3/8/2020

1

CHAPTER 6

Intermolecular Forces Attractions between Particles

Scientists are interested in how matter behaves under unusual

circumstances. For example, before the space station could

be built, fundamental research into materials properties had to

be undertaken.

1

2

3/8/2020

2

Solids and Liquids

Intermolecular Forces

Ion-ion, Ion-dipole, dipole-dipole and H-

bonding, dipole-induced dipole, induced

dipole-induced dipole

Liquids

Vapor pressure and

temperature, Critical

T & P, Surface

tension and

viscosity

Phase Diagrams

Show relation of solid,

liquid, and gas phases

with change in T and

P

Solids

Unit cells, metal

structures, formulas

and structures of ionic

compounds,

Molecular, network,

and amorphous solids

Properties of Solids

Lattice energy, heat of

fusion, melting point

Summary of Intermolecular Forces

3

4

3/8/2020

3

Chapter Outline

▪ 6.1 London Dispersion Forces: They’re Everywhere▪ Nonpolar molecules – polarizability and temporary dipoles

▪ 6.2 Interactions Involving Polar Molecules▪ Polar molecules – ion-dipole, dipole-dipole, hydrogen bonds

▪ 6.3 Trends in Solubility

▪ 6.4 Phase Diagrams: Intermolecular Forces at Work

▪ 6.5 Some Remarkable Properties of Water

AlkanesWhy do increasing lengths increase the boiling point?

5

6

3/8/2020

4

London Dispersion Forces

“Temporary Dipoles”

▪ Even nonpolar molecules and

uncombined atoms have attractive

forces between them, otherwise

they would never condense or

solidify.

▪ Similar to a covalent bond, the

electron of an atom in one

molecule is attracted to the

nucleus of an atom in the adjacent

molecule and visa-versa

▪ This creates “temporary dipoles” or

London Dispersion Forces (named

after the physicist Fritz London)

Polarizability - a measure of the extent to which the

electron cloud of an atom or molecule can be

distorted by an external electric charge.

• In general, larger atoms

or molecules are more

easily polarizable than

smaller ones (more shells,

etc), and so experience

larger London forces

• Sometimes London forces

are called “induced dipole-

induced dipole”

London Dispersion Forces

7

8

3/8/2020

5

Effect of Increasing Polarizability and Increasing

Molecular Mass on Physical Properties

Combined together, these explain why boiling points of

nonpolar molecules like alkanes increase with size.

Explains the trends in boiling points for the

Noble Gases and the halogens.

9

10

3/8/2020

6

The Importance of Shape – Branched AlkanesWhich alkane has greater attractive intermolecular forces ?

• These molecules all have the same chemical formulas,

C5H12 ,but different shapes. These are known as “isomers”.

• The boiling points decrease as the isomers become more

branched.

The Importance of Shape – Branched AlkanesWhich alkane has greater attractive intermolecular forces ?

11

12

3/8/2020

7

Viscosity: the measure of a fluid’s

resistance to flow

http://www.reologie.ro/an-introduction-to-

rheology-and-viscosity/

Viscosities of some liquid alkanes

hexane

octane

decane

Molar

Mass

(g/mol)

Viscosity

@ 20oC

(cP)

86 0.29

114 0.54

142 0.92

13

14

3/8/2020

8

Chapter Outline

▪ 6.1 London Dispersion Forces: They’re Everywhere▪ Nonpolar molecules – polarizability and temporary dipoles

▪ 6.2 Interactions Involving Polar Molecules▪ Polar molecules – dipole-dipole and hydrogen bonds, ion-dipole

▪ 6.3 Trends in Solubility

▪ 6.4 Phase Diagrams: Intermolecular Forces at Work

▪ 6.5 Some Remarkable Properties of Water

Dipole-Dipole InteractionsAttractive forces between the dipoles of polar molecules.

Acetic acid

CH3COOH

-

+

15

16

3/8/2020

9

• Very strong type of dipole-dipole interactions

• Requires H atom covalently bonded to the very

electronegative atoms O, N, or F

Hydrogen Bonds

H2O NH3 HF

Boiling Points of Binary Hydrides

17

18

3/8/2020

10

adenine thymine

white = hydrogen

blue = nitrogen

black = carbon

red = oxygen

The Double Helix of DNA is held together

by hydrogen bonding

The polymer Nylon is also held together

by hydrogen bonding

19

20

3/8/2020

11

Interactions Involving Polar Molecules

▪ Ion-Dipole:

• Interaction between an ion and the partial

charge of a molecule with a permanent dipole.

▪ Sphere of Hydration:

• Cluster of water molecules surrounding an ion

as it dissolves in aqueous solution.

• Sphere of solvation if solvent other than H2O.

Ion-Dipole Interactions

21

22

3/8/2020

12

Ion-Dipole Interactions:

Sphere of Hydration

23