Intermolecular Forces
a.k.a. van der Waal’s Forces
+ O -
H
States of Matter
Attraction between molecules increasing
Intermolecular Forces
a.k.a. van der Waal’s Forces
• Attractive force between molecules
• Much weaker than chemical bonds within
molecules
Types of Intermolecular Forces
• London Dispersion Forces: an attractive
force acting between all molecules
* attraction of electrons of one atom to
nucleus of another atom
* very weak force
* increases in strength as molar mass
increases (more electrons)
London Forces
• A temporary dipole is induced Two non polar molecules.
in a non polar molecule due to Electrons Electrons
electron movements. repelled
moving
• These INDUCE similar but
opposite forces in neighboring d+ d- d+ d-
molecules which cause weak
momentary attractions.
Temporary dipole Induced dipole
• These are the WEAKEST
Weak short lived
attractive forces that exist attraction
between molecules.
• London forces get stronger as Example:
the molecule/atom gets bigger I2 & CCl4
because there are more
electrons that can create
dipoles..
Keith Warne
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• Dipole-Dipole Forces: attractive force
acting between oppositely charged ends of
polar molecules.
* medium strength
* strongest when molecules close together
Dipole - Dipole Forces
• Attractive forces that exist
between molecules that have dipole dipole
permanent dipoles.
• These exist in any polar + -
-
+
H Cl H Cl
substance.
• In order to create a dipole or polar
Attractive force
molecule, the molecule must have:
– Polar bonding
– An asymmetrical shape (not
symmetrical)
Example:
• The following would exhibit Dipole
H-Cl, H-Br, H2S
Dipole forces
H2S yes – polar bonds &
asymettrical, CO2 No - symmetrical,
C2H4 No – no polar bonds. Keith Warne
Keith Warne
www.TeachBomb.com
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• Hydrogen Bonding: attractive force
between positive hydrogen atom of one
molecule and N, O or F in another molecule
* strongest force
* NOT a chemical bond
Predict IMF acting between
molecules of:
• F2
- London dispersion
• PH3
- dipole-dipole, London dispersion
• CH4
- London dispersion
• H2O
- Hydrogen bonds, dipole-dipole, London
dispersion
Properties of Liquids
Boiling Point: Temperature at which a liquid
becomes a gas
* It represents the amount of energy to
overcome the intermolecular forces holding
the liquid molecules together
* Substances with greater intermolecular
forces have higher boiling points
Boiling point is higher for molecules
with greater intermolecular forces
Hydrogen Bonding in Ice
Oxygen
atoms
Hydrogen
bonds
Hydrogen
atoms
http://commons.wikimedia.org/wiki/File:Hex_ice.GIF licence behind image
Ion-Dipole Forces
Ion - dipole forces-
H
• arise from the attraction
+ H
between an ion and the
oppositely charged pole of a
O
polar molecule
H
-
• Ionic substance (sodium
+
chloride) dissolving in a
polar solvent (water).
+
Na - O
H
-
H
O
Example:
Salt & Water
+
H
Keith Warne
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Dissolution (dissolving)
Salt (NaCl) dissolves in
water spontaneously.
H
+ O - + -
H
-
+
+
-
-
Na+
Cl
The charged ends of the dipoleKeith
would
Warne
be attracted to
the opposite charge on the ionic solid.
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The dissolution process
+
Solvent molecules (water) are
attracted to the solute
-
particles as they have -
similar forces of attraction Cl - +
Na+
between them.
-+
Na+
Solvent: water -hydrogen -
bonding (electrostatic forces Cl + -
- strong) +
-
+
Solute: Sodium Chloride - ionic bonds strong
electrostatic attraction of oppositely charged ions.
-
Since the forces in the SOLVENT are similar to
those in the SOLUTE the solvent particles are
able to substitute for and break up the forces in
the solute material - which is then literally ripped
Keith Warne
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Iodine + Water
Iodine is only sparingly soluble in water?
• Water contains hydrogen bonds,
• iodine contains london forces.
• The bonds have very different strengths.
• Iodine molecules can not substitute for water
molecules in the solution - there is very weak
attraction between the solvent particles (H 2O)
and solute particles (I2).
• Dipole - induced dipole forces exist between
water and iodine molecules.
KMnO4 + Water
Potassium permanganate is very soluble in water.
• KMnO4 is an ionic substance.
• Water contains hydrogen bonds,
• ION - DIPOLE FORCES EXSIST between
these two substances.
• These are relatively strong intermolecular
forces and that accounts for the solubility
of ionic substances in polar solvents.
KMnO4 + Hexane
Potassium permanganate is insoluble in hexane.
• KMnO4 is an ionic substance
• Hexane is non-polar - v.d. Waals (london) forces
• Ion - induced dipole forces would exist between solvent molecules - these
are VERY weak interactions
• The solubility is VERY LOW.
Ethanol + Iodine
Iodine is soluble in Ethanol (C2H5OH)
INTERMOLECULAR FORCES
• Iodine - v.d. Waals (London) forces
• Ethanol - hydrogen bonds and london forces
• Dipole - induced dipole forces would exist between solvent and solute as
well as v.d.Waals (London) forces
• The london forces between ethanol molecules could be substituted for
london forces in the iodine and so iodine is soluble in ethanol.
Ethanol + KMnO4
Potassium permanganate is soluble in Ethanol (C2H5OH).
The ion-dipole interactions between KMnO4 and ethanol molecules are strong
enough to cause dissolution of the ionic salt.
Solubility Rules
• Polar solutes will be soluble in ………………solvents.
• Non-polar solutes will be soluble in …………… solvents.
• Non-polar solutes will NOT be soluble in ………... solvents.
• Polar solutes will NOT be soluble in ………………. solvents.
…………………………………….
Solubility Rules
• Polar & Ionic solutes will be soluble in polar solvents.
• Non-polar solutes will be soluble in non-polar solvents.
• Non-polar solutes will NOT be soluble in polar solvents.
• Polar & Ionic solutes will NOT be soluble in non-polar solvents.
LIKE DISSOLVES LIKE!
Homework Questions
• P. 109 # 1-3, 5
• P. 117 # 1-5
