2.4 Intermolecular Forces

Intramolecular Forces: the attractive forces between atoms within a molecule the attractive forces between ions within an ionic crystal e.g. ionic bond, covalent bonds strong Intermolecular Forces: the attractive forces between molecules e.g. Van der Waals forces and hydrogen bonds weak (in comparison to intramolecular forces) i.e. much less energy to melt H2O (inter) than for it to decompose into H2 and O2 (intra)

Intermolecular Forces if covalent bonds were the only forces at work, most molecular compounds would be gases as there would be no attraction between molecules strong enough to group the molecules as liquids or solids

van der Waals Forces Dipole-dipole London dispersion

Dipole-dipole forces of attraction between oppositely charged ends of polar molecules. the strength of the dipole-dipole force is dependent on the polarity of the molecule

London Dispersion attractive forces between all molecules, including nonpolar molecules result of temporary displacements of the electron cloud around atoms in a molecule (extremely short-lived dipoles) therefore weaker than dipole-dipole strength depends on number of electrons in a molecule

Hydrogen Bonding strong dipole-dipole force between the positive H atom of one molecule and the lone pair of electrons on the negative atom (N, O, or F) on another molecule

Properties of Gases and Liquids very weak intermolecular forces acting on gases in liquids, intermolecular forces account for some special properties.

Surface Tension molecules in a liquid are attracted by molecules on all sides Penny drop, alka seltzer

Cohesion and Adhesion Cohesion: The attraction between like molecules Adhesion: The attraction between unlike molecules. Adhesion between water and glass is higher than cohesion between water molecules. Cohesion between mercury is higher than adhesion between mercury and glass