Mission B6 - Intermolecular Bonding

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Mission B6 - Intermolecular Bonding

• Aim B6 How do IMFs Change the Boiling Points

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Bonding in non-Molecules

• Ionic solids
• held by ionic bonds between all the ions
• Metallic solids
• are atoms of a metal bound to each other in a sea
  of mobile electrons
• Network solids
• diamond and sand have all their atoms bound
  together
• All have high melting points due to these bonds

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Intermolecular Forces

• Individual molecules in a molecular solid are
  held together by covalent bonds
• BUT molecular solids are not!
• There are three types of molecule - to - molecule
  attractions in molecular solids called
  intermolecular forces or IMFs
• London forces (aka Van der Waals forces)
  electrons of one atom can attract the protons of
  another in a very weak bond
• Dipole-dipole interactions the mini magnet
  effect
• H-bonds a special case of the mini magnet
  effect

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The Weakest - London Forces

• London Dispersion Forces
• Protons have attractions for electrons
• both their own and other atoms
• A very weak bond occurs between atoms due to
  these attractions
• Example
• helium molecules
• monoatomic molecules

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The Weakest - London Forces

• London Dispersion Forces
• The larger the molecules
• The more electrons / protons
• The greater the attraction
• between the two molecules
• The greater the boiling point
• Examples
• BP of Neon -246oC
• BP of Argon -186oC
• BP of Krypton -152oC
• BP of Xenon -107oC

Increasing molar masses
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London Forces

• London forces
• Tend to raise melting and boiling points
• Due in part to masses of molecules
• RELATIONSHIP
• The larger the molecules
• the greater the London
• forces, and the higher
• the MP and BP
• Which of these two molecules
• has stronger London forces?

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A little Bit Stronger- Dipole-Dipole Interactions

• A polar covalent bond occurs
• between each H and O in
• water H2O
• But not an even sharing
• Due to electronegativity differences
• Electrons spend more time around oxygen
• Oxygen becomes slightly (-), H becomes slightly
  ()
• A mini magnet or dipole is formed

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Dipole-Dipole Interactions

• Dipole- dipole interactions
• Positive ends of dipole
• attracted to the negative ends of other dipoles
• Again, electrostatic effect or mini magnet
  effect

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Polarity and Molecular Symmetry

• Polar molecules are dipoles they are
  asymmetrical molecules
• Shape affects polarity!
• Linear shapes
• with different atoms attached
• Ex hydrogen chloride (HCl)
• Angular shapes
• bent
• Ex water (H2O)
• Pyramidal shapes
• like a tripod
• Ex ammonia (NH3)
• Note all have polar bonds too!

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Polarity and Molecular Symmetry

• Nonpolar molecules are symmetrical
• They DO NOT form dipoles!
• Linear shapes
• with same atoms on each end
• Ex carbon dioxide (CO2)
• Tetrahedral shapes
• Four sided pyramid shape
• All corners have same elements
• Ex methane (CH4)
• Note even though they have polar bonds, the
  whole molecule is nonpolar due to symmetry!

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Dipole-Dipole Interactions

• Keep in mind!
• London Forces occur in all molecules
• Many molecules also have dipole-dipoles
• Question what will happen to the boiling point
  of similar sized particles, one that is nonpolar
  with only London Forces, and the other with polar
  molecules with dipole-dipole interactions?
• HIGHER MELTING / BOILING POINTS

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The ultimate force HYDROGEN BONDING

• Hydrogen bonding
• Special form of dipole-dipole interaction
• Occurs in molecules with
• Hydrogen atoms
• And small atoms
• Much stronger form of dipole bonding
• Reason for the boiling point of water being only
  100oC

Hydrogen sulfide molecule
Water molecule
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Boiling points of hydrogen compounds
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When Ions and Molecules Collide! Dissolving -
Molecule-Ion Attractions

• NaCl dissolves in water (a given)
• This is due to the attraction of the () and (-)
  ends of the polar water molecules to the Na ions
  and Cl- ions dissolved in the water
• Hydration - water molecules surround the ions,
  separating them and thus dissolving the solid

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Molecule-Ion Attractions Dissolving

• Sodium chloride and other ionic substances dont
  dissolve in oil!
• No hydration occurs
• Oil is nonpolar
• Main reason for oil and water not mixing
• Always remember Like dissolves like
• Polar dissolves polar
• Salt, sugar, in water
• Nonpolar dissolves nonpolar
• Oil paint in turpentine, styrofoam in acetone