1. Intermolecular Forces Microscopic Properties Rule the Macroscopic World

2. What causes them? • Two things affect intermolecular forces: • Charge • Distance • Bigger distance equals LESS attraction! • More charge equals MORE attraction! • Coulomb’s Law • F ~ charge/distance2 • Distance has more effect since it is squared.

3. IMF’s versus Bonds • Bonds are permanent attractions, IMF’s are temporary

4. Types of Bonds • Covalent • Sharing of electrons in permanent tug of war. • Ionic • Stealing electrons and then being attracted by opposite charges

5. Types of IMF’s • Ion Dipole • Hydrogen Bonding • Dipole-Dipole • London Dispersion forces

6. Ion Dipole Interactions • Between an ion and a dipole! • Ion = permanently charged particle • Dipole = a polar molecule • Almost exclusively seen in aqueous solutions.

7. Dipole-Dipole Interactions • Between two dipoles! • Dipoles are polar molecules that have no net charge, but the charge is distributed unequally. • Positive end of one attracts negative end of the other.

8. London Dispersion Forces • ALL covalent compounds experience this force. • It is between the electrons in one atom and the nucleus of another atom. • More mass and/or more atoms means more dispersion forces.

9. Be Very Careful! • More mass doesn’t cause an increase in LDF! • More mass infers that there are more P and N, which MEANS there are more electrons. • It is an increase in the number of e- which causes a larger e- density which leads to a stronger temporary dipole. •  You can use molar mass to figure it out since a larger molar mass implies a bigger density but you can't use the words molar mass in your justificationin a Free Response Question

10. Hydrogen Bonding • Occurs in molecules in which hydrogen is having FON! • The most electronegative atoms are F, O and N. With a hydrogen, there is excessive polarization. • This excessive polarization attracts the lone pair of electrons.

11. Strengths of IMF’s • Strongest: Ion-Dipole (very strong) Hydrogen Bonding (strong) Dipole-Dipole Dispersion Forces (very weak)

12. Effects of IMF’s • More IMF’s or stronger IMF’s affect the following properties: • Boiling point • Melting Point • Viscosity (thickness) • Vapor Pressure • Surface tension

13. Boiling Point • Stronger IMF • Higher boiling point • Weaker IMF • Lower boiling point • Compare CS2 to CO2

14. CS2 to CO2 Both are linear Both are nonpolar covalent Both have LDF CS2 has stronger LDF – larger molecule means larger e- density

15. Melting Point • Stronger IMF • Higher melting point • Weaker IMF • Lower melting point • Think of red rover… • Compare HBr to Cl2

16. HBr to Cl2 • Both are liner • HBr has LD, Dipole • Cl2 has LD, nonpolar • HBr has stronger IMF – higher melting point

17. Viscosity • Stronger IMF • Higher viscosity • Weaker IMF • Lower viscosity • More attached to each other, the hard to flow, like solids vs. liquids. • Compare CH3OCH3 to CH3CH2OH

18. CH3OCH3 to CH3CH2OH CH3OCH3 LD, Dipole CH3CH2OH LD, HB Therefore CH3CH2OH has stronger IMF and a higher viscosity

19. Vapor Pressure the pressure exerted by a vapor, particularly a vapor in contact with its liquid form • Stronger IMF • Lower vapor pressure • Weaker IMF • Higher vapor pressure • Again…think red rover. • Compare H2S to H2O2

20. H2S to H2O2 • H2S LD, Dipole • H2O2 LD, HB • H2O2 has stronger IMF therefore lower vapor pressure

21. Surface Tension • Resistance of a liquid to increase in its surface area • Stronger IMF • Higher surface tension • Weaker IMF • Lower surface tension • Compare H2CO to H2O

22. H2CO to H2O • H2CO LD, Dipole • H2O LD, HB • Hydrogen bonding is a stronger IMF than Dipole, therefore H2O has a stronger surface tension

23. Summing it Up • Stronger IMF • Higher boiling point • Higher melting point • Higher viscosity • Higher surface tension • Lower vapor pressure • Weaker IMF • Lower boiling point • Lower melting point • Lower viscosity • Lower surface tension • Higher vapor pressure