AP Chemistry
Unit 5 – States of Matter
Lesson 8 – Intermolecular Forces
Book Section: 11.1-11.2, 11.8
Intermolecular Forces
• Intermolecular forces are interactions, or “mini-bonds”
between molecules, as opposed to within a molecule.
• Intermolecular forces are weaker than bonds.
Intermolecular Forces
• Intermolecular forces are strong enough to control physical
properties like boiling & melting points, vapor
pressures, and viscosities.
• Collectively, intermolecular forces are called van der Waals
forces.
van der Waals forces
• 4 types:
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–
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Ion-Ion (like in ionic bonds)
Ion-dipole
Dipole-dipole
Hydrogen bonding
London dispersion forces
Ion-Dipole Interactions
• This is a van der Waals force between an ion in
solution and a polar molecule.
– Example: Na+ and Cl- in water
• The strength of these forces are what makes it
possible for ionic substances to dissolve in
polar solvents (like water)
Dipole-Dipole Interactions
• Polar molecules are
attracted to each other.
– The positive end of one
molecule is attracted to the
negative end of another
molecule (and vice versa)
– Only important when
molecules are close
together (liquids & solids,
or high pressure gases)
Dipole-Dipole Interactions
• The more polar the molecule, the higher its boiling
point.
Hydrogen Bonding
• The dipole-dipole
interactions experienced
when H is bonded to N, O,
or F are unusually strong.
• We call these interactions
hydrogen bonds.
Hydrogen Bonding
• Hydrogen bonding arises in part from the high
electronegativity of nitrogen, oxygen, and fluorine.
London Dispersion Forces
• London dispersion forces result from
instantaneous dipoles from random motion
of electrons.
London Dispersion Forces
• London dispersion forces are present in all
molecules.
• The strength of the London forces depend on
the size of the molecule.
London Dispersion Forces
• The strength of the London forces tends to
increase with increased molecular weight.
• Larger atoms have larger electron clouds which
are easier to polarize.
London Dispersion Forces
• The strength of the London forces tends to
increase with increased molecular weight.
• Larger atoms have larger electron clouds which
are easier to polarize.
Ranking Intermolecular Forces
• Strongest
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Ion-Ion Forces (ionic bonds)
Ion-Dipole Forces
Hydrogen Bonding
Dipole-Dipole Forces
London Dispersion Forces
• Weakest
• These forces determine boiling and melting points
– more intermolecular forces, the harder it is to
physically separate the molecules.
Intermolecular Forces Problem
• The dipole moments of acetonitrile, CH3CN, and
methyl iodide, CH3I, are 3.9 D and 1.62 D,
respectively.
– Which of these substances has greater dipole-dipole
attractions among its molecules?
– Which of these substances has greater London
dispersion attractions?
– The boiling points of CH3CN and CH3I are 354.8 K and
315.6 K, respectively. Which substance has the greater
overall attractive forces?
Intermolecular Forces Problem
• The dipole moments of acetonitrile,
CH3CN, and methyl iodide, CH3I, are 3.9 D
and 1.62 D, respectively.
– Which of these substances has greater dipole-dipole
attractions among its molecules? CH3CN
– Which of these substances has greater London
dispersion attractions? CH3I
– The boiling points of CH3CN and CH3I are 354.8 K
and 315.6 K, respectively. Which substance has the
greater overall attractive forces? CH3CN
Intermolecular Forces Problem
• In which of the following substances is
hydrogen bonding likely to play an
important role in determining physical
properties: methane (CH4), hydrazine
(H2NNH2), methyl fluoride (CH3F), or
hydrogen sulfide (H2S)?
Intermolecular Forces Problem
• In which of the following substances is
hydrogen bonding likely to play an
important role in determining physical
properties: methane (CH4), hydrazine
(H2NNH2), methyl fluoride (CH3F), or
hydrogen sulfide (H2S)?
• Hydrazine (H2NNH2)
Intermolecular Forces Problem
• List the substances BaCl2, H2, CO, HF, and
Ne in order of increasing boiling points.
Intermolecular Forces Problem
• List the substances BaCl2, H2, CO, HF, and
Ne in order of increasing boiling points.
• Lowest:
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–
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H2 (London) – MM = 2 g/mol
Ne (London) – MM = 10 g/mol
CO (dipole-dipole)
HF (hydrogen bonding)
BaCl2 (ion-ion)
• Highest
Homework: 11.10-22 even, 26, 60, 62,
66
• This week:
– Tuesday: Molar Mass of a Condensable Vapor Lab
• Next week:
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Monday: Predicting Physical Properties (11.2-11.3)
Tuesday: Molar Mass of a Condensable Vapor Lab
Wednesday: Phase Changes (11.4)
Thursday: Phase Diagrams (11.6), Review Q & A for Test
Friday: Unit 6: Solutions – Solubility (13.1-13.3)
• Due Dates:
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Volume-Temperature Behavior of Gases: 11/30
States of Matter Exam: Monday, 12/6
Molar Mass of Condensable Vapor: 12/8
Problem Set 4: 12/10