LECTURE

Intermolecular Forces and Physical Properties

IB Chemistry Power Points

Topic 4

Bonding

www.pedagogics.ca

Great thanks to

JONATHAN HOPTON & KNOCKHARDY PUBLISHING

www.knockhardy.org.uk/sci.htm

Much taken from

AN INTRODUCTION TO

BONDING

and

SHAPES OF MOLECULES

Intermolecular Forces

Intermolecular forces collectively describe the attractions BETWEEN the unit particles that make up an element or compound.

The nature of the intermolecular forces depends on the structure of the substance in question.

WARNING: be very specific in your language usage when answering “explain” type questions. The stronger the intermolecular forces, the greater the forces of attraction. This affects properties of substances such as melting and boiling point.

Complex Structures and Intermolecular ForcesIn general - intermolecular forces in complex structures are strong, chemical bonds that involve valence electrons.

• metallic bonds (in metallic structures)

• ionic bonds (in ionic compounds)

• covalent bonds (in giant covalent network structures)

Simple Structures and Intermolecular ForcesIn general - intermolecular forces in simple molecules are weak, electrostatic attractions between particles.

• Van Der Waals forces

• Dipole – Dipole interactions

• hydrogen “bonds”

Intermolecular Forces – Ionic Bonds

Ionic compounds are generally visualized as solids consisting of anions and cations held together by electrostatic attractions in a crystal lattice structure.

In molten NaCl, the ions have sufficient energy to overcome (“break”) the ionic bonds such that the ions are no longer held in fixed positions (note: they are still attracted to each other)

Intermolecular Forces – Metallic Bonds

Metallic structures are generally visualized as solids consisting of fixed cations held in place by mutual attractions for a “sea” of valence electrons.

Intermolecular Forces – Metallic Bonds

Metallic bonds are ELECTROSTATIC attractions between positive metal ions and negative valence electrons.

Heating a metal leads to an increase in the space between the metal ions (thermal expansion). Increased energy of ions, increases vibration, overcomes intermolecular forces, and allows them to move apart. When melting occurs, the ions are no longer “fixed” in position.

Intermolecular Forces – Giant Covalent Structures

The intermolecular forces in covalent networks (giant molecules, macromolecules) are covalent bonds.

In diamond, each carbon atom is covalently bonded to 4 other carbon atoms. Collectively, these 4 bonds create extremely strong intermolecular forces.

It is difficult to imagine a molten diamond – where the bonds have been broken.

more on macromolecules - allotropes

How the atoms are bonded together in macromolecules can affect the properties of the substance. Different bonded forms are called allotropes. For example, three allotropes of pure carbon are shown below.

Diamond Graphite C60

Buckminsterfullerene

more on macromolecules

Pure silicon and silicon dioxide (quartz) have similar structures to diamond.

Silicon Silicon Dioxide

Basic Structures and Physical Properties

Simple Molecular Structures and Intermolecular Forces

The intermolecular forces between simple molecules are much weaker than the covalent bonds that bind the atoms together to make the molecule itself.

Be very careful with language use here.

The strong intermolecular forces in ionic, metallic, and giant covalent structures are chemical bonds. The weak intermolecular forces between simple molecules are NOT chemical bonds but are sometimes referred to as “physical bonds”.

Simple Molecular Structures and Intermolecular ForcesIntermolecular forces between simple covalent molecules are collectively called Van der Waals forces.

Some texts, and the IBO often refer to only the weakest type of these forces as VDW forces (be aware)

VDW forces - an electrostatic attraction between opposite dipoles in two different molecules.

Non-Polar Molecules – weak VDW forces

attractions from temporary separations of charge

force of attraction increases with molecular weight Mr

Dipole-Dipole attraction between oppositely charged regions of neighboring POLAR molecules. For example HCl

Polar Molecules – stronger attractions

Hydrogen “bonding” – strongest attractions

Hydrogen bonding in Kevlar

Hydrogen bonding occurs between positive hydrogen dipoles and the lone pairs of oxygen and nitrogen atoms.

Look for O-H and N-H bonds in molecules!

Remember - Not a chemical “bond”

Hydrogen Bonding in Water

Hydrogen Bonding between Ammonia and Water

Physical Properties

Melting, boiling points, volatility, electrical

conductivity, solubility

Melting point, boiling point, volatilityThe stronger the intermolecular forces, the

greater the forces of attraction.

Results in increased melting and boiling points, and decreased volatility (ease of evaporation).

In general, metallic, ionic, and giant covalent structures have very high m.p., b.p. and low volatility.

Trends in these properties in simple covalent molecules are important to understand.

The greater the attraction between dipoles the more energy must be put in to separate molecules resulting in higher boiling points.

Covalent Molecules and Boiling Points

Mr °CCH

416 -161

SiH4

32 -117

GeH4

77 -90

SnH4

123 -50

NH3

17 -33

PH3

34 -90

AsH3

78 -55

SbH3

125 -17

Mr °CH

2O 18 +100

H2S 34 -61

H2Se 81 -40

H2Te 130 -2

HF 20 +20HCl 36.5 -85HBr 81 -69HI 128 -35

Boiling pointsof hydrides

Those in red illustrate hydrogen bonding

BOILING POINTS OF HYDRIDES

Mr

BO

ILIN

G P

OIN

T /

C°

100

0

-160

14050 100

H2O

HF

NH3

The higher than expected boiling points of NH3, H2O and HF are due to intermolecular HYDROGEN BONDING

Electrical Conductivity

Conductivity means “movable charge”.

Metals conduct: valence electrons are free to move

Molten ionic compounds, and aqueous solutions conduct: ions are free to move

Simple covalent structures do not conduct

Giant covalent structures do not conduct (exception silicon and graphite)

Summary

Conductivity means “movable charge”.

Metals conduct: valence electrons are free to move

Molten ionic compounds conduct: ions are free to move

Simple covalent structures do not conduct

Giant covalent structures do not conduct (exception silicon and graphite)