Compounds and Molecules CHAPTER 8 Compounds and Molecules 8.3 Intermolecular Forces

We have seen in Chapter 3 that molecules in solids and liquids are held together by intermolecular forces In this section, we are going to discuss the different types of intermolecular forces that exist, why they are present and how strong they are. Not all liquid molecules, for instance, will experience the same intermolecular forces. What are these forces? Where do they come from? Do all molecules feel them?

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.

Types of intermolecular attractions 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

Types of intermolecular attractions 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

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.

Like water, formaldehyde is a polar molecule 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

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

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

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

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

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 least polar most polar

Types of intermolecular attractions 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

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

Water molecules are held together by a network of hydrogen bonding

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 Iceberg photo courtesy of NOAA

H-bonds keep the water molecules together 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.

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

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

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

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

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

Types of intermolecular attractions 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

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

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 Polarizability is greater in a molecule with an elongated shape than in a molecule that is globular. pentane

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

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 If molecules attract strongly, they are less likely to be in the gas phase. More energy will be required to overcome these attractive intermolecular forces. Therefore, the boiling point will be higher. lower

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.)

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

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

Types of intermolecular attractions 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