Intermolecular Forces: Liquids, and Solids

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Intermolecular Forces Liquids, and Solids
Chapter 13
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A Molecular Comparison of Liquids and Solids
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Intermolecular Forces
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Intermolecular Forces

• Ion-Dipole Forces
• Interaction between an ion (Na) and a dipole
  (water).
• Strongest of all intermolecular forces

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Intermolecular Forces
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Intermolecular Forces

• Dipole-Dipole Forces
• Interaction between an dipole on one molecule and
  a dipole on an adjacent molecule.
• Dipole-dipole forces exist between neutral polar
  molecules.
• Weaker than ion-dipole forces

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Intermolecular Forces
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Intermolecular Forces

• London Dispersion ForcesInduced Dipole Induced
  Dipole
• Weakest of all intermolecular forces.
• It is possible for two adjacent nonpolar
  molecules to affect each other.
• The nucleus of one molecule (or atom) attracts
  the electrons of the adjacent molecule (or atom).
• This attraction causes the electron clouds become
  distorted.
• In that instant a polar molecule (dipole) is
  formed (called an instantaneous dipole).

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Intermolecular Forces
London Dispersion Forces
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Intermolecular Forces

• Hydrogen Bonding
• A special case of dipole-dipole forces.
• This intermolecular force is very strong.
• Strongest of the three Van der Waals forces
  (Hydrogen bonding, Dipole-dipole, London
  forces,)
• H-bonding requires H bonded to an electronegative
  element (most important for compounds of F, O,
  and N).

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Intermolecular Forces
Hydrogen Bonding
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Some Properties of Liquids

• Viscosity
• Viscosity is the resistance of a liquid to flow.
• A liquid flows by sliding molecules over each
  other.
• The stronger the intermolecular forces, the
  higher the viscosity.

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Some Properties of Liquids

• Surface Tension
• The surface of a liquid behaves as a membrane or
  barrier.
• This is due to the unequal attractive forces on
  molecules as the surface.
• Surface molecules are only attracted inwards
  towards the bulk molecules.

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Some Properties of Liquids

• Surface Tension
• Cohesive forces bind molecules to each other.
• Adhesive forces bind molecules to a surface.

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Some Properties of Liquids

• Surface Tension
• Meniscus is the shape of the liquid surface.
• If adhesive forces are greater than cohesive
  forces, the liquid surface is attracted to its
  container more than the bulk molecules.
  Therefore, the meniscus is U-shaped (e.g. water
  in glass).
• If cohesive forces are greater than adhesive
  forces, the meniscus is curved downwards.

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Some Properties of Liquids
Surface Tension Capillary Action - When a narrow
glass tube is placed in water, the meniscus pulls
the water up the tube.
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Properties of Liquids

• Vaporization
• Also called evaporation
• A process in which a substance is transfromed
  from a liquid to a gas.
• Standard molar enthalpy of vaporization (DHovap)
• The energy required to convert one mole of a
  liquid at its boiling point to a gas.
• The resulting gas will exert a pressure on a
  system.

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Properties of Liquids

• Vapor Pressure
• This is the pressure exerted by a substance in
  the gas phase.
• As a liquids temperature increases, its vapor
  pressure increases.

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Properties of Liquids
Vapor Pressure Volatile A substance which has a
low boiling point Or A substance which has a
high vapor pressure at a low temperature
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Properties of Liquids

• Vapor Pressure and Boiling Point
• Liquids boil when the external pressure equals
  the vapor pressure.
• Two ways to get a liquid to boil increase
  temperature or decrease pressure.
• Normal boiling point is the boiling point at 760
  mmHg (1 atm).

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Properties of Liquids

• Vapor Pressure and Boiling Point
• Vapor pressure, temperature and enthalpy of
  vaporization can be related to each other
  using Clausius-Clapeyron equation

P pressure T temperature R gas law DHovap
enthalpy of vaporization
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Properties of Liquids
Vapor Pressure and Boiling Point
The Clausius-Clapeyron equation makes more sense
when it is rearranged into the slope intercept
form.
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Properties of Liquids
Vapor Pressure and Boiling Point
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Structures of Solids

• Unit Cells
• Crystalline solid well-ordered, definite
  arrangements of molecules, atoms or ions.
• Crystals have an ordered, repeated structure.
• The smallest repeating unit in a crystal is a
  unit cell.
• Three-dimensional stacking of unit cells is the
  crystal lattice.

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Structures of Solids
Unit Cells
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Structures of Solids
Unit Cells
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Structures of Solids
Cell Occupancy
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Structures of Solids
Cell Occupancy
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Structures of Solids
Cell Occupancy
Site Occupancy
Corner 1/8
Edge 1/4
Face 1/2
Center 1
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Structures of Solids
Cell Occupancy
Zinc (grey) 4 Center 4 atoms
Sulfur (yellow) 8 corners 6 faces 1 atom 3 atoms
Zn4S4 ? ZnS
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Structures of Solids

• Close Packing of Spheres
• A crystal is built up by placing close packed
  layers of spheres on top of each other.
• There is only one place for the second layer of
  spheres.
• There are two choices for the third layer of
  spheres
• Third layer eclipses the first (ABAB
  arrangement). This is called hexagonal close
  packing (hcp).
• Third layer is in a different position relative
  to the first (ABCABC arrangement). This is
  called cubic close packing (ccp).

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Structures of Solids
Close Packing of Spheres
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Structures of Solids

• Close Packing of Spheres
• Each sphere is surrounded by 12 other spheres (6
  in one plane, 3 above and 3 below).
• Coordination number the number of spheres
  directly surrounding a central sphere.

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Structures of Solids
Other Kinds of Solid Materials
Molecular Solids These are crystalline substances
in which the building blocks are composed of
molecules in place of ions. Example Table
Sugar
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Structures of Solids
Other Kinds of Solid Materials
Network Solids These are crystalline substances
in which the building blocks are atoms and all
the atoms are connected by covalent
bonds. Example Diamond
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Phase Diagrams

• Phase diagram plot of pressure vs. temperature
  summarizing all equilibria between phases.

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Phase Diagrams
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Phase Diagrams
Triple point - Temperature and pressure at which
all three phases are in equilibrium. Critical
point Point above which the liquid and gas
phases are indistinguishable. Critical
temperature - The minimum temperature for
liquefaction of a gas using pressure Critical
pressure - Pressure required for liquefaction
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Homework
2, 16, 18, 24, 30, 32, 40, 44