Ch8 Sec3
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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 What are these forces? Where do they come from? Do all molecules feel them? ‹#› 8.3 Intermolecular Forces A quick look at water 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. ‹#› 8.3 Intermolecular Forces Types of intermolecular attractions Between polar molecules Between nonpolar molecules London dispersion weak Dipole-dipole Hydrogen bonding Intermolecular attractions strong Intermolecular attractions are also called van der Waals attractions ‹#› 8.3 Intermolecular Forces Types of intermolecular attractions Between polar molecules Between nonpolar molecules London dispersion weak Dipole-dipole Hydrogen bonding Intermolecular attractions strong Intermolecular attractions are also called van der Waals attractions ‹#› 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. ‹#› 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 ‹#› 8.3 Intermolecular Forces Dipole-dipole attractions Molecules that are more polar will attract more strongly. less Dipole-dipole attractions ‹#› 8.3 Intermolecular Forces Dipole-dipole attractions Molecules that are more polar will attract more strongly. less Dipole-dipole attractions ‹#› 8.3 Intermolecular Forces Dipole-dipole attractions Molecules that are more polar will attract more strongly. less Boiling point Molecules that attract more strongly will have a higher boiling point. lower ‹#› 8.3 Intermolecular Forces Dipole-dipole attractions Molecules that are more polar will attract more strongly. less Boiling point Molecules that attract more strongly will have a higher boiling point. lower ‹#› 8.3 Intermolecular Forces Dipole-dipole attractions Higher polarity molecules attract more strongly and have a higher boiling point Boiling points least polar ‹#› propane 1-propanol 1,3-propanediol –42oC 97oC 214oC most polar 8.3 Intermolecular Forces Types of intermolecular attractions Between polar molecules Between nonpolar molecules London dispersion weak Dipole-dipole Hydrogen bonding Intermolecular attractions strong Intermolecular attractions are also called van der Waals attractions ‹#› 8.3 Intermolecular Forces Hydrogen bonding Electronegativity There is a moderate difference in electronegativity between H and F, O and N (0.94 to 1.88) ‹#› 8.3 Intermolecular Forces Hydrogen bonding Water molecules are held together by a network of hydrogen bonding ‹#› 8.3 Intermolecular Forces 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 ‹#› 8.3 Intermolecular Forces 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. ‹#› 8.3 Intermolecular Forces Hydrogen bonding Surface tension from hydrogen bonds allows a water strider to “walk” on water ‹#› 8.3 Intermolecular Forces 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 ‹#› 8.3 Intermolecular Forces Hydrogen bonding Hydrogen bonding plays a crucial role in DNA and protein structures The protein structure is stabilized with H bonds ‹#› 8.3 Intermolecular Forces Hydrogen bonding Paper glue is a mixture of polyvinyl acetate (PVA) and water In “wet” glue, polymer molecules are lubricated by water ‹#› 8.3 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 ‹#› 8.3 Intermolecular Forces Types of intermolecular attractions Between polar molecules Between nonpolar molecules London dispersion weak Dipole-dipole Hydrogen bonding Intermolecular attractions strong Intermolecular attractions are also called van der Waals attractions ‹#› 8.3 Intermolecular Forces London dispersion Isolated hydrogen molecules are nonpolar A temporary, very small polarity can be induced when nonpolar molecules are close enough ‹#› 8.3 Intermolecular Forces London dispersion Molecules with a larger surface area will attract more ____strongly. less propane A temporary, very small polarity can be induced when nonpolar molecules are close enough pentane ‹#› 8.3 Intermolecular Forces London dispersion Molecules with a larger surface area will attract more ____strongly. less propane A temporary, very small polarity can be induced when nonpolar molecules are close enough pentane ‹#› 8.3 Intermolecular Forces London dispersion Molecules with a larger surface area will attract more ____strongly. less Boiling point Molecules that attract more strongly will have a higher boiling point. lower ‹#› 8.3 Intermolecular Forces London dispersion Molecules with a larger surface area will attract more ____strongly. less Boiling point Molecules that attract more strongly will have a higher boiling point. lower (It takes more energy to overcome the intermolecular forces.) ‹#› 8.3 Intermolecular Forces London dispersion Molecules with larger surface area attract more strongly and have a higher boiling point Boiling points least surface area ‹#› propane butane –42oC 0oC pentane 36oC most surface area 8.3 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 8.3 Intermolecular Forces Types of intermolecular attractions Between polar molecules Between nonpolar molecules London dispersion weak Dipole-dipole Hydrogen bonding Intermolecular attractions strong Intermolecular attractions are also called van der Waals attractions ‹#› 8.3 Intermolecular Forces
