3.6 intermolecular forces, electronegativity and bond properties

Describe hydrogen bonding

• strongest form on intermolecular bonding

• Type of permanent dipole- permanent dipole bonding

• Molecule must contain highly electronegative atom with a lone pair

• H atoms overlap to must be covalently bonded

Describe the hydrogen bonding in ammonia

Describe the hydrogen bonding in water

What properties does hydrogen bonding cause water to have?

• high melting and boiling points

• High surface tension

• A lower density in ice compared to water

Explain why ice is less dense than water

• the water molecules are packed in 3D H-bonded network in a rigid lattice

• Each oxygen atom is surrounded by hydrogen atoms

• This way of packing the molecules in a solid and the relatively long bond lengths of the hydrogen bonds means that the water molecules are slightly further apart than in the liquid form

When is a covalent bond polar? What does it cause?

When 2 atoms in a covalent bond have different electronegativities:

• the electron distribution is asymmetric

• Partial charges form

Define dipole moment

• Measure of how polar a bond is

• Arrow points to delta negative of dipole

Some molecular have polar bonds but why are they not polar overall?

The molecule is arranged in such a way that individual dipole moments cancel each other out

Define Vander Waal’s forces

Intermolecular forces between molecular entities other than those due to bond formation

State the types of vander Waals’ forces

1. ID-ID forces (London Forces)

2. PD-PD forces

   1. Including hydrogen bonding

In general ionic, covalent and metallic bonding are … than intermolecular forces

Stronger

Explain ID-ID forces

• instantaneous dipole-induced dipole forces

• Electron charge cloud in non polar molecules is constantly moving

• This can cause a temporary dipole to arise

  • This can induce a dipole on neighbouring molecules

ID-ID forces increase with:

• Increasing number of electrons (and atomic number) in the molecule

• Increasing the places where the molecules come close together

How do contact points affect id-id forces and thus boiling point

Increased number of contact points means more id id forces therefore higher boiling point

Explain pd-pd forces (Permanent dipole- permanent dipole

• Polar molecules have permanent dipoles

• The molecule will always have a negatively and positively charged end

Compare the boiling points of butane and propanone

• Butane and propanone have the same number of electrons

• Butane is a non polar molecule so will have id-id forces

• Propanone is a polar molecule so will have pd-pd forces

• Therefore, more energy is needed to break the intermolecular forces between propanone.

• Therefore propanone has higher bp

Hydrogen bonds are … than pd pd forces

Stronger