Intermolecular Forces

Section 6.5

Introduction

We will consider ionic and covalent bonds between atoms

If there are no attractive forces between molecules, then all substances would be gases

There must be some force that attracts molecules to other molecules that does not create a real bond

These are known collectively as intermolecular forces

Types of Intermolecular Forces

In order of increasing strength: van der Waals' forces (AKA London forces or

dispersion forces) Dipole-dipole forces Hydrogen bonding

van der Waals' Forces

Exist between all species Is the only intermolecular force between non-

polar species Result of temporary (instantaneous) dipoles

when one side of the molecule becomes partially negative because of the random motion of electrons

Continued

This results in a weak dipole moment that then attracts other molecules

Strength of this force increases with molar mass as more electrons are available for temporary dipoles

Effect of van der Waals' Forces on Boiling Point

Higher boiling point means more energy is needed to break the intermolecular forces

As the molar mass increases, the boiling point increases

As the surface area increases, the boiling point increases (again, more electrons available)

More elongated the molecule, the stronger the van der Waals' forces, so the higher the boiling point

Continued

Sometimes the van der Waals' force can be quite strong as in the case of some polymers that have high mass and are very long molecules

Dipole-dipole forces Dipole moment: a measure of the polarity of a

molecule Arrows are used to represent the polarity of the

bond (heading toward the partially negative part)

The permanent dipoles formed cause electrostatic attraction between molecules that have the permanent dipoles

Stronger than van der Waals' forces in molecules of similar size

Continued

Effect of Dipole-dipole Forces on Boiling Point

Compared to molecules of similar mass with only van der Waals' forces, much higher boiling point

Polar molecules have van der Waals' forces and dipole-dipole forces

Stronger intermolecular force: higher boiling point

Hydrogen Bonding

Not a “true” bond Occurs when hydrogen is bonded to highly

electronegative, small atoms, like N, O, or F Creates a very high dipole moment as the more

electronegative atom attracts the electrons, leaving the hydrogen very partially positive

Can be thought of as partway between a dipole-dipole force and a dative covalent bond

Continued

For maximum strength, the 2 atoms and the hydrogen should be in a straight line

When X and Y are N, O, or F: Xδ-: hydrogen bond Hδ+ - Yδ-

Usually much stronger than other intermolecular forces

H-bonding

Effect of H-bonding on Boiling Point

Consider HF and HCl: HF has van der Waals' forces, dipole-dipole

forces, and H-bonding HCl has van der Waals' forces and dipole-

dipole forces Boiling point of HF is higher than HCl More energy is needed to break the

intermolecular forces

Consider H2O and H

2S

H-bonding present in H2O, but not in H

2S

Boiling point of water is higher than H2S

H-bonding allows water to form tetrahedral shapes when it is a liquid

H-bonding allows water to form hexagons when it freezes, so ice is less dense than liquid water

H-bonding in water also forms temporary hexagon arrays on the surface of water, giving it a high surface tension

Consider NH3 and PH

3

NH3 can hydrogen bond with itself and with

the water when the ammonia is aqueous

PH3 can only hydrogen bond with water, not

with itself Ammonia has the higher boiling point

Biological Importance

The weak bond between the nitrogenous bases in nucleic acids is a H-bond

Occurs between thymine and adenine as well as between cytosine and guanine

Easily broken by enzymes