FORCES BETWEEN MOLECULES: TEMPORARY AND PERMANENT DIPOLES

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FORCES BETWEEN MOLECULES TEMPORARY AND
PERMANENT DIPOLES
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DIPOLES AND INTERMOLECULAR FORCES
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What are dipoles?

• A dipole is simply a molecule (or part of a
  molecule) with a positive end and a negative end.

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There are different types of dipoles

• Permanent dipoles
• Instantaneous dipoles
• Induced dipoles

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Instantaneous dipoles

• The following shows how instantaneous dipoles
  occur

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Electron cloud evenly distributed no dipole
Cl
Cl
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At some instant, more of the electron cloud
happens to be at one end of the molecule than the
other, electron density is constantly changing.
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At some instant, more of the electron cloud
happens to be at one end of the molecule than the
other molecule has a dipole.
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An un-polarised Cl2 molecule finds itself next to
an HCl molecule with a permanent dipole
H
Cl
Cl
Cl
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Electrons get attracted to the positive end of
the HCL dipole, inducing a dipole in the Cl2
molecule
Cl
Cl
H
Cl
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How a dipole is induced in an atom of Xenon
This atom is not yet polarised, but its electrons
are repelled by the dipole next to it
This atom is instantly polarised
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so it becomes polarised
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DIPOLES AND INTERMOLECULAR FORCES
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If a molecular substance contains dipoles, they
can attract each other. All intermolecular forces
arise from the attractive forces between dipoles.
There are three kinds of attraction

• Permanent dipole-permanent dipole
• Permanent dipole-induced dipole
• Instantaneous dipole-induced dipole

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Summary
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INSTANTANEOUS DIPOLE-INDUCED DIPOLE ATTRACTIONS

• Are forces of attraction which operate between
  all molecules and between the isolated atoms in
  Noble gases
• Are much weaker than other types of
  intermolecular forces
• Are the result of electrostatic attraction
  between temporary dipoles and induced dipoles
  caused by movement of electrons

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Note

• The strength of the forces is related to the
  number of electrons present and hence to the size
  of the molecule (or isolated atom)

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PERMANENT DIPOLE PERMANENT DIPOLE ATTRACTIONS

• Are additional electrostatic forces of attraction
  between polar molecules
• Are stronger than instantaneous dipole-induced
  dipole forces for molecules of equivalent size

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So

• If we have two substances, one polar and one
  non-polar, whose molecules have the same mass
  (and so approximately the same number of
  electrons), the polar substance will have a
  higher boiling point than the non-polar substance.