14.1 Molecular Substances Intermolecular Forces

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14.1 Molecular SubstancesIntermolecular Forces

• Forces between molecules determine the melting
  and boiling point of substances.
• Ionic compounds have high melting and boiling
  points due to their electrostatic attraction.
• Molecular substances are held together by one of
  three forces.
• Dispersion
• Dipole
• Hydrogen Bond

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Dispersion Forces

• Dispersion forces, also called London Forces, are
  a result of the temporary location arrangement of
  the electrons.
• Electrons have a probability of being located on
  one side of the atom.
• This side of the atom becomes temporarily charged
  negative while the other side becomes positive.
• This is a temporary situation
• This is the weakest of the forces

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Temporary Dipole
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Dipole Forces

• Polar molecules have a permanent asymmetrical
  distribution of electrons.
• Although the total molecule is neutral, it has a
  positive and negative side.
• The opposite attraction between /- sides of the
  molecule is permanent and thus will cause polar
  substances to have a higher melting and boiling
  point than non-polar substances.
• Dipole forces are the next strongest.

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Forces Between Polar Molecules
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Hydrogen Bonding

• The strongest intermolecular force is a subset of
  dipole forces.
• If hydrogen is bonded to oxygen, nitrogen, or
  fluorine the molecule has the strongest of
  polarities.
• This is called a hydrogen bond.
• The hydrogen is highly positive and the negative
  side is similarly negative.
• These substances have the highest melting and
  boiling points.

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Hydrogen Bonding in Water
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Heating Curves

• A heating Curve is a graphical representation of
  the phase change of a substance.

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Heating Curves

• A to B is heating the solid
• B to C is melting the solid
• C to D is heating the liquid
• D to E is vaporizing the liquid
• E to F is heating the gas

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Heating Curves

• The amount of energy from B to C and from D to E
  is dependent on the intermolecular forces between
  atoms or molecules.
• The slope of AB or CD or EF depends on the
  specific heat of the substance.

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Evaporation Vapor Pressure

• Evaporation is a natural process in which a
  liquid converts to a gas in an endothermic
  process.
• H2O(l) ? H2O(g) ?H
• Evaporation is a cooling process
• The energy is supplied from the environment.
• Fun Fact! Lake Tahoe evaporates 335 million of
  water daily

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• If we can contain evaporation by enclosing our
  liquid source (think of placing a lid on a pot) a
  pressure would begin to build within the air
  space.
• Eventually the pressure will cause condensation
  at the same rate as evaporation.
• The pressure this happens at is called the vapor
  pressure of water.
• At vapor pressure, Vp, the system is at dynamic
  equilibrium.
• This pressure is above atmospheric pressure
• Total Pressure Vp 1 atm.