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Intermolecular forces - those
forces of attraction and repulsion
that exist between adjoining
molecules
Technically, interparticle forces is
the better term because that allow
us to consider ion-dipole forces;
however, most of the time we hear
intermolecular forces (IMF).
• Dipole-dipole interactions (or
attractions) are the forces between
neighboring polar molecules. The
negative dipole of one molecule is
attracted to the positive dipole of
another causing the molecules to stay
together.
• The strongest type of dipole-dipole
interactions is called hydrogen
“bond”ing. This occurs in molecules in
which hydrogen is bonded to F, O, or N.
• The most familiar molecules with hydrogen
bonding are H2O, HF, NH3, and alcohols like
CH3CH2OH.
• The force between ions and dipoles (full
charges and partial charges such as NaCl in
water) is the strongest type of interparticle
force.
• The weakest type of force which exists
between ALL particles is called London
dispersion forces (LDF). What causes these?
Since electrons are always in motion, at any
given time, the electrons in one molecule may
repel the electrons in a neighboring molecule.
• So what are London dispersion forces?
They are weak, temporary,
unpredictable forces of attraction and
repulsion that exist between all
molecules. They are the ONLY forces
that exist between nonpolar species.
• The larger a molecule is, the more
electrons it has. Accordingly, with more
electrons, the molecule is more
polarizable and will have greater LDF.
• Vapor pressure is the pressure of gas above
a liquid.
• Between which types of molecules - those
with LDF, those with dipole-dipole
interactions, or those with hydrogen bonding will the vapor pressure be highest? Lowest?
Why?
• Vapor pressure is lowest over the substance
with the greatest IMF because fewer particles
are able to escape to the gaseous phase.
Vapor pressure is highest over the substance
with the weakest IMF because lots of
molecules can escape to vapor.
• Clausius-Clapeyron equation
• ln (VPT1) = ∆Hvap
•
VPT2
R
( 1 T2
1 )
T1
• Since heat of vaporization is an energy
term, we use the energy R value which
is 8.31 J/K-mol.