2.2.11 Intermolecular Forces

Question 1

What are IM forces and why do they occur?

Answer 1

An attractive force between neighboring molecules. IM = ‘between molecules.’ They occur due to random movements of electrons within the shells of the atoms in molecules.

Question 2

Name two types of IM forces.

Answer 2

1. H bonding

2. Van der walls forces (including weaker London dispersion forces)

Question 3

What forces does the term ‘Van der waals’ forces describe?

Answer 3

1. Dipole-dipole interactions including permanent dipole-induced dipole interactions and permanent dipole-permanent dipole interactions and
2. London dispersion forces which are instantaneous induced dipole-dipole interactions.

Question 4

What are permanent dipole-induced dipole interactions?

Answer 4

There is a delta + and a delta -, there is a permanent dipole as permanent bonds are present. They shift nearby electrons and cause them to become slightly polar - induced. Half of the other non polar molecule is delta + and half is delta -.

Question 5

What are permanent dipole-permanent dipole interactions?

Answer 5

Molecules with permanent dipoles are attracted to each other. Ie a delta - Cl will be attracted to the delta + Na of another NaCl molecule.

Question 6

What is another term for London forces?

Answer 6

Dispersion forces.

Question 7

How do dispersion forces occur?

Answer 7

Caused by constant random movement of electrons in atoms’ shells - all could be on one side of the atom. So:

1. At any moment, there will be an instantaneous dipole across the molecule.
2. This induces a dipole in neighboring molecules, like a chain reaction.

Question 8

What determines the size of dispersion forces? What is the correlation?

Answer 8

The size of London forces increases with increasing numbers of electrons. Greater e- count = larger induced dipoles , greater attractive forces between molecules.

Question 9

What is the effect of London forces on boiling points? Where can you see this and how?

Answer 9

Non polar molecules just have London forces, which are weak and so have low melting pts. Best shown in noble gas group:
1. No of e-‘s increases, London forces do
2. So more electrons down group = higher boiling point
ONLY AS NO DIPOLES ARE PRESENT
ONLY AS NO DIPOLES ARE PRESENT
ONLY AS NO DIPOLES ARE PRESENT
Otherwise it would be impossible to liquidify noble gasses.