Transcript Intermolecular Forces

Electronegativity and
Polarity
Nonpolar Covalent Bond
• When electrons are shared between 2
atoms, a covalent bond is formed.
• If the atoms are identical, e.g. Cl2, the
electrons are shared equally (nonpolar)
Polar Covalent Bond
• If the electrons are shared between 2 different
atoms, e.g. HBr, the sharing is unequal
• The bonding electrons spend more time near the
more electronegative atom
H
Br
Electronegativity Differences
• The absolute value of the difference in
electronegativities of two bonded atoms
provides a measure of polarity of a bond.
• The greater the difference, the more polar the
bond.
0 to 0.4
0.41 to 1.69
> 1.7
Nonpolar covalent
Polar covalent
Ionic
Electronegativity Difference
Polar Molecules
• Note: Not all molecules with polar bonds
are polar molecules
Practice
• P. 253 # 1-3
• P. 255 # 6-11
Intermolecular Forces
Intramolecular Forces:
– The attractive forces between atoms and ions within
a molecule
– e.g ionic, covalent, metallic
– Strong
Intermolecular Forces:
– The attractive forces between molecules
– E.g. Van der Waals forces (London dispersion
forces, dipole-dipole forces), 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)
• Strength depends on
number of electrons in a
molecule
• Therefore weaker than
dipole-dipole
London Dispersion Forces
• Strength depends on
– number of electrons, more electrons more
chances for instantaneous dipoles
– molecular shape
bp = 36˚C
28˚C
Tetris Analogy
9.5˚C
Intermolecular Forces and
Boiling Point
• The stronger the intermolecular
forces the higher the boiling point.
• The more polar the molecule, the
higher the boiling point
• More electrons = more LDF = higher
boiling point
Boiling Points
• Unexpectedly high boiling points of
hydrogen compounds containing
nitrogen, oxygen, and fluorine
• Additional forces must exist
Hydrogen Bonding
• Strong dipole-dipole
force between the
positive H atom and the
lone pair of electrons
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
• But, molecules at the
surface are only
attracted down or
sideways.
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
Practice
• P. 260 # 1-4
• P. 264 # 9, 11, 12
• P. 266 #1-7