PR - THE POLYMER REVOLUTION

Forces between Molecules

Dipoles and Intermolecular Forces

Permanent Dipole - Permanent Dipole Attractions

Forces Between Molecules

When liqids change to gases, energy is required to bring this about. This energy is required to overcome forces between the molecules, i.e. to separate them. In the series of alkanes, the larger the molecule, the higher the boiling point. Straight-chained alkanes have a higher boiling point than branched chains. The intermolecular forces between the molecules arise due to the presence of dipoles in the molecules.

A dipole arises when charge is not shared equally between two atoms. This is shewn in hydrogen chloride below:

δ+		δ-
H	-	Cl

When a molecule has a dipole it is said to be polarised. There are several ways a molecule can become polarised.

Permanent Dipoles: This occurs when a molecule has two atoms bonded together which have substantially different electronegativities. Hydrogen chloride has a permanent dipole because chlorine is more electronegative than hydrogen.
Instantaneous Dipoles: Molecules containing atoms of similar electronegativities bonded together do not have a permanent dipole, e.g. chlorine, Cl₂. However, an instantaneous dipole is set up due to the continuous movement of electrons. The dipole only lasts for an instant before changing its position.
Induced Dipoles: If an unpolarised molecule finds itself next to a dipole, the unpolarised molecule may get a dipole induced in it. This would happen if hydrogen chloride comes near say chlorine.

δ+		δ-		δ+		δ-
Cl	-	Cl		H	-	Cl

Dipoles and Intermolecular Forces

All intermolecular forces arise from the attractive forces between dipoles. There are three types:

Permanent dipole - permanent dipole, where two or more permanent dipoles attract each other, e.g. in HCl.
Permanent dipole - induced dipole, where a permanent dipole induces a dipole in another molecule, then they attract each other, e.g. HCl and Cl₂.
Instantaneous dipole - induced dipole, where an instantaneous dipole induces a dipole in another molecule, e.g. Cl₂. These are often called Van der Waals forces, and these attractions occur between all molecules.

How do we know if there is a dipole present in a molecule? Bond polarity depends on any difference in electronegativity between the atoms in the bond. A table of electronegativity values for different elements was drawn up by Pauling.

Atom	Electronegativity
F	4.0
O	3.5
Cl	3.0
N	3.0
Br	2.8
I	2.5
S	2.5
C	2.5
H	2.1

Elements in the top right of the Periodic Table are the most electronegative.

Note that carbon and hydrogen have similar electronegativities, hence a C - H bond is taken as non-polar.

Permanent Dipole - Permanent Dipole Attractions

These will usually occur in molecules which contain the elements, F, O, N and Cl. Examples of this are aldehydes, ketones, esters, chlorinated hydrocarbons, PVC.

In propanone:

Propanone structure and intermolecular forces

Hydrogen Bonding

Hydrogen bonding is a particularly strong intermolecular force that involves three features:

a large dipole between a hydrogen atom and a highly electronegative atom, e.g. O, N or F
the small hydrogen atom which can get very close to other atoms
a lone pair of electrons on another O, N or F atom, with which the positively charged H atom can line up.

Hydrogen bonds are much stronger than other types of intermolecular forces. They are however much weaker than covalent bonds.

Water is a liquid due to the hydrogen bonds between the molecules.