Transcript Intermolecular Forces - St. Jean de Brebeuf Home Page

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
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