Intermolecular Forces

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Intermolecular Forces

• Chem 211, F 2006
• Harris

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Four types of IFs

• Van der Waals (induced dipole other)
• weak, but numerous!
• Dipole-Dipole forces
• moderate
• Hydrogen bonding
• strong
• Ion-Ion forces (or just ionic bonding)
• strongest

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Melting and boiling points

• Each compound will have
• Physical properties
• Chemical properties
• Melting and boiling points are related to
  intermolecular forces
• Stronger IFs mean higher mps and bps
• Need to outline fundamentals of both

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First, sources of organics
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What does it take to melt?

• Solid atoms or molecules are rigidly bound to
  one another
• Liquid atoms or molecules are mobile
• Difference is due to energy
• High energy molecules move, spin or vibrate
  faster to separate from each other
• Melting point marks the transition from
• solid to liquid
• static to dynamic but still experiencing
  attraction

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What does it take to boil?

• Liquid atoms or molecules are mobile, but still
  in contact with each other
• Gas atoms are much more mobile, completely free
  of each other
• Difference is due to energy
• Two stages
• Separate from each other - IFs
• Accelerate to high speed - Molecular Weight
• boiling point marks the transition from
• Liquid to gas
• Dynamic with interactions to completely free of
  interactions

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bp increases with MW
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Ion-Ion forces

• Opposite charges attract ionic comps
• Stronger Intermolecular Forces are the
  microscopic explanation for
• Higher melting points
• Higher boiling points
• Example NaCl vs Na or Cl2

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Ionic forces are very strong!

• Cl2, Chlorine gas (van der Waals forces)
• mp -101oC, bp -34oC
• Na, Sodium metal (metal-metal bonds)
• mp 98oC , bp 881.4oC
• NaCl, Sodium Chloride (ionic bonds)
• mp 804oC, bp ?

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Hydrogen Bonds are strong

• These are part covalent/part dipole bonds
• H.O-H bond is strongest when bond angle is 180oC
• This is the reason life can exist on earth
• CH4, NH3, H2O, Cl2
• mps -183, -78, 0, -101
• bps -161, -33, 100, -34
• Ok, and water density

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Amines and alcohols, high bps
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Dipole Dipole interactions

• Actually Dipole other
• Dipole Dipole
• Acetone/acetone
• Dipole ionic
• Acetone/NaCl
• Dipole Hydrogen bonding
• Acetone - water
• Dipole Induced Dipole
• Acetone - hexane

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Compare R to RCl, RCHO
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Van der Waals orinduced dipole other

• Should be called London forces
• Present in all molecules
• Used to explain attractions between non-polar
  molecules
• More surface area stronger force
• Marbles, pencils, plates

15
Compare R to RCl, RCHO