TOPIC: Intermolecular Forces How do particle diagrams of liquids

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TOPIC Intermolecular ForcesHow do particle
diagrams of liquids solids compare to those of
gases?
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Describe relative positions and motions of
particles in each of 3 phases
SOLID LIQUID GAS
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• Why do some substances exist as gases, some as
  liquids, and some as solids at room temp?

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• Part of answer has to do with forces between
  separate molecules (called intermolecular forces)

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Intermolecular forces between molecules. They are
weaker. Intramolecular forces are between
individual atoms (we will learn this later)

Intermolecular forces
Intramolecular forces
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Intermolecular Forces-IMF

• Inter means between or among
• Intermolecular forces forces between
  neighbouring compounds

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• All molecules have Dispersion forces (the regents
  calls these Van der Waals)
• 2 other types of forces (IMF)
• Dipole-Dipole forces
• Hydrogen bonds
• -if one of these are present, they are more
  important.

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Most atoms dont have a charge, unless they are
ions, so we often refer to them as having partial
charges and write them like this
This separation of Charge is responsible For
the forces Between the molecules
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• 1. Dispersion Forces (van der waals)
• weakest IMF
• occur between nonpolar molecules
• Click here for animation (slide 4 of 13)

• Nonpolar means no poles (/-)
• Cant tell one end of molecule from other end
• electrons are evenly distributed

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• instantaneous and momentary
• fluctuate
• results from motion of electrons
• if charge cloud not symmetrical will induce
  asymmetry in neighbors charge cloud!

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4 categories of Nonpolar Molecules(you need to
memorize)

• Noble Gas molecules
• He, Ne, Ar, Kr, Xe, Rn
• diatomics if both atoms are same (7)
• H2, N2, O2, Cl2, F2, I2, Br2
• Pure Hydrocarbons (CxHy)
• CH4, C2H6, C3H8
• small symmetrical molecule
• CO2, CF4, CCl4

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Dispersion Forces and Size

• Dispersion forces ? with molecule size
• larger the electron cloud, the greater the
  fluctuations in charge can be
• Rn gt Xe gt Kr gt Ar gt Ne gt He
• I2 gt Br2 gt Cl2 gt F2
• C8H18 gt C5H12 gt C3H8 gt CH4

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Boiling point of N2 is 77 K (-196C) IMF are
very weak dispersion forces
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• 2. Dipole-dipole forces
• intermediate IMF
• occur between polar molecules (they have a
  partial charge at each pole one is typically
  much larger than the other)
• Click here for animation (slide 3 of 13)

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Dipole-dipole Forces Polar Molecules
Molecule shows permanent separation of charge
has poles one end partly (-) one end partly
()
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(No Transcript)
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Polar Molecules
Polar means molecule has poles () (-)
geometry and electron distribution are not
symmetrical
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• 3. Hydrogen bonds
• strongest IMF
• occur between molecules that have
• H-F H-O or H-N bonds ONLY

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Hydrogen Bonding
H-O
N-H Occurs between molecules with H-F,
H-O, or H-N bonds
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Hydrogen Bonding

• Hydrogen bonding is extreme case of
• dipole-dipole bonding
• F, O, and N are all small and electronegative
• strong electrons attraction
• H has only 1 electron, so if being pulled away H
  proton is almost naked
• H end is always positive
• F, O, or N end is always negative

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Strength of Hydrogen Bonding

• Fluorine most electronegative element, so
• H-F bonds are most polar and exhibit
• strongest hydrogen bonding
• H-F gt H-O gt H-N
• (H-bondingsound like FON to me!!!)

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• Hydrogen bonding
• strongest IMF
• influences physical props a great deal
• H-F gt H-O gt H-N

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Strongest Intermolecular Force
Hydrogen Bonding
Dipole-Dipole
Dispersion
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Indicate type of IMF for each molecule

• NH3
• Ar
• N2
• HCl
• HF
• Ne
• O2
• HBr
• CH3NH2

• Hydrogen bonding
• Dispersion forces
• Dispersion forces
• Dipole-dipole forces
• Hydrogen bonding
• Dispersion
• Dispersion
• Dipole-dipole
• Hydrogen bonding

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H
O
H
H
O
H
H-Bonding strongest IMF much harder to pull
molecules apart
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H
C
H
H
H
H
C
H
H
H
Dispersion Forces weakest IMF much easier to
pull molecules apart
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IMF vs. Physical Properties

• If IMF ? then
• Boiling point ?
• Melting point ?
• Heat of Fusion ?
• Heat of Vaporization ?
• while
• Evaporation Rate ?

Change from solid to liquid w/o changing temp
Change from liquid to gas w/o changing temp
Rate at which conc. will go from liquid to gas
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• Why do some substances exist as gases, some as
  liquids, and some as solids at room temp?

1 reason IMF
2 reason temperature (avg. KE)
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IMF vs. Temp

• IMF more important as temp is lowered
• Low temperature low evaporation rate
• High temperature high evaporation rate

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Intermolecular forces determine phase

• Competition between strength of IMF KE
  determines phase

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• If IMF are strong, substance will be solid or
  liquid at room temp
• Particles want to clump together
• If IMF are weak, substance will be gas at
• room temp
• Particles free to spread apart

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Its a balancing act!
Kinetic Energy High (fast)
Intermolecular Forces weak
this substance a gas at room temperature
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Intermolecular Forces strong
Kinetic Energy Low (slow)
this substance a condensed phase
(solid/liquid)
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REMEMBER

• Temp average KE
• If we change T we change KE
• Increase KE will help pull molecules apart
  (overcome IMF)