Chapter 11 BLB 11th

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Intermolecular Forces, Liquids, and Solids

• Chapter 11 BLB 11th

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Questions?

• Why does water have such anomalous properties?
• Why do you add salt to the water before cooking
  pasta?
• How can ice melt below 0C?
• How does antifreeze work?

Answer Solutions have different properties than
pure solvents.
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11.2 Intermolecular Forces

• The attractive forces between molecules or
  between ions and molecules
• Most properties of liquids and solids (and
  solutions) are due to the strength of the
  intermolecular forces present.
• Boiling point
• Melting (freezing) point
• ?Hvap

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Types of Intermolecular Forces

• In order from strongest to weakest
• Ion-dipole attraction of an ion and the partial
  charges of a polar molecule (hydration of ionic
  salts)
• Dipole-dipole attraction between two polar
  molecules (water, HF, alcohols)
• London Dispersion (induced dipole)
  instantaneous attraction between two nonpolar
  molecules due the polarizability of the electron
  cloud.

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Ion-dipole
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Dipole-dipole
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London dispersion
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Hydrogen bonding

• Special type of dipole-dipole interaction
• Occurs between a H atom attached to an
  electronegative atom (N, O, F) on one molecule
  and a lone pair on an electronegative element on
  another molecule. (whew!)
• Common in water(!) and alcohols (COH)
• Very strong collectively

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Hydrogen-bonding examples
Between same molecule
Between different molecules
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Protein structure(a-helix)
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DNA(double helix)
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Hydrogen-bonding between base pairs in DNA
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Why is water so weird?

• High specific heat (only NH3 higher)
• High ?Hfus (only NH3 higher)
• Highest ?Hvap
• Highest surface tension
• Higher boiling point than expected
• Ice floats on water.
• Answer Strong network of hydrogen bonding

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Hydrogen bonding in ice
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Boiling Points (See Table 11.3)
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11.3 Some Properties of Liquids

• Viscosity resistance to flow
• Surface tension a measure of the inward forces
  that must be overcome in order to expand the
  surface area of a liquid
• Cohesive between molecules
• Adhesive attraction between substance and a
  surface (glass ? water)
• Capillary action

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11.5 Vapor Pressure

• In an open container
• liquid ? gas (evaporation)
• In a closed container
• liquid ? gas (dynamic equilibrium)
• Vapor pressure pressure exerted by the vapor in
  equilibrium with the liquid or solid in a closed
  container at a given temperature

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Vapor Pressure
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Vapor Pressure, cont.

• As T ?, vapor pressure ?.
• When vapor pressure atmospheric pressure,
  boiling occurs.
• When pressure 1 atm normal boiling point
• Volatile substances have a higher vapor pressure.

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Vapor Pressure
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11.6 Phase Diagrams

• Show graphically the relationship between the
  different phases of a substance.
• Solid, liquid, gas regions
• Normal melting and boiling points
• Triple point
• Critical point
• Slope of solid-liquid line

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Generic Phase Diagram
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H2O vs. CO2
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H2O