How Many Pennies?
• Fill a cup of water so that water is level with the top
of cup. Carefully add a penny to the cup of water.
Continue until the water spills over the edge.
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–
–
–
Predict the number of pennies it will take.
Actual number of pennies required.
Sketch a picture of this phenomena.
Provide an explanation of what you have
observed.
Ch. 10.2 Intermolecular Forces
• Intermolecular forces are attractive forces between
molecules.
– Become significant when particles are close
together.
– Influence properties such as boiling point.
– Intermolecular forces vary in strength but are
generally weaker than ionic and covalent bonds.
Dipole- Dipole Forces
• One type of intermolecular force exists between polar
molecules.
• Polar molecules are created when the polar bonds in
a molecule are asymmetrically arranged. This
creates partially positive and negative regions in the
molecule.
• Polar molecules are also called dipoles.
• The negative region in one polar molecule attracts
the positive region in adjacent molecules.
• Such forces of attraction between polar molecules
are known as dipole-dipole forces.
Dipole – Dipole Interactions
Occur when polar
molecules are
attracted to each
other.
Hydrogen Bonding
• Some hydrogen-containing compounds have unusually
high boiling points. This is explained by a particularly
strong type of intermolecular force.
• Hydrogen bonding – intermolecular force that exists
between a hydrogen atom covalently bonded to a
highly electronegative atom (N, O, or F) in one molecule
and highly electronegative atom (N, O, or F) in another
nearby molecule.
• Hydrogen bonds are usually represented by dotted
lines connecting the hydrogen-bonded
hydrogen to the electronegative atom to
which it is attracted.
Hydrogen Bonding In Water
London Dispersion Forces
• Even noble gas atoms and nonpolar
molecules can experience weak
intermolecular attraction.
• In any atom or molecule—polar or
nonpolar—the electrons are in continuous
motion.
• As a result, at any instant the electron
distribution may be uneven. A momentary
uneven charge can create a positive pole at
one end of an atom or molecule and a
negative pole at the other.
London Dispersion Forces
• This temporary dipole can then induce a dipole in an
adjacent atom or molecule. The two are held together
for an instant by the weak attraction between
temporary dipoles.
• The intermolecular attractions resulting from the
constant motion of electrons and the creation of
instantaneous dipoles are called London dispersion
forces.
• London forces tend to increase with molar mass.
• We can identify the types of intermolecular
forces present in a substance by considering its
composition and structure.
• London dispersion forces are found in ALL
substances. Strength of these attractions
varies directly with molar mass.
• Dipole-dipole forces and hydrogen bonding can
also add to the force of attraction. Hydrogen
bonding is the strongest of the intermolecular
forces.
What Do You Think?
• In each of the following select the substance with the
higher boiling point. Justify your answer.
(a) CH4 or CCl4
(b) NH3 or PH3