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