Intermolecular Forces
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Intermolecular Forces

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A set of slides created to teach Intermolecular Forces to students following the South African National Science curriculum (NSC CAPS) in Cape Town.

A set of slides created to teach Intermolecular Forces to students following the South African National Science curriculum (NSC CAPS) in Cape Town.

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Intermolecular Forces Intermolecular Forces Presentation Transcript

  • Intermolecular Forces BHS Physical Science K Warne
    • Two different types of bonds occur in substances.
    • Intramolecular
      • Between hydrogen atoms and oxygen atoms inside the molecules Covalent bonds
    • Intermolecular
      • Between two different water molecules Hydrogen bonds
    Intermolecular forces O  +  - H H O  +  - H H
  • Bond Polarity in Water The oxygen atom has greater electronegativity so it is surrounded by greater electron density than the hydrogen atoms. O H H  -  +  + The water molecule is a DIPOLE - it has two oppositely charged “poles”.  +  -  +  - O H H
    • The oppositely charged poles on the water molecules attract each other.
    • This electrostatic attraction constitutes Hydrogen bonding.
    • It is the strongest form of intermolecular attraction.
    • Hydrogen bonding exists only between molecules in which hydrogen is bonded to a very electronegative atom ( H- O -X, H- N X 2 or H- F ).
    • Result in abnormally high boiling points.
    Hydrogen Bonding Example: Water O  +  - H H O  +  - H H O  +  - H H
  • Hydrogen Bonding in Ice Back to Bond Types
  • Molecular solids Iodine - I 2 Strong covalent bonds Weaker intermolecular bonds Covalently bonded molecules held together by weaker intermolecular bonding.
    • PROPERTIES
    • Low melting points
    • Brittle
    • Soluble in non-polar solvents
    • Non - conducting
  • IMF vs Mp & Bp Mp & Bp increases with SIZE and molecule MASS Halogen X 2 (diatomic) Molecular Mass (M r g.mol -1 ) Mp / Bp ( o C) Flourine F 2 pale yellow gas 19 x 2 = 38 -220 / -188 Chlorine Cl 2 pale green gas 35.5 x 2 = 71 -101/ -35 Bromine Br 2 red volatile liquid 80 x 2 = 160 -7 / 59 Iodine I 2 purple solid - sublimes 127 x 2 = 254 114 / 184 . . : X . . . . . : X . . .
  • Density
    • Density = mass/volume (g.cm -3 )
    High density – solids Many particles per cm 3 Low density – gases Few particles per cm3 1cm x 1cm x 1cm = 1cm 3
  • Effect of Temp
    • Temperature is a measure of average kinetic energy .
    • As temperature rises more particles have high energy.
    Maxwell-Boltzman Curve Average E k Increases
  • Effect of Temp
    • Temperature is a measure of average kinetic energy.
    • As temperature rises – particles move faster – and further apart – substances expand – used in a thermometer.
    Maxwell-Boltzman Curve Average E k
  • Viscosity
    • Viscosity is a measure of how thick (viscous) and sticky a liquid is.
    • Viscosity reduces the ability of a liquid to flow.
    • Liquids that flow readily (water) have a low viscosity.
    • Viscosity is a function of (depends on) the attractive forces of the molecules of the liquid.
    • Strong forces – high viscosity
    • Temperature also greatly affects viscosity: as temperature increases, viscosity decreases.
    Kinetic energy enables particles to overcome forces.