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Introduction There are ionic, giant covalent, and simple
molecular covalent bonds between atoms If there are no attractive forces between
molecules, then all substances would be gases There must be some force that attracts
molecules to other molecules that does not create a real bond
These are known collectively as intermolecular forces
Types of Intermolecular Forces In order of increasing strength: van der Waals' forces (AKA London forces or
dispersion forces) Dipole-dipole forces Hydrogen bonding
van der Waals' Forces
Exist between all species Is the only intermolecular force between
non-polar species Result of temporary (instantaneous) dipoles
when one side of the molecule becomes partially negative because of the random motion of electrons
Van der Waals Continued This results in a weak dipole moment that
then attracts other molecules Strength of this force increases with molar
mass as more electrons are available for temporary dipoles
Effect of van der Waals' Forces on Boiling Point
Higher boiling point means more energy is needed to break the intermolecular forces
As the molar mass increases, the boiling point increases
As the surface area increases, the boiling point increases (again, more electrons available)
More elongated the molecule, the stronger the van der Waals' forces, so the higher the boiling point
Continued Sometimes the van der Waals' force can be
quite strong as in the case of some polymers that have high mass and are very long molecules
Dipole-dipole forces Dipole moment: a measure of the polarity of a
molecule Arrows are used to represent the polarity of
the bond (heading toward the partially negative part)
The permanent dipoles formed cause electrostatic attraction between molecules that have the permanent dipoles
Stronger than van der Waals' forces in molecules of similar size
Effect of Dipole-dipole Forces on Boiling Point
Compared to molecules of similar mass with only van der Waals' forces, much higher boiling point
Polar molecules have van der Waals' forces and dipole-dipole forces
Stronger intermolecular force: higher boiling point
Hydrogen Bonding Not a “true” bond Occurs when hydrogen is bonded to highly
electronegative, small atoms, like N, O, or F Creates a very high dipole moment as the more
electronegative atom attracts the electrons, leaving the hydrogen very partially positive
Can be thought of as part way between a dipole-dipole force and a dative covalent bond
Continued For maximum strength, the 2 atoms and the
hydrogen should be in a straight line When X and Y are N, O, or F: Xδ-: hydrogen bond Hδ+ - Yδ-
Usually much stronger than other intermolecular forces
Effect of H-bonding on Boiling Point
Consider HF and HCl: HF has van der Waals' forces, dipole-dipole
forces, and H-bonding HCl has van der Waals' forces and dipole-
dipole forces Boiling point of HF is higher than HCl More energy is needed to break the
intermolecular forces
Consider H2O and H2S
H-bonding present in H2O, but not in H2S Boiling point of water is higher than H2S H-bonding allows water to form tetrahedral
shapes when it is a liquid H-bonding allows water to form hexagons when
it freezes, so ice is less dense than liquid water H-bonding in water also forms temporary
hexagon arrays on the surface of water, giving it a high surface tension
Consider NH3 and PH3
NH3 can hydrogen bond with itself and with the water when the ammonia is aqueous
PH3 can only hydrogen bond with water, not with itself
Ammonia has the higher boiling point
More to Consider
Considering CH3OCH3 and CH3CH2OH CH3OCH3 has the hydrogens bonded to the
carbons, not the oxygen, so no hydrogen bonding occurs
CH3CH2OH has the hydrogen bonded to the oxygen, so H-bonding occurs
CH3CH2OH has a higher boiling point than CH3OCH3
Another Consideration CH3CH2CH3, CH3CHO, and CH3CH2OH All have about the same mass CH3CH2CH3 cannot hydrogen bond with itself or with
water CH3CHO cannot hydrogen bond with itself, but can
with water CH3CH2OH can hydrogen bond with itself and water b. p. trend: CH3CH2CH3 < CH3CHO < CH3CH2OH
Biological Importance The weak bond between the nitrogenous
bases in nucleic acids is a H-bond Occurs between thymine and adenine as well
as between cytosine and guanine Easily broken by enzymes