Intermolecular forces of attraction

Learning ObjectivesDifferentiate between polar and non polar molecules

State the names of different types of intermolecular forces of attraction and differentiate between them

Explain the terms dipole –dipole , instantaneous dipole – induced dipole attractions and hydrogen bonding

Describe the effects of attractive forces on the physical properties of substances

( This presentation is to enhance your understanding of the topic . This is only part of the syllabus)

Polarity

Separation of charge

An asymmetrical difference in electronegativity along a bond or in a molecule

The polar molecules with + and - ends and non polar molecules without dipoles

O

HH

N

HH

H

CCO O

Al

Cl

Cl

Cl

Cl

Cl

Cl

Cl

S

HH

Van der Waals Forces

Small, weak interactions between molecules

 

Van der Waals Forces

Intermolecular: between molecules (not a bond)

 

Intramolecular: bonds within molecules (stronger)

What is being attracted?

+ attracted to -

electrostatic attraction

e- s of one atom are attracted by another atom’s nucleus

e-

e-+ +

Evidence of VDW Forces?Non-polar molecules can exist as gases,

liquids and solids.

Example: CO2

CO

O

CO O CO

O

CO O

CO

O

CO O CO

O

CO OCO

O

CO O CO

O

CO O

3 Types of Van der Waals Forces

1)    dipole-dipole

2)    permanent dipole - induced dipole

3) Instantaneous dipole – induced dipole

Dipole-DipoleTwo polar molecules align so that + end of one molecule and - end of another molecule are next to each other giving rise to electrostatic attraction between the two molecules. Let’s look at:

fluromethane (CH3F) and ethane (C2H6) on the next slide

Fluoromethane (CH3F) – boiling point = 194.7 K

H H

H C F H C F

H H

polar or non-polar?

- -

Ethane (C2H6) – boiling point = 184.5 K

H H H H

H C C H H C C H

H H H H

polar or non-polar?

Dipole-Dipole

Temporary Dipole- temporaryDipole

Permanent Dipole - Induced Dipole

A permanent dipole can induce (cause) a temporary dipole in a

Neighbouring non-polar molecule

The molecules then line up So that + end of one molecule and -

end of another molecule attract each other.

Example

H Cl+ Are-e-

e-e-

e-

e-

e-e- e-e-

e- e-

e-

e-e-

e-e-

e-

A Permanent DIPOLE ( polar molecule)

INDUCEDDIPOLE non- polar molecule

Permanent Dipole – Induced Dipole (weak and short-lived)

Dispersion Forces or instantaneous dipole – induced dipole forces

A temporary dipole forms in a non-polar molecule…

which leads to…a temporary dipole to form in ANOTHER

non-polar molecule

Dispersion is the ONLY intermolecular attraction that occurs between non-polar molecules

Dispersion Forces

Cl-Cle-e-

e-

e-

e-

e-

e- e-e- e-

e-e-

e-

e-e-

e-

e-e-

Non-polar

INDUCEDDIPOLE

TEMPORARY DIPOLE

Non-polar

Cl-Cle- e- e-

e-

e-

e-e- e- e-

e-

e-e- e-e-

e-

e- e-

Dispersion (weakest and very short-lived)

Review Intermolecular forces

Dipole – Dipole

between two polar molecules

Dipole – Induced Dipole

between a polar & a non-polar molecule

Instantaneous dipole – induced dipole or Dispersion

between two non-polar molecules

Hydrogen BondingIs the STRONGEST Intermolecular Force!!Occurs only if hydrogen in a molecule is bonded to a F, O or a N atom

Ice Liquid

Hydrogen Bonding Hydrogen bonds keep water in the liquid

state over a wide range of temperatures

than is found for any other molecule of its

Size

Hydrogen bonded to either of F, O or N,

(the 3 most electronegative elements in

the periodic table); is highly deficient in

electrons and so is easily attracted by the

lone pair of electrons on F, O or N of

another molecule

Hydrogen bonds account for the high boiling point of water

Expansion of Ice

Ice expands when water freezes compared to

most substances that contract on freezing

This is due to the presence of hydrogen

bonding between water molecules

Ice as a result gets an open structure, which

makes it less dense than liquid water.

Ponds Freezing

Solid water (ice) has a lower density than liquid water

Why can liquid water change to vapor at room temperature?

High energy molecules escape leaving the

low energy molecules behind

Evaporation occurs at all temperatures

# p

arti

cles

low KE ave KE high KE

Volatile Substances

Easily evaporate

Have weak attractive forces between the

particles

Have low boiling point

Have high vapor pressure

Non-volatile substances

Do not easily evaporate

Have strong attractive forces between

the particles

Have high boiling point

Have Low vapor pressure

Boiling

Boiling occurs when

Vapor Pressure = Atmospheric Pressure

When Vapor Pressure = 760 mmHg,

Boiling Point of water = 100oC

Boiling and Evaporation

What is the difference between evaporation and boiling?

Evaporation: occurs at any temperature; high energy molecules escape

Boiling: occurs when vapor pressure = atmospheric pressure