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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?. A quick look at water. - PowerPoint PPT Presentation
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CHAPTER 8
Compounds and
Molecules
8.3 Intermolecular Forces
2 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?
3 8.3 Intermolecular Forces
A tiny drop of waterIntermolecular 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 dispersion Dipole-dipole Hydrogen bonding
Types of intermolecular attractions
Betweenpolar molecules
Betweennonpolar molecules
Intermolecular attractions are also called van der Waals attractions
Intermolecular attractionsweak
5 8.3 Intermolecular Forces
strong
London dispersion Dipole-dipole Hydrogen bonding
Types of intermolecular attractions
Betweenpolar molecules
Betweennonpolar molecules
Intermolecular attractions are also called van der Waals attractions
Intermolecular attractionsweak
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.
10 8.3 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
11 8.3 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
12 8.3 Intermolecular Forces
Dipole-dipole attractions
Higher polarity molecules attract more strongly and have a higher boiling point
propane 1-propanol 1,3-propanediol
least polar
most polar
–42oC 97oC 214oCBoiling points
13 8.3 Intermolecular Forces
strong
London dispersion Dipole-dipole Hydrogen bonding
Types of intermolecular attractions
Betweenpolar molecules
Betweennonpolar molecules
Intermolecular attractions are also called van der Waals attractions
Intermolecular attractionsweak
14 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)
15 8.3 Intermolecular Forces
Hydrogen bonding
Water molecules are held together by a network of
hydrogen bonding
16 8.3 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
17 8.3 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
18 8.3 Intermolecular Forces
Hydrogen bonding
Surface tension from hydrogen bonds allows a water strider to “walk” on water
19 8.3 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
20 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
21 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
22 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
23 8.3 Intermolecular Forces
strong
London dispersion Dipole-dipole Hydrogen bonding
Types of intermolecular attractions
Betweenpolar molecules
Betweennonpolar molecules
Intermolecular attractions are also called van der Waals attractions
Intermolecular attractionsweak
24 8.3 Intermolecular Forces
London dispersion
Isolated hydrogen molecules are nonpolar
A temporary, very small polarity can be created when nonpolar molecules are close enough
25 8.3 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
26 8.3 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
27 8.3 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
28 8.3 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 stronger intermolecular forces.)
more
Molecules with a larger surface area
will attract ____strongly. less
29 8.3 Intermolecular Forces
Molecules with larger surface area attract more strongly and have a higher boiling point
propane butane pentane
least surface area
–42oC 0oC 36oCBoiling points
most surface area
London dispersion
30 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
31 8.3 Intermolecular Forces
Intermolecular attractionsweak strong
London dispersion Dipole-dipole Hydrogen bonding
Types of intermolecular attractions
Betweenpolar molecules
Betweennonpolar molecules
Intermolecular attractions are also called van der Waals attractions
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