Matter Trends and Chemical Bonding
3.4
Intermolecular Forces
Learning Goals
• We are learning to…
– Define intermolecular forces
– Identify the types of intermolecular forces
– Identify how intermolecular forces affect a
compounds physical properties
Last Class
• Determined how to identify Polar and nonpolar molecules based on the polarity within
their bonds and the symmetry of the charges
– Asymmetrical charge distribution = Polar
– Symmetrical charge distribution = Non-polar
Why do molecular compounds have lower
boiling and melting points than ionic
compounds?
Why are ionic compounds solids while molecular
compounds can be found as solids, liquids or
gasses?
Why is carbon dioxide a gas while water is a
liquid?
Intermolecular
Forces!
Intermolecular Forces
• Intramolecular forces – the forces within a
molecule
– ie: covalent/ionic bonds
• Intermolecular forces – the attraction
between molecules
– much weaker than intramolecular forces
Intermolecular Forces
• intermolecular forces between molecules
determines:
–
–
–
–
–
–
Physical state (s, l, g)
Melting point
boiling point
Surface tension
Hardness/texture
Solubility
• The stronger the intermolecular force, the higher
all of the above properties will be
Intermolecular Forces
There are three types:
1. London Forces (aka Dispersion Forces or van
der Waal's Forces)
2. Dipole-dipole interactions
3. Hydrogen bonds
Relative strength:
Hydrogen bonds > Dipole dipole > London Forces
London Dispersion Forces
• Very weak forces of attraction resulting from:
– A temporary imbalance in the position of
electrons
– The weak residual attraction of the nuclei in one
molecule for the electrons in another molecule
• The more electrons that are in the molecule,
the stronger the London forces will be
• The only intermolecular force between nonpolar molecules
Dipole-dipole interactions
• Stronger than London forces
• Occur between polar molecules – have
permanent net dipoles
• The partial positive charge on one molecule is
electrostatically attracted to the partial
negative charge on another molecule.
Hydrogen Bonds
• Occur between polar molecules in which
hydrogen is covalently bonded to fluorine,
oxygen or nitrogen – the 3 most EN atoms
• Stronger than London forces and dipole-dipole
interactions
– The electrostatic attraction between H and F, O, or
N on neighbouring molecules is greater than for
other polar molecules
Intermolecular Forces
On Melting/boiling point…
• Melting/boiling results from a weakening of
the attractive forces between molecules
• The stronger the intermolecular force, the
more energy required to melt/boil the
substance
• Group IV hydrides CH4, SiH4, GeH4, SnH4
– Polar or non-polar?
– Intermolecular force?
– Trend as you go down the group?
– Why?
• Group VII hydrides HF, HCl, HBr, HI
– Polar or non-polar?
– Intermolecular force?
– Trend as you go down the group? Exception?
– Why?
Intermolecular Forces
On Solubility…
“Like Dissolves Like”
• Non-polar solutes dissolve in non-polar solvents
• Polar solutes dissolve in polar solvents
– Forces of attraction between molecules of solute and
solvent.
– Sugar (C6H12O6) is a polar molecule – dissolves in
water (H2O) and alcohol but not in hexane (C6H14) or
carbon tetrachloride (CCl4)
– Ionic solutes dissolve in polar solvents
Homework
Page 115
# 1, 2, 4, 5