Intermolecular Forces

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Intermolecular Forces

• Chemistry 11
• Ms. McGrath

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Intramolecular and Intermolecular Forces

• The forces that bond atoms to each other within a
  molecule are called intramolecular forces.
  Covalent bonds are intramolecular forces.
• In comparison, the forces that bond molecules to
  each other are called intermolecular forces.

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Intermolecular Forces

• Because pure covalent compounds have low melting
  and boiling points, intermolecular forces must be
  very weak compared with the intramolecular forces
  (ie., it does not take very much energy to break
  the bonds that hold the molecules to each other).

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Intermolecular Forces

• Intermolecular forces were studied extensively by
  Johannes van der Waals (1837 1923) and they are
  therefore often referred to as van der Waals
  forces.

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Intermolecular ForcesDipole Dipole Forces

• Dipole polar molecule
• Dipoles will change their direction so that their
  oppositely charged ends are near to one another.
• The electrostatic attraction between the
  oppositely charged ends of the polar molecules
  are called dipole-dipole forces.

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Intermolecular ForcesDipole Dipole Forces

• As a result of these dipole dipole forces of
  attraction, polar molecules will tend to attract
  one another more at room temperature than
  similarly sized non-polar molecules would.
• The strength of these dipole-dipole forces and
  the amount of energy required to separate them is
  indicated by the temperature at which the
  substance changes phase its melting point and
  its boiling point.

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Intermolecular ForcesIon Dipole Forces

• The force of attraction between an ion and a
  dipole (polar molecule).
• NaCl disolves in water because the polar charges
  on the water molecules are strong enough to
  overcome the forces that bind the ions together.

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Intermolecular ForcesIon Dipole Forces

• In aqueous solution, ionic solids dissolve at
  their negative ends and positive ends as the
  water molecules become oriented with the
  corresponding oppositely charged ions that make
  up the ionic compound, pulling them away from the
  solid into solution.

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Intermolecular ForcesDispersion (London) Forces

• Weakest intermolecular force between non polar
  molecules
• It is a temporary attractive force that results
  when the electrons in two adjacent atoms occupy
  positions that make the atoms form temporary
  dipoles

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Intermolecular ForcesDispersion (London) Forces

• The shared pairs of electrons in a covalent bond
  are constantly vibrating.
• The bond vibrations, which are part of the normal
  condition of a non-polar molecule, cause
  momentary, uneven distribution of charge.

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Intermolecular ForcesDispersion (London) Forces

• In other words, a non-polar molecule becomes
  slightly polar for an instant, and continues to
  do this on a random but on-going basic. At the
  instant that one non-polar molecule is in a
  slightly polar condition, it is capable of
  inducing a dipole in a nearby molecule.

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Intermolecular ForcesDispersion (London) Forces

• An intermolecular force of attraction results.
• The degree of force is related to the number of
  electrons as well as the size and shape of the
  molecule larger molecules tend to display
  stronger London dispersion forces.

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Intermolecular ForcesHydrogen Bonding

• Is the attractive force between the hydrogen
  attached to a highly electronegative atom (F, O,
  N) and the hydrogen attached to a highly
  electronegative atom (F, O, N) of a different
  molecule.
• The molecule involved with hydrogen bonding
  should have at least one lone pair. More lone
  pairs stronger hydrogen bonds.

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Intermolecular ForcesInteresting facts

• Hydrogen Bonding
• If a compound experiences strong intermolecular
  forces such as hydrogen bonding, it will have a
  higher melting and boiling point. This is because
  hydrogen bonding helps to stick the molecules
  together, making it harder to pull them apart.

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Intermolecular ForcesInteresting facts

• Like Dissolves Like
• Non polar solutes dissolve in non polar solvents
• (ex solid I2 and liquid Br2)
• Ionic solutes dissolve in polar solvents
• (ex NaCl and H2O)