Intermolecular Forces

Prentice Hall © 2003Chapter 11 Intermolecular Forces

Prentice Hall © 2003Chapter 11 Ion-Dipole Forces Interaction between an ion and a dipole (e.g. water). Strongest of all intermolecular forces. Intermolecular Forces

Prentice Hall © 2003Chapter 11 Dipole-Dipole Forces Exist between neutral polar molecules. Polar molecules need to be close together. Weaker than ion-dipole forces. There is a mix of attractive and repulsive dipole-dipole forces as the molecules tumble. If two molecules have about the same mass and size, then dipole-dipole forces increase with increasing polarity. Intermolecular Forces

Prentice Hall © 2003Chapter 11 Dipole-Dipole Forces Intermolecular Forces

Prentice Hall © 2003Chapter 11 Dipole-Dipole Forces Intermolecular Forces

Prentice Hall © 2003Chapter 11 London Dispersion Forces Weakest of all intermolecular forces. It is possible for two adjacent neutral molecules to affect each other. The nucleus of one molecule (or atom) attracts the electrons of the adjacent molecule (or atom). For an instant, the electron clouds become distorted. In that instant a dipole is formed (called an instantaneous dipole). Intermolecular Forces

Prentice Hall © 2003Chapter 11 London Dispersion Forces Polarizability is the ease with which an electron cloud can be deformed. The larger the molecule (the greater the number of electrons) the more polarizable. London dispersion forces increase as molecular weight increases. London dispersion forces exist between all molecules. London dispersion forces depend on the shape of the molecule. Intermolecular Forces

Prentice Hall © 2003Chapter 11 London Dispersion Forces The greater the surface area available for contact, the greater the dispersion forces. London dispersion forces between spherical molecules are lower than between sausage-like molecules. Intermolecular Forces

Prentice Hall © 2003Chapter 11 London Dispersion Forces Intermolecular Forces

Prentice Hall © 2003Chapter 11 London Dispersion Forces Intermolecular Forces

Prentice Hall © 2003Chapter 11 Hydrogen Bonding Special case of dipole-dipole forces. By experiments: boiling points of compounds with H-F, H-O, and H-N bonds are abnormally high. Intermolecular forces are abnormally strong. Intermolecular Forces

Vapor Pressure and IF The stronger the intermolecular forces of the compound, the higher the boiling point of the compound because it will take more energy to break the bonds. It therefore follows that the stronger the intermolecular forces, the less volatile the compound will be, which means that its vapor pressure will be low.

Vapor Pressure Compounds with high vapor pressure want to turn to gas or vapor. Compound with low VP don’t want to convert to vapor easily unless more energy is added. Therefore compounds with LOW VP have High Boiling Points because their intermolecular forces are stronger) and compounds with HIGH VP have LOW Boiling points (because their intermolecular forces are weaker).

Summary Strong Intermolecular forces  High Boiling Points  compounds have LOW VAPOR PRESSURE and are LESS VOLATILE (don’t evaporate easily) Weak Intermolecular forces  LOW Boiling Points  compounds have HIGH VAPOR PRESSURE and are MORE VOLATILE (evaporate easily)