CHAPTER 8

1. CHAPTER 8 Compounds and Molecules 8.3 Intermolecular Forces

2. We have seen in Chapter 3 that molecules in solids and liquids are held together by intermolecular forces What are these forces? Where do they come from? Do all molecules feel them?

3. A quick look at water Intermolecular attraction A tiny drop of water As a liquid, water molecules can move around but intermolecular forces keep them from separating completely to become a gas.

4. Types of intermolecular attractions Between polarmolecules Between nonpolar molecules Hydrogen bonding London dispersion Dipole-dipole Intermolecular attractions are also called van der Waals attractions weak Intermolecular attractions strong

5. Types of intermolecular attractions Between polar molecules Between nonpolar molecules London dispersion Dipole-dipole Hydrogen bonding Intermolecular attractions strong weak Intermolecular attractions are also called van der Waals attractions

6. Dipole-dipole attractions Like water, formaldehyde is a polar molecule The polar covalent C=O bond makes the entire molecule polar We say the molecule has a dipole dipole-dipole attraction: the attractions between the positive part of one polar molecule and the negative part of another polar molecule.

7. Dipole-dipole attractions Like water, formaldehyde is a polar molecule Dipole-dipole attractions Dipole-dipole attractions cause formaldehyde to condense into a liquid at room temperature

8. Dipole-dipole attractions more Molecules that are more polar will attract strongly. less Dipole-dipole attractions

9. Dipole-dipole attractions more Molecules that are more polar will attract strongly. less Dipole-dipole attractions

10. Dipole-dipole attractions more Molecules that are more polar will attract strongly. less Boiling point Molecules that attract more strongly will have a boiling point. higher lower

11. Dipole-dipole attractions more Molecules that are more polar will attract strongly. less Boiling point Molecules that attract more strongly will have a boiling point. higher lower

12. Dipole-dipole attractions Higher polarity molecules attract more strongly and have a higher boiling point propane 1-propanol 1,3-propanediol Boiling points –42oC 97oC 214oC most polar least polar

13. Types of intermolecular attractions Between polar molecules Between nonpolar molecules London dispersion Dipole-dipole Hydrogen bonding Intermolecular attractions strong weak Intermolecular attractions are also called van der Waals attractions

14. Hydrogen bonding Electronegativity There is a moderate difference in electronegativity between H and F, O and N (0.94 to 1.88)

15. Hydrogen bonding Water molecules are held together by a network of hydrogen bonding

16. Iceberg photo courtesy of NOAA Hydrogen bonding One special property of water: Ice is less dense than water in the liquid form There is more space in between water molecules in ice Water in the liquid form

17. Hydrogen bonding Why a drop of water doesn’t “lie flat” on a hard surface: H-bonds keep the water molecules together In reality water molecules are much, much smaller than on the drawing! surface tension: a force acting to pull a liquid surface into the smallest possible area.

18. Hydrogen bonding Surface tension from hydrogen bonds allows a water strider to “walk” on water

19. Hydrogen bonding Hydrogen bonding plays a crucial role in DNA and protein structures Hydrogen bonds DNA uses hydrogen bonds to hold the two strands together

20. Hydrogen bonding Hydrogen bonding plays a crucial role in DNA and protein structures The protein structure is stabilized with H bonds

21. Hydrogen bonding Paper glue is a mixture of polyvinyl acetate (PVA) and water In “wet” glue, polymer molecules are lubricated by water

22. Hydrogen bonding Paper glue is a mixture of polyvinyl acetate (PVA) and water In “wet” glue, polymer molecules are lubricated by water As glue dries, many more H-bonds form between the polymer molecules, so the glue hardens

23. Types of intermolecular attractions Between polar molecules Between nonpolar molecules London dispersion Dipole-dipole Hydrogen bonding Intermolecular attractions strong weak Intermolecular attractions are also called van der Waals attractions

24. London dispersion Isolated hydrogen molecules are nonpolar A temporary, very small polarity can be induced when nonpolar molecules are close enough

25. London dispersion Molecules with a larger surface area will attract ____strongly. more less propane A temporary, very small polarity can be induced when nonpolar molecules are close enough pentane

26. London dispersion Molecules with a larger surface area will attract ____strongly. more less propane A temporary, very small polarity can be induced when nonpolar molecules are close enough pentane

27. London dispersion Molecules with a larger surface area will attract ____strongly. more less Boiling point Molecules that attract more strongly will have a boiling point. higher lower

28. London dispersion Molecules with a larger surface area will attract ____strongly. more less Boiling point Molecules that attract more strongly will have a boiling point. higher lower (It takes more energy to overcome the intermolecular forces.)

29. London dispersion Molecules with larger surface area attract more strongly and have a higher boiling point propane butane pentane Boiling points –42oC 0oC 36oC least surface area most surface area

30. London dispersion The shape of the molecule also matters! About the same surface area Stronger attraction Weaker attraction Higher boiling point Lower boiling point

31. Types of intermolecular attractions Between polar molecules Between nonpolar molecules London dispersion Dipole-dipole Hydrogen bonding Intermolecular attractions weak strong Intermolecular attractions are also called van der Waals attractions