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
- + -+-
+ - + -
Molecules are electrically neutral overall but organize themselves by attractions of head to tail dipole orientation
Force of attraction between molecules is a dipole-dipole attraction
Dipole-dipole forces are smaller than ion-ion forces Hvap .20 kJ/mol
Hydrogen Bonding
A Special Dipole-Dipole Interaction
Hydrogen Bonding
The energy of the H-bonddepends on the electronegativityof the X-atom
F > O > N . Cl
:X-H ....... :X-H- + +
olecule Hvap (kJ/mol)
C non-polar molecule
Heats of Vaporization
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
Top Related