole attraction

explain LDF

in any molecule, at any particular instant, as its electrons move around, they both may be closer to one end than the other. the result will be a buildup of negative charge on one end, and an a positive charge on the other end, establishing a temporary dipole.

because we know like repels like, the negative end of a molecule may cause another molecule to have a positive end, and then the toe ends will be attracted to one another.

factors influencing LDF strength

size and shape of the molecule

how does size impact LDF

larger molecules will have more electrons within them, which gives them the ability to create stronger temporary dipoles, which means stronger LDF, and higher boiling points

overall: big molecule stick together more strongly

how does shape impact LDF

while two molecules may have the same number of electrons, they may have different BP’s, depending on their shapes.

long, flat molecules will have a lot of surface area to encourage contact (and temp. dipoles) with other molecules.

on the other hand, spherical or compact (lots of branches) molecules wont have much surface area, so their bonds will occur less often and be weaker.

overall, long and flat = strong LDF = higher BP, small and spherical = weak LDF = lower BP