Intermolecular Forces: relationships between molecules

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Intermolecular Forcesrelationships between
molecules

• Chemistry 11
• Mrs. Kay

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Polarity and Shape

• Polar bonds do not necessarily always create a
  polar molecule.
• The shape of the molecule and the polarity of
  each bond is considered before stating if a
  molecule is considered POLAR or NON-POLAR
• We will watch a 5 minute explanation to help us
  understand this difficult concept
• http//www.mhhe.com/physsci/chemistry/chang7/esp/f
  older_structure/bo/m4/s2/index.htm

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Polarity

• The atoms are sharing electrons, one atom will
  attract the electrons shared more closely, making
  it slightly more negative, and the other atom
  slightly more positive.
• The overall shape is polar
• POLAR Shapes always include bent and pyramidal

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Electronegativity

• Atom is more electronegative, it is pulling
  electrons closer to itself.
• Atom is more electropositive, its electrons have
  been pulled farther away.

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Non-polar Molecules

• A non-polar molecule is one that the electrons
  are distributed more symmetrically and thus does
  not have an abundance of charges at the opposite
  sides. The charges all cancel out each other.

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Summary of Polarity of Molecules

• Linear
• When the two atoms attached to central atom are
  the same the molecule will be Non-Polar
• When the two atoms are different the dipoles will
  not cancel, and the molecule will be Polar
• Bent
• The dipoles created from this molecule will not
  cancel creating a net dipole and the molecule
  will be Polar

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Summary of Polarity of Molecules

• Pyramidal
• The dipoles created from this molecule will not
  cancel creating a net dipole and the molecule
  will be Polar
• Trigonal Planar
• When the three atoms attached to central atom are
  the same the molecule will be Non-Polar
• When the three atoms are different the dipoles
  will not cancel, resulting in a net dipole, and
  the molecule will be Polar

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Tetrahedral

• When the four atoms attached to the central atom
  are the same the molecule will be Non-Polar
• When three atoms are the same, and one is
  different, the dipoles will not cancel, and the
  molecule will be Polar

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Summary of Polarity of Molecules
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Examples to Try

• Determine whether the following molecules will be
  polar or non-polar
• SI2
• CH3F
• AsI3
• H2O2

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Intermolecular Forces

• Forces binding atoms in a molecule are due to
  chemical bonding
• Intramolecular forces forces that bond the
  atoms to each other within the molecule.

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Two ways to form a solution

• 1. If there are strong mutual forces of
  attraction between solute and solvent particles
  in order to separate solute particles from each
  other and solvent particles from each other.
• 2. If there are very weak forces of attraction
  holding solute particles together and holding
  solvent particles together.

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Dipole-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 ends is
  dipole-dipole force

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Ion Dipole Forces

• The force of attraction between an ion and a
  polar molecule.
• NaCl breaks up because the ion dipole with water
  is stronger than the attraction of Na to Cl-

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London (dispersion) 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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Hydrogen Bonding

• It is the attractive force between the hydrogen
  attached to an electronegative atom of one
  molecule and an electronegative atom 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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Like Dissolves Like

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