Noncovalent Interactions and
Forces Between Molecules
Intermolecular Forces (IMFs)
Molecules are attracted to one another. If not
only gases would exist.
Attraction between molecules is always the
result of opposite charges whether temporary or
permanent.
Noncovalent interactions between molecules are
referred to as intermolecular forces.
IMFs do not result from sharing electron pairs
between atoms therefore they are weaker.
IMFs account for MPs, BPs and other properties.
Noncovalent
Noncovalentinteractions
interactions
London
London forces
forces
Attractions between
all types of
molecules, even
nonpolar ones
Dipole-dipole
Dipole-dipole
forces
forces
Attractions between
dipoles of polar
molecules
Hydrogen
Hydrogen
bonding
bonding
Attraction of
covalently bonded H
to lone pairs on N, O,
or F atoms in other
molecules or in the
same molecule (if the
molecule is large
enough)
London forces
Dispersion forces occur in all molecular
substances
Result from the attraction of positive and
negative ends of induced (nonpermanent) dipoles
in adjacent molecules
Induced dipole is caused in one molecule when
the electrons of a neighboring molecule are
momentarily unequally distributed (temporary
dipole)
http://chemwiki.ucdavis.edu/@api/deki/files/13929/=forces.jpg
London forces increase with increased number of
electrons in a molecule
Boiling points increase as the total number of
electrons increases
Boiling point of a liquid depends on the nature and
strength of IMFs
Dipole-dipole Forces
Polar molecules have permanent dipoles that
create a dipole-dipole attraction.
Dipoles attract when partial positive region one
molecule is close to the partial negative region of
another
In general, the more polar the molecule the
higher the boiling point
http://www.mikeblaber.org/oldwine/chm1045/notes/Forces/Intermol/dip_dip.gif
YOUR TURN
Draw a sketch of four CO molecules to indicate
dipole-dipole forces between the CO molecules.
(use dipole symbol )
Which forces must be overcome to
a) Evaporate gasoline?
b) Melt solid butane?
c) Boil ICl (iodine monocloride)
Hydrogen Bonding
Special dipole-dipole force
Attraction between a partially positive hydrogen
atom and a lone electron pair on a small, very
electronegative atom (N, O, F)
Hydrogen bonding is strong because hydrogen is
small and the partial positive charge is
concentrated in a very small volume so the
hydrogen is pulled very close to the electronegative
atom
http://1.bp.blogspot.com/-JPpJrFoCFn8/T4pa6ulZ7SI/AAAAAAAAAM8/X_JQbay7IfY/s1600/hydrogen.gif