Intermolecular Forces

What is an intermolecular force?

An intermolecular force is an attractive force between neighbouring molecules.

Bond Type [Intermolecular forces]                     Relative Strength
Ionic and covalent bond                                    1000
Hydrogen bonds                                                 50
Dipole-dipole forces                                           10
Van-der-Waals forces                                         1

• Van-der-Waals forces exist between all covalent molecules, whether polar or non-polar.

What causes Van-der-Waals forces?

• The uneven distribution of electrons creates an instantaneous dipole on one atom.
• This induces dipoles on nearby molecules.
• Molecules are now attracted to each other by weak forces
• The greater the number of electrons, the stronger the Van-der-Waals force.
• The more Van-der-Waals forces, the higher the melting and boiling point.

 

 

Permanent Dipole

 

Permanent dipole-dipole forces: is a weak attractive force between permanent dipoles in neighbouring POLAR molecules.

 

 

 

• There is a dipole-dipole interaction between a partially positive atom and a partially negative atom of another molecule.
• This example shows the permanent dipole between two hydrochloric acid molecule



Hydrogen Bonds

A hydrogen bond is a strong dipole-dipole attraction between:

• An electron deficient hydrogen atom
• And a lone pair of electrons on a highly electronegative atom on a different molecule          (e.g. N, O, F)

It is the strongest intermolecular force.

 

 

 

 

 

 

Ice Lattice (open network of water molecules)

• Hydrogen bonds hold the H2O molecules apart
• Each oxygen atom has 4 bonds (2 covalent, 2 hydrogen bonds)
• Hydrogen bonds are slightly longer
• Open structure made up of rings of 6 oxygen atoms
• Covalent bonds are stronger than the hydrogen bonds.

Q: Suggest why ice has a higher melting point than solid ammonia. (2)

A:

• Ice has stronger hydrogen bonds than ammonia
• O has two lone pairs/N has one OR YOU COULD SAY - O is more electronegative than N

Here is the electronegativity's of the elements in the periodic table:

 

 

Q: Nitrogen can form a fluoride, NF₃ which has a permanent dipole. Explain why NF₃ has a permanent dipole. (2)
A:

• It has a permanent dipole because F is more electronegative than N.
• Also the shape of the molecule is non-symmetrical as NF₃ is non-symmetrical as it is trigonal pyramidal.
• Therefore the dipoles do not council each other out so it has a permanent dipole.