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