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

4 8.3 Intermolecular Forces strong London dispersionDipole-dipoleHydrogen bonding Types of intermolecular attractions Between polar molecules Between nonpolar molecules Intermolecular attractions weak Intermolecular attractions are also called van der Waals attractions

5 8.3 Intermolecular Forces strong London dispersionDipole-dipoleHydrogen bonding Types of intermolecular attractions Between polar molecules Between nonpolar molecules Intermolecular attractions are also called van der Waals attractions Intermolecular attractions weak

6 8.3 Intermolecular Forces 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 8.3 Intermolecular Forces 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 8.3 Intermolecular Forces Dipole-dipole attractions more less Dipole-dipole attractions Molecules that are more polar will attract strongly.

9 8.3 Intermolecular Forces Dipole-dipole attractions more less Dipole-dipole attractions Molecules that are more polar will attract strongly.

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

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

Intermolecular Forces Dipole-dipole attractions Higher polarity molecules attract more strongly and have a higher boiling point propane1-propanol1,3-propanediol least polar most polar –42 o C97 o C214 o C Boiling points

Intermolecular Forces strong London dispersionDipole-dipoleHydrogen bonding Types of intermolecular attractions Between polar molecules Between nonpolar molecules Intermolecular attractions are also called van der Waals attractions Intermolecular attractions weak

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

Intermolecular Forces Hydrogen bonding Water molecules are held together by a network of hydrogen bonding

Intermolecular Forces Hydrogen bonding Iceberg photo courtesy of NOAA 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

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

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

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

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

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

Intermolecular Forces 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

Intermolecular Forces strong London dispersionDipole-dipoleHydrogen bonding Types of intermolecular attractions Between polar molecules Between nonpolar molecules Intermolecular attractions are also called van der Waals attractions Intermolecular attractions weak

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

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

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

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

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

Intermolecular Forces Molecules with larger surface area attract more strongly and have a higher boiling point propanebutanepentane least surface area –42 o C0oC0oC36 o C Boiling points most surface area London dispersion

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

Intermolecular Forces Intermolecular attractions weakstrong London dispersionDipole-dipoleHydrogen bonding Types of intermolecular attractions Between polar molecules Between nonpolar molecules Intermolecular attractions are also called van der Waals attractions