Intermolecular Forces

1
Intermolecular Forces

• Forces Between Molecules

2
Intermolecular Forces

• Electrical forces between molecules causing one
  molecule to influence another
• Heats of vaporization give a measure of the
  strength of attractions present between molecules
• the energy required to separate molecules when
  changing from liquid to gas state

3
Ionic Compounds

• The forces of attractions in ionic compounds are
  the electrostatic force between ions
• A relatively strong force
• ?Hvap /100 kJ/mol

4
Molecular Compounds

• Polar Molecules

5
Polar Molecules

• Force of attraction between molecules is a
  dipole-dipole attraction
• Dipole-dipole forces are smaller than ion-ion
  forces
• ?Hvap .20 kJ/mol

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Molecules are electrically neutral overall but
organize themselves by attractions of head to
tail dipole orientation
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6
Hydrogen Bonding

• A Special Dipole-Dipole Interaction

7
Hydrogen Bonding
The energy of the H-bond depends on the
electronegativity of the X-atom F gt O gt N . Cl
?-
?
?-
?
X-H ....... X-H
8
Heats of Vaporization
non-polar molecule
9
Molecular Compounds

• Non-Polar Molecules

10
Non-Polar Molecules

• Non-polar molecules do not possess permanent
  dipoles
• Force of attraction between molecules is a London
  Force

11

• ?Hvap increases with increasing numbers of
  electrons

12
Principles of Solubility

• Solubility is dependent on intermolecular forces

13
Liquid-Liquid

• like dissolves like
• liquids with similar structures (similar type
  magnitude intermolecular forces) will be soluble
  in each other in all proportions.

14
Example

• Both are held together by London Forces
• When a pentane molecule passes into a volume of
  hexane molecules, there is no significant
  environment change

hexane
pentane
15
Oil Slicks

• Non-polar substances have little water solubility
• Water molecules are held together by H-bonds
• Non-polar are held together by London Forces
• H-bonds must be broken to dissolve appreciable
  quantities of non-polar substances in water

16
Oil Slicks

• For substances to be soluble, there must be
  compensation for any forces broken in the
  dissolution process.
• Since there is no compensating force between a
  non-polar molecule and a water molecule, enough
  energy is not available to break the H-bonds

17
Water Solubility of Polar Molecules

• Water will dissolve some polar molecules
• CH3OH and CH3CH2OH are capable of forming H-bonds
• Intermolecular forces between these alcohols and
  water are similar to those forces in pure alcohol
  and pure water.

18
Water Solubility of Alcohols

• Solubility decreases as length of carbon chain
  increases
• As the chain gets longer, more H-bonds in the
  water must be broken to make room for the
  alcohol.
• Not enough H-bonds can be reformed to compensate

19
Non-Polar Slightly Polar Substances

• Most soluble in solvents of low polarity
• Least soluble in H-bonding solvents

20
The DDT Story

• Soluble in non-polar or slightly polar solvents
• Concentrates in fatty tissue of fish, birds
  game
• Quite water insoluble
• isnt washed out of contaminated soil

21
Solid-Liquid

• Solids always have limited solubility in liquids
• due to differences in the magnitudes of
  intermolecular forces in solid vs. liquid state
• at 25oC a solid has much stronger intermolecular
  forces than a liquid

22
Solid-Liquid

• The closer a solid is to its mp, the better its
  intermolecular forces will match up with a liquid
• Typically, solubility increases as the
  temperature increases
• Low mp solids tend to exhibit greater solubility
  than high mp solids