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 - - + + Molecules are electrically neutral overall but organize themselves by attractions of head to tail dipole orientation - - + - +

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 > O > 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 • isn’t 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