Forces between molecules (3.1.3.7)

What are the three main types of intermolecular forces in order of strength?

1) Hydrogen bonding

2) Permanent dipole-dipole forces

3) London forces

How are London forces induced?

1) In non-polar molecules electrons are constantly and randomly moving

2) During this the electrons can be on one side of the molecule more than the other

3) This causes a temporary dipole to form, inducing dipoles in neighbouring molecules

What affects the strength of London forces?

The more electrons in a molecule, the higher the chance temporary dipoles will form, making the London forces stronger so more energy is required to break them.

What are polar molecules and what causes permanent dipole-dipole forces?

Polar molecules - asymmetrical molecules that have significant differences in electronegativity between the atoms

If the molecule is symmetrical there is an even distribution of electron density so the dipoles cancel out

Common compounds with permanent dipoles have C-Cl, C-F, C-Br, H-Cl, C=O

Polar molecules have permanent dipoles so the molecule will always have a positively and negatively charged end

What are the conditions needed for hydrogen bonding to occur?

1) Hydrogen must be bonded to nitrogen, oxygen or fluorine, the most electronegative elements

2) There must be a lone pair of electrons as the hydrogen bonds form between the hydrogen atom and lone pair

Why does NH₃ have a lower melting point than H₂O?

NH₃ only has one lone pair so 1 hydrogen bond can form per molecule, where as H₂O has two lone pairs so 2 hydrogen bonds can form per molecule

Why is ice less dense than water?

In ice, water molecules are held further apart as there are more hydrogen bonds which are more ordered