Chemistry Notes
Intermolecular Forces
Intermolecular Forces
What is an Intermolecular Force?
• Force between molecules (weak force)
• Differs from an intramolecular force
(strong force)
Relative Magnitude of Forces
Intermolecular
Forces
Covalent
Bonds
400 kcal
Intermolecular Forces
H-Bonds
Dipoledipole
London
Dispersion
12-16 kcal 2-0.5 kcal Less than
1 kcal
Notice: covalent bonds are almost 40
times the strength
Intermolecular Forces
What creates an Intermolecular force?
• Unequal distribution of electrons
– Created as a result of differences in:
• Electronegativity
Bonded hydrogen atoms showing
equal charge distribution
Hydrogen and bromide bond with an
unequal charge distribution.
What creates an Intermolecular
force?
• The unequal
distribution of
electrons
causes HBr to
be attracted to
another HBr
Br
Br
Intermolecular Forces
Main types of IMF we will discuss:
• London Dispersion Forces
• Dipole-Dipole
• Hydrogen Bonds
London Dispersion Forces
• Is the result of temporary charge imbalance
– Exists due to the random movement of
electrons
• When imbalance is present, nucleus of
another atom is attracted to partially
charged part of the atom or molecule
• Found in ALL molecules
– polar and nonpolar
• Weakest IMF
London Dispersion Forces
Dipole-Dipole
• Exists between polar molecules
• Polar molecules have partial positive
charges at the end of the molecules
• The stronger the polarity the stronger the
dipole force
Dipole-Dipole
Dipole-Dipole
Dipole-Dipole forces exist between neutral
polar molecules.
Hydrogen Bonds
• Exist between a hydrogen that is bonded to
a highly electronegative atom and unshared
pair of electrons on another highly
electronegative atom
• H-bonds are possible only with N, O, and F.
– Seen in Water,
– Ammonia, and DNA
• Strongest IMF
Hydrogen Bonds
The molecules
which have this
extra bonding are
N,O, and F
NOTICE where
the H-bond is
occurring. See
previous slide.
Hydrogen Bonds
Hydrogen
Oxygen
Notice: H’s electrons are virtually pulled out of it’s 1s sublevel
Hydrogen Bonds
Water
Notice: the virtually empty 1s orbitals on H are attracted to
the unshared pair of electrons on the O from another H2O.
The evidence for hydrogen bonding
Many elements form compounds with hydrogen referred to as "hydrides". If you plot the boiling points
of the hydrides of the Group 4 elements, you find
that the boiling points increase as you go down the
group.
The increase in boiling point happens because the
molecules are getting larger with more electrons,
and so dispersion forces become greater.
The evidence for hydrogen bonding
If you repeat this exercise with the hydrides of elements in
Groups 5, 6 and 7, something odd happens.
• Although for the most part the trend is exactly the same as
in group 4 (for the same reason), the boiling point of the
hydride of the first element in each group is abnormally
high.
• In the cases of NH3, H2O and HF there must be some
additional IM forces of attraction, requiring significantly
more heat energy to break.
• These relatively powerful IM forces are described as
hydrogen bonds.
Hydrogen Bonds
Overview of IMF
Other IMF Types
Summary
Be able to…
• Explain what an IMF is and talk about their
relative strengths
• Explain what causes IMF
• Describe the different types of IMF
• Go in depth about H-bonds
• Draw diagrams that represent the IMF
types