2.2.11 Intermolecular Forces

Question 1

Why do intermolecular forces occur?

Answer 1

Due to constant random movements of the electrons within the shells of the atoms in molecules. They do not involve any sharing or transfer of electrons.

Question 2

How do permanent dipole-induced dipole interactions occur?

Answer 2

If a molecule has a permanent dipole and it is near to other molecules that are non-polar, it is able to cause electrons in the shells in the nearby molecule to shift slightly. This causes the non-polar molecule to become slightly polar and then an attraction occurs.

Question 3

How do permanent dipole-permanent dipole interactions occur?

Answer 3

Molecules with permanent dipoles will be attracted to other molecules with permanent dipoles. Opposite ends will be attracted to each other.

Question 4

What force can attract non-polar molecules to each other?

Answer 4

London (dispersion) forces.

Question 5

How are London (dispersion) forces created?

Answer 5

• London (dispersion) forces are caused by the constant random movement of electrons in the atoms’ shells. This movement unbalances the distribution of charge within the electron shells.
• At any moment, there will be an instantaneous dipole across the molecule.
• The instantaneous dipole induces a dipole in neighbouring molecules, which in turn induce further dipoles of their neighbouring molecules.
• The small induced dipoles attract one another, causing weak intermolecular forces known as London (dispersion) forces or instantaneous dipole-induced dipole forces.

Question 6

How does number of the electrons affect London (dispersion) forces?

Answer 6

The greater the number of electrons, the larger the induced dipoles and the greater the attractive forces between molecules. This results in greater London (dispersion) forces.

Question 7

What are London (dispersion) forces?

Answer 7

Attractive forces between induced dipoles in neighbouring molecules.