Forces Between Molecules

Full Notes

Intermolecular forces have all been covered in more detail here.

This page is just what you need to know for AQA A-level Chemistry :)

Intermolecular forces exist between molecules because of unequal electron distribution within molecules. There are three types:

• Induced dipole–dipole forces (Van der Waals / London / Dispersion forces).
• Permanent dipole–dipole interactions.
• Hydrogen bonding.

For similar sized molecules, hydrogen bonding is strongest, then permanent dipole–dipole, with induced dipole–dipole being weakest.

1. Induced Dipole–Dipole (Van der Waals / London / Dispersion) Forces

Induced Dipole–Dipole forces cccur between all molecules, but are the only force in non-polar molecules (e.g., O₂, CO₂, CH₄).

They are caused by temporary fluctuations in electron density creating instantaneous dipoles.

• Electrons are constantly moving within molecules and unequal electron distribution around a molecule creates an instantaneous dipole that can induce a dipole on a neighbouring molecule.
• Two opposite dipoles from different molecules are attracted to each other. This creates a weak force of attraction between the two molecules.

Larger molecules have more electrons in them and this means stronger induced dipole-dipole forces can arise (due to greater partial charges with the temporary dipoles).

2. Permanent Dipole–Dipole Forces

Permanent dipole-dipole forces cccur between polar molecules with permanent dipoles.

Oppositely charged partial charges from neighbouring molecules are attracted to each other. The forces are stronger than induced dipole-dipole interactions because the dipoles are permanent, not temporary.

• The greater the partial charges in molecules (higher the polarity), the stronger the permanent dipole-dipole forces of attraction

Molecules that can form permanent dipole-dipole forces have higher boiling points compared to similar sized molecules with only Van der Waals forces between them.

Example: HCl (Hydrogen Chloride)

In hydrogen chloride (HCl), chlorine (Cl) is more electronegative than hydrogen (H), creating a permanent dipole, δ⁺H — Clδ⁻.

Opposite partial charges from neighbouring molecules are attracted to each other, meaning in HCl there are permanent dipole-dipole forces between molecules.

3. Hydrogen Bonding

Hydrogen bonding is a unique type of intermolecular force that only occurs when H is directly bonded to F, O, or N atom (all highly electronegative atoms).

There is a strong attraction force between the H from an N-H, O-H or F-H bond and the lone pair of electrons on another N, O or F atom. The proton in the hydrogen atoms nucleus is left exposed on one side when bonded to N, O or F and this means a lone pair of electrons from another N, O or F atom is able to form strong forces of attraction to it)

Examples: H₂O, NH₃, HF — all have much higher boiling points than expected. However, because each molecule of water can form hydrogen bonds with up to four other molecules, the hydrogen bonding in water is stronger than for HF and NH₃

Matt’s exam tip

If an exam question asks you to draw hydrogen bonding between two OH groups (such as between water molecules), always make sure you draw and label the hydrogen bond with a dotted line that has an angle of 180 degrees between the oxygen, hydrogen and oxygen. Always show the lone pair of electrons on the oxygen and include partial charges.
For example -

Intermolecular Forces and Boiling Points

The stronger the intermolecular forces between molecules in a substance, the higher its melting and boiling point. More energy is required to overcome the stronger attractions, meaning a higher temperature is needed to break apart the molecules.

For similar sized molecules, induced dipole-dipole forces are weaker than permanent dipole-dipole forces and hydrogen bonding is strongest.

Sometimes the size of the molecule can have a bigger impact on melting and boiling point than polarity.

For example: The boiling points of the hydrogen halides increases from HCl to HI.

The polarity of the molecules decrease, meaning weaker permanent dipole-dipole forces however the size of the molecules increase, meaning stronger induced dipole-dipole forces. (HF has a relatively high boiling point because hydrogen bonding exists between molecules).

5. Hydrogen Bonding in Water and Ice

Low Density of Ice

In ice, hydrogen bonds hold water molecules in an open hexagonal structure, making it less dense than liquid water.

When ice melts, molecules have enough energy to overcome some of the hydrogen bonding between them and molecules can move closer, increasing density.

Anomalous Boiling Points

H₂O, NH₃, and HF have abnormally high boiling points due to hydrogen bonding.

For example: H₂O has a melting point of 0^oC whereas H₂S, which has a similar structure and shape, has a melting point of only -85.5^oC. This is because the strongest type of intermolecular force in H₂O is hydrogen bonding, whereas in H₂S it is only permanent dipole-dipole forces.

Summary

• Three types of intermolecular forces: London (induced dipole), permanent dipole, hydrogen bonding.
• Strength order: London < permanent dipole < hydrogen bonding.
• Hydrogen bonding explains anomalies such as ice’s low density and high boiling points of H₂O, NH₃, and HF.
• Stronger intermolecular forces = higher melting and boiling points.