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Compounds and Molecules
CHAPTER 8 Compounds and Molecules 8.3 Intermolecular Forces
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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?
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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.
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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
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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
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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.
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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
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Dipole-dipole attractions
more Molecules that are more polar will attract strongly. less Dipole-dipole attractions
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Dipole-dipole attractions
more Molecules that are more polar will attract strongly. less Dipole-dipole attractions
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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
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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
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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
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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
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Hydrogen bonding Electronegativity There is a moderate difference in electronegativity between H and F, O and N (0.94 to 1.88)
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Water molecules are held together by a network of hydrogen bonding
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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
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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.
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Hydrogen bonding Surface tension from hydrogen bonds allows a water strider to “walk” on water
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Hydrogen bonding plays a crucial role in DNA and protein structures
Hydrogen bonds DNA uses hydrogen bonds to hold the two strands together
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Hydrogen bonding Hydrogen bonding plays a crucial role in DNA and protein structures The protein structure is stabilized with H bonds
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Hydrogen bonding Paper glue is a mixture of polyvinyl acetate (PVA) and water In “wet” glue, polymer molecules are lubricated by water
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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
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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
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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
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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
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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
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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
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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.)
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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
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London dispersion The shape of the molecule also matters!
About the same surface area Stronger attraction Weaker attraction Higher boiling point Lower boiling point
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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
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