Intermolecular forces

What are intramolecular forces?

These are forces within a molecule and are typically covalent bonds.

What are intermolecular forces?

These are forces between molecules and the 3 types of intermolecular forces are:

• London forces (instantaneous dipole-induced dipole)
• Permanent dipole-dipole interactions
• Hydrogen bonds

The term van der Waals’ forces is also used to describe intermolecular forces, however, this term has also been used to describe just London forces, so it is best avoided.

• For more specific information on this, go to this revision note: ‘Intermolecular Forces - Introduction’

London forces (instantaneous dipole-induced dipole)

Also known as instantaneous dipole-induced dipole forces (or induced dipole-dipole forces or dispersion forces)).

These forces exist between all molecules and act between instantaneous and induced dipoles and can be explained as follows:

1. Electrons moving within a molecule cause an instantaneous dipole as the charge is not evenly spread across the molecule
2. This instantaneous dipole induces a dipole on a neighbouring molecule
3. The induced dipole will also induce other dipoles on neighbouring molecules, the dipoles are then attracted to each other:

The electron cloud is continually moving, so the dipoles are temporary.

• More information: ‘Intermolecular Forces - Introduction’

Permanent dipole-permanent dipole interactions

Permanent dipoles exist in polar molecules. The δ- end of the dipole on one molecule will be attracted to the δ+ end of the dipole on another molecule giving rise to permanent dipole-permanent dipole interactions.

• More information: ‘Intermolecular Forces - Introduction’

Hydrogen bonds

These are the strongest type of intermolecular force.

A hydrogen bond is a particular type of permanent dipole-permanent dipole interaction that occurs between molecules that contain:

• An electronegative atom with a lone pair of electrons (O, N or F)
• A hydrogen atom attached to O, N or F, e.g. H一O, H一N or H一F

The hydrogen bond exists between the lone pair of electrons on the O, N or F atom and the δ+ H atom in another molecule and is shown by a dashed line. For example, the hydrogen bond between water molecules is shown as: 

• More information: ‘Intermolecular Forces - Introduction’

What compounds form hydrogen bonds?

The following compounds are able to form hydrogen bonds:

• Alcohols
• Ammonia
• Amines
• Carboxylic acids 
• Hydrogen fluoride
• Proteins 
   
• More information: ‘Molecular Interactions'

What are the relative strengths of forces?

Intermolecular vs intramolecular forces

Intramolecular forces are stronger than intermolecular forces

Intermolecular forces

Hydrogen bonds are the strongest, followed by permanent dipole-permanent dipole interactions and finally, London (instantaneous dipole-induced dipole) forces are the weakest. 

For small molecules with the same number of electrons, permanent dipole-permanent dipole interactions are stronger than London forces.

• More information: ‘Intermolecular Forces - Introduction’

Why does water show anomalous properties?

The physical properties of water are influenced by hydrogen bonding.

High melting point & boiling point

Water has both London forces and hydrogen bonds between its molecules. A lot of energy is required to break the hydrogen bonds, so it has a much higher boiling point and melting point than would be expected if only London forces existed between the molecules.

If the enthalpy change of vaporisation of water (i.e. the energy required to boil a substance) is compared with other Group 16 hydrides, water does not follow the trend of the other hydrides - it requires much more energy to vaporise than would be expected, based on the other Group 16 hydrides. This is because hydrogen bonds are present in water, but not in the other hydrides.

Density

In general, solids are denser than their liquids as the particles are more closely packed.

However, ice is less dense than water as hydrogen bonds hold the water molecules apart in an open lattice. Hydrogen bonds are relatively long, so the molecules are further apart in liquid water than in ice. So ice will float on water.

• More information: ‘Molecular Interactions’

Physical Properties and intermolecular forces

How does chain length affect the boiling point of alkanes?

As chain length increases, the number of electrons increases. This increases the magnitude of the instantaneous and induced dipoles so the London forces are greater in larger molecules. More energy is required to overcome them so the boiling point increases with the chain length of alkanes.

What is the effect of branching in the carbon chain on the boiling temperatures of alkanes?

The more branching an alkane has, the lower its boiling point as there is less contact between adjacent molecules. This means the London forces are weaker than in a straight chain alkane, so less energy is needed required to overcome them.

What is the volatility of alcohols compared to alkanes with a similar number of electrons?

Alcohols have a higher boiling point than alkanes with a similar number of electrons. Alcohols have both London forces and hydrogen bonds present whereas alkanes only have London forces acting between molecules. As the hydrogen bonds are stronger than London forces, more energy is required to separate alcohol molecules than is required to separate alkane molecules.

What is the trend in boiling temperatures of the hydrogen halides HF to HI?

The boiling point of hydrogen halides is shown in the table:

From HCl to HI, the boiling point increases as the molecules get larger and the London forces become stronger. HF has the highest boiling point because, in addition to London forces, hydrogen bonds are also present between the HF molecules which require a large amount of energy to overcome.

• More information: ‘Anomalous Properties of Water’

How are solvents chosen?

As a general rule, non-polar substances dissolve in non-polar solvents and polar substances dissolve in polar solvents. However, giant covalent molecules are generally insoluble as the energy required to overcome all of the covalent bonds within the lattice structure is too great.

Non-polar molecules often dissolve in non-polar solvents, e.g. hydrocarbons, as they form London forces between the solvent and the solute.

Polar covalent molecules often dissolve in polar solvents due to the permanent dipole-permanent diploe interactions or hydrogen bonds which are formed between the solvent and the solute.

Alcohols become less soluble as the carbon chain becomes longer as the polar part of the molecule becomes a much smaller part of the molecule.

Halogenoalkanes contain a polar bond but are only partially soluble in water as they do not form hydrogen bonds as there are no H一O, H一N or H一F bonds within the molecule.

Most ionic compounds dissolve in polar solvents, such as water as the polar molecule can break down the ionic lattice. The polar molecules are then attracted to and surround the ions.

• More information: ‘Solvent Choice’

Common questions and answers

Does diamond have intermolecular forces? 

A diamond has a giant covalent lattice so is not made up of individual molecules but atoms covalently bonded together, so diamond does not have intermolecular forces.

Does magnesium have intermolecular forces?

Magnesium is a metal, so exists as a giant metallic lattice. It is not made of molecules, therefore it does not have intermolecular forces.

Does sodium chloride have intermolecular forces?

Sodium chloride is a giant ionic lattice so is made from oppositely charged ions held together by strong ionic bonds, it is not made from molecules, therefore, it does not have intermolecular forces.

Does polyethene have intermolecular forces?

Polyethene is a polymer and is essentially a very long hydrocarbon molecule. It has London forces between each polymer chain as the molecule.

This is a quick summary of some key concepts on intermolecular forces - remember to go through the full set of revision notes, which are tailored to your specification, to make sure you know everything you need for your exams!