Transcript Intermolecular Forces

Intermolecular Forces
Forces Between Molecules
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
Ionic Compounds
 The forces of attractions in ionic
compounds are the electrostatic force
between ions
 A relatively strong force
 Hvap /100 kJ/mol
Molecular Compounds
Polar Molecules
Polar Molecules
 Force of attraction between molecules is a
dipole-dipole attraction
+
-
+
-
Molecules
- - are+ electrically
- neutral overall but organize
+
themselves by attractions of head to tail dipole orientation
 Dipole-dipole forces are smaller than ion-
ion forces
 Hvap .20 kJ/mol
Hydrogen Bonding
A Special Dipole-Dipole Interaction
Hydrogen Bonding
- +
- +
:X-H ....... :X-H
The energy of the Hbond
depends on the
electronegativity
of the X-atom
F > O > N . Cl
Heats of Vaporization
olecule
Hvap (kJ/mol)




C

non-polar molecule
Molecular Compounds
Non-Polar Molecules
Non-Polar Molecules
 Non-polar molecules do not possess
permanent dipoles
 Force of attraction between molecules is a
London Force
Hvap increases with increasing numbers of
electrons
Molecule
Hvap (kJ/mol)
F2
6.5
Cl2
20.4
I2
41.9
Principles of Solubility
Solubility is dependent on
intermolecular forces
Liquid-Liquid
 “like dissolves like”
 liquids with similar structures (similar type
& magnitude intermolecular forces) will be
soluble in each other in all proportions.
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
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
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
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.
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
Non-Polar & Slightly Polar
Substances
 Most soluble in solvents of low polarity
 Least soluble in H-bonding solvents
The DDT Story
 Soluble in non-polar
or slightly polar
solvents
 Concentrates in fatty
tissue of fish, birds &
game
 Quite water insoluble
– isn’t washed out of
contaminated soil
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
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