Intermolecular Forces

3.5 Other bonding possibilities

There are three types of intermolecular force van der Waals dipole-dipole forces hydrogen bonding

Dipoles:

Individual bonds can be polar, but molecules with polar bonds can also have a dipole moment caused by all of the polar bonds in the molecule.

What is a turning moment?

Turning moments are found where asymmetric forces operate

Van der Waals forces

What happens in non-polar species?

In the noble gases, there is no “molecular stickiness” so how can they be liquefied?

Other forces exist – called dispersive forces. They are very important in non-polar molecules and atoms, but exist in all atoms and molecules.

Consider helium. Where would you be likely to find the electrons at a moment in time?

Transient dipoles

The movement of the electrons, even in the He atom, cause an instantaneous dipole to be formed.

The time-averaged dipole moment of the atom is still zero.

This dipole, however short lived, can induce a dipole in a neighbouring atom, causing a force.

This force is always attractive but even shorter ranged (and weaker) than permanent dipole-induced dipole forces.

1 – Van der Waals’ Forces

The electrons in an atoms are moving around – at any moment in time they are unlikely to be evenly spread.

This gives the atom or molecule a temporary dipole.

This dipole moment will induce a temporary dipole in a neighboring atom by attracting/repelling its electron charge cloud.

Dipoles

What causes the intermolecular force?

An unequal distribution of electron density due to the high e-negativity of Chlorine. It is an electrostatic force

Uncharged molecule can still have an electric dipole moment.

Electric Dipoles arise from opposite but equal charges separated by a distance.

Molecules that possess a dipole moment are called Polar molecules

Dipole –dipole forcesDipole-Dipole forces exist between neutral polar molecules

Dipoles affect the boiling points of a substance – the reason why water is a liquid at room temperature

2 – Dipole-dipole forces

Polar molecules (molecules with permanent dipoles) will attract other molecules with permanent dipoles.

Dipoles and symmetry

A polar molecule is one with a permanent dipole moment. A polar molecule must have a slightly positive end opposite a slightly negative one.

What about symmetrical molecules?

If a molecule is 'spherical' enough, then each end of the molecule will have the same properties and in must be non-polar.

(Ext) Induced dipoles are the reasons for induced charge in electrostatics.

At the molecular level, the proximity of a charged particle can distort the electron cloud of another neutral atom and the two will stick together. This is called an induced dipole

3.6 Hydrogen bondingIf van der Waals forces act between all molecules and atoms (ie they are ubiquitous), what range do hydrogen bonds have and are they as strong?

• H-bonds are a special case of permanent dipole-dipole interactions.

• They are stronger than van der Waals forces and around 10% as strong as covalent bonds

• Molecules with hydrogen bonds have higher boiling points than molecules that don’t.

What are the two prerequisites for H-bonding?

A lone pair of electrons on the electronegative atom.

A hydrogen atom covalently bonded to an electronegative atom … N, O or F.

If only one of these conditions is met, you don’t get hydrogen bonding.

TaskDecide what these molecules are and whether they will take part in H-bonding

Ammonia

Has hydrogen bonds.Nitrogen is very electronegative, and it has one lone pair of electrons in ammonia

Methane

No hydrogen bonds. Carbon is not very electronegative, and it has no lone pairs of electrons in methane.

Water

Has hydrogen bonds.Oxygen is very electronegative, and it has two lone pairs of electrons in water

Hydrogen bonding in water

3 – H-bonds

These are a special case of dipole-dipole forces.

They occur when H is bonded to N, O or F (which are very electronegative).

As H only has the two electrons in the covalent bond, if they are pulled away from the H atom, the H nucleus is exposed.

The “H-bond” is the attraction between a lone pair on the N, O or F to the + H on a neighbouring molecule.

AQA way of drawing H bonds

from lone pair on N, O, F to + on H, e.g. NH3

H

NH

H

H

NH

H

AQA way of drawing H bonds

H

O

H

H

O

H

from lone pair on N, O, F to + on H, e.g. H2O

AQA way of drawing H bonds

FH FH

from lone pair on N, O, F to + on H, e.g. HF

Also in:

alcohols (O-H bonds)

carboxylic acids (O-H bonds)

amines (N-H bonds)

proteins (N-H bonds)

Boiling points of period 2 and 3 hydrides

Why do the hydrides of N, O and F buck the trend?

http://liakatas.org/chemblog/?page_id=17#Videos

HSW – Hydrogen bonds in action

Why does ironing get rid of creases in fabrics and paper?

The heat supplied, breaks the hydrogen bonds between the polymers in the fabrics. They re-form when the heat is removed

Protein chains

Summary:

Summary of intermolecular bondingEnergy/kJ

mol-1

Van der Waals