Intermolecular Forces Forces Between Molecules

Intermolecular Forces 4 Electrical forces between molecules causing one molecule to influence another 4 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 4 The forces of attractions in ionic compounds are the electrostatic force between ions  A relatively strong force   H vap / 100 kJ/mol

Molecular Compounds Polar Molecules

Molecules are electrically neutral overall but organize themselves by attractions of head to tail dipole orientation 4 Force of attraction between molecules is a dipole-dipole attraction 4 Dipole-dipole forces are smaller than ion- ion forces   H vap. 20 kJ/mol

Hydrogen Bonding A Special Dipole-Dipole Interaction

Hydrogen Bonding The energy of the H- bond depends on the electronegativity of the X-atom F > O > N. Cl :X-H :X-H -- ++ ++

non-polar molecule Heats of Vaporization

Molecular Compounds Non-Polar Molecules

4 Non-polar molecules do not possess permanent dipoles 4 Force of attraction between molecules is a London Force

 H vap increases with increasing numbers of electrons

Principles of Solubility Solubility is dependent on intermolecular forces

Liquid-Liquid 4 “like dissolves like” 4 liquids with similar structures (similar type & magnitude intermolecular forces) will be soluble in each other in all proportions.

Example 4 Both are held together by London Forces 4 When a pentane molecule passes into a volume of hexane molecules, there is no significant environment change hexane pentane

Oil Slicks 4 Non-polar substances have little water solubility –Water molecules are held together by H-bonds –Non-polar are held together by London Forces 4 H-bonds must be broken to dissolve appreciable quantities of non-polar substances in water

Oil Slicks 4 For substances to be soluble, there must be compensation for any forces broken in the dissolution process. 4 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 4 Water will dissolve some polar molecules 4 CH 3 OH and CH 3 CH 2 OH are capable of forming H-bonds 4 Intermolecular forces between these alcohols and water are similar to those forces in pure alcohol and pure water.

Water Solubility of Alcohols 4 Solubility decreases as length of carbon chain increases 4 As the chain gets longer, more H-bonds in the water must be broken to make room for the alcohol. 4 Not enough H-bonds can be reformed to compensate

Non-Polar & Slightly Polar Substances 4 Most soluble in solvents of low polarity 4 Least soluble in H-bonding solvents

The DDT Story 4 Soluble in non-polar or slightly polar solvents 4 Concentrates in fatty tissue of fish, birds & game 4 Quite water insoluble –isn’t washed out of contaminated soil

Solid-Liquid 4 Solids always have limited solubility in liquids –due to differences in the magnitudes of intermolecular forces in solid vs. liquid state –at 25 o C a solid has much stronger intermolecular forces than a liquid

Solid-Liquid 4 The closer a solid is to its mp, the better its intermolecular forces will match up with a liquid 4 Typically, solubility increases as the temperature increases 4 Low mp solids tend to exhibit greater solubility than high mp solids