Intermolecular Forces

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Intermolecular Forces
Intermolecular forces
• The attraction and repulsion forces between
molecules
– (intramolecular forces are the forces within the
molecule – bonding)
• Cotton vs plastic raincoat
• Broken down into three types
Van der Waals Forces
• Intermolecular forces
• Forces holding molecules in order
• But very weak compared to the covalent
bonding between elements
• Energy to boil water vs the amount to
decompose water
Dipole-Dipole Force
• Force between dipoles or polar molecules
• Strength of the force depends on the polarity
of the molecule
London Force
• Force between nonpolar molecules
– While the overall molecule does not have a dipole
the electrons moving within the molecule will
create a momentary dipole
– The more electrons the molecule has the higher
the London Force
Predicting boiling points
• Can use the number of electrons as a
prediction of the boiling point.
• This is because the intermolecular forces are
related to how difficult the molecules are to
separate
Isoelectronic Molecules
• Iso – meaning the same
• electronic – electrons
• These molecules would have the same
number of electrons and the London forces
will be similar
• However this does not mean that the boiling
points would be the same.
• Remember the boiling point depends on the
London forces AND the dipole-dipole
Summary
• More polar -> stronger dipole-dipole -> higher
boiling point
• More electrons -> larger London Forces ->
higher boiling point
• Using this knowledge to predict relative
boiling points for certain cases
Lab Exercise 3.4
Hydrogen bonding
• Hydrogen (proton) is shared with a lone pair.
• Hydrogen needs to be bonded with an atom with
a high electronegativity
• Need to have at least one lone pair on the atom
bonded to Hydrogen to give hydrogen bonding to
another molecule
• Three possible structures: HF (g), anything with –
OH and –NH.
• Example is sugar C12H22O11(s), or better written as
C12H14O3(OH)8, we can see there are 8 –OH
groups to hydrogen bond
Examples
• DNA and proteins
• Water, ice
– Reason for different densities
Physical Properties of Liquids
• Intermolecular forces (IMF) contribute to
various
• Surface tension – in the liquid IMF are in all
directions but at the surface it is only away
from the surface.
• Cohesion – attraction between like molecules
• Adhesion – attraction between unlike
molecules
Properties Cont’d
• How easily the liquid evaporates
– Weak forces – easy to evaporate – volatility
Summary
• Intermolecular forces are the attraction and
repulsion of positive and negative charges
• All molecules have London forces – with
momentary diploes
• Polar molecules have dipole-dipole forces
• Hydrogen bonding occurs when a hydrogen is
attracted to the lone pair from an adjacent
molecule
• IMF affect many various physical properties

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