1d – Intermolecular Forces

To examine the effects of intermolecular forces on properties of a substance

To know how Van der Waals forces arise and their effect

To Know how permanent dipole/dipole interactions occur and their effect on properties

To know how hydrogen bonding arises and it’s importance for properties of some substances

Noble gases have full outer electron shells

They do not need to combine with other atoms.

They are said to be monatomic.

However, the monatomic gases do form weak inter-atomic bonds at very low temperatures.

Group 0 are all gases and exist as individual atoms.

He

++

Sometimes the electrons can end up on oneside of the atom, i.e. the electron cloud can wobble

This means that one side of the atom is morenegative than the other side. i.e. 2 ‘electric poles’ are formed, otherwise called a dipole.

A temporary dipole is therefore formed.

Van der Waals forces

These charges are given the symbol δ ‘delta’

A dipole can induce other atoms to form dipoles, resulting indipole –dipole attraction.

++

++

δ-

δ+

δ-

δ+

δ-

δ+

b.p / K

B.p.’s increase as the size of the atom increases

This happens because the Van der Waals’ forces increases with increasing size of atoms.

4 27

87

121

166

0

20

40

60

80

100

120

140

160

180

Helium

NeonArgon

Krypton

Xeon

A SMALL MOLECULE

COVALENT SUBSTANCE

covalent bonds (strong)

intermolecular attractions

(weak)

covalent bonds (strong)

what causes them?

intermolecular bonds(weak)

covalent bonds (strong)

van der Waal’s

forces

=

MUCH WEAKERTHAN

boilingpoint

boilingpoint

WHY?

wee, light molecule

weak attraction

big, heavy molecule

strong attraction

lower boiling point higher boiling point

the heavier the molecule

the stronger the intermolecular attraction

the higher the boiling point.

F F

F F

Strong covalentbond

Weak Van der Vaals force

Strong intra-molecular bonding and weak inter-molecular bonding exist in this diatomic molecule.F 2 m.p. -220o C or 53 K

A covalent bond is formed when a pair of electrons are shared. The atoms in a covalent bond are held together by electrostatic forces between positively charged nuclei and negatively charged electrons.

9+

Fluorine atom

9+ 9+

Fluorine molecule F2

diatomic

Halogens b.p.’s

b.p./ K

As the size of the halogen atom increases, so does the size of the van der waals’ forces between the halogen molecule.

0

50

100

150

200

250

300

350

400

450

500

Fluorine

Chlorine

Bromine

Iodine

85

238

332

457

The differing electronegativities of different atoms in a molecule and the spatial arrangement of polar covalent bonds can cause a molecule to form a permanent dipole.

+

-

-

-

-

4 polar covalent C-Cl bonds in CCl4 tetrahedral shape

-

-

-

+

3 polar covalent C–Cl bonds and1 polar covalent C-H bond in CHCl3

No permanent dipoleSymmetrical molecule

Permanent dipoleAsymmetrical molecule

NON-POLAR molecule POLAR molecule

e.g. also CO2e.g. also H2O

Both propanone and butane have the same formula mass of 58however, butane boils at – 1 oC while propanone boils at 56oC

Butane has no permanent dipoles, so only Van der Waals forces between molecules. So has a lower boiling point.

Propanone is a polar molecule as it has a permanent dipole, so has polar-polar attraction as well as Van der Waals’ forces between molecules.

+

-

C

O

CCH

H

H

H

H

H

C C C C

H

H

H

H

H H

H

H

H

H

b.p. 56 o C

b.p. -1 o C

Hydrogen bonding is a special type of dipole-dipole attraction in which hydrogenatom acts as a bridge between two very electronegative atoms. It is the strongest of the weak inter-molecular forces.

A - H + B -

A and B are electronegative atoms, such as F, O or N. Such atoms possess one or more lone pair of electrons.

Proteins consist of long chain atoms containing polar >C=O and H-N< bonds.Hydrogen bonds help give proteins their shape.

The Hydrogen atom is in a straight line between A - and B - . The bond strength is stronger than other forms of permanent dipole-permanent dipole interaction but weaker than a covalent bond.

HF+

-

boiling points of hydrides

-200

-150

-100

-50

0

50

100

150

bp

t oC

Gp 4

Gp 5

Gp 6

Gp 7

Group4 5 6 7

boiling points of hydrides

-200

-150

-100

-50

0

50

100

150

bpt o

C

Gp 4

Gp 5

Gp 6

Gp 7

1st row 2nd row 3rd row 4th row

Group4 5 6 7

The intermolecular attractionis much stronger than you

would expect it to be.

This extra strong attraction is given the name

THE HYDROGEN BOND.

But what causes it?

A Apure covalentbond

A A

A B-+

polar covalentbond

pure covalentbond

H X-+

A special type of polar covalent bond

X = a highly electronegative element

+ - + - + -

HYDROGEN BONDS(HYDROGEN BONDING)

X = a very electronegative atom

H X H X H X

covalent bonds

Covalent substances that have hydrogen bonding

have higher

boiling points

have higher melting pointsoften have harder crystals

than you would otherwise expect.

O

H

H

-

-

+

+

WATER

Oxygen has 2 lone pairs of electrons which can forma hydrogen bonds with two hydrogen atoms.

Water has a high surface tension. The molecules on the surfacehave in effect, hydrogen bonds. This has the effect of pullingthe surface molecules closer together.

Water has a its greatest density at a temperature of 4oC. When, as water cools further, the molecules start to move further apart, due to the hydrogen bonding,until a more open structure formed at its freezing point. So ice floats!!

Pond Skater

Ice Skater:

Each water molecule, in theory, could be surrounded by 4 hydrogen bonds

O

H H

-

++

O

H

HO

H

HO

H

OHOHH

H

OH

sugars

glucose

amino acids

N CH C

H

OH

OH

H

an amino acid

N C

H

C

H O

R1

N C

H

C

H O

R4

N C

H

C

H O

R3

N C

H

C

H O

R5

N C

H

C

H O

R2

proteins

N

NN

N

NH2

O

HO

HH

HH

PO

O-

HO

O-

DNA

3 billion

haemoglobin

DNA