Simple molecular structures

Question 1

What holds together simple molecular structures?

Answer 1

Weak inter molecular forces, those being:
1. Van der Waals’ forces
2. Permanent dipole-dipole interactions
3. Hydrogen bonding

Question 2

Van der Waals’ forces

Answer 2

Van der Waals’ forces are weak forces of attraction that hold the non-polar molecules together
* Due to constant motion of molecules, electron cloud on one side of the molecule is more than the other side
* This gives rise to a dipole
* This dipole induces dipoles on neighbouring molecules

Question 3

What affects Van der Waals’

Answer 3

• The larger the molecule (more electrons) the stronger the Van der Waals forces
• The more branched an organic molecule, the weaker the van der waals forces

Question 4

Permanent Dipole-Dipole attractions

Answer 4

• When 2 polar molecules arrange, opposite charges align to give small, very weak electrostatic attractions between molecules.
• Holds together simple molecular structures of polar molecules together
• Stronger than Van der Waals

H - Cl H - Cl H - Cl H - Cl
δ+ δ- | δ+ δ- | δ+ δ- | δ+ δ-

Question 5

Hydrogen Bonding

Answer 5

• Occurs between 2 molecules when one molecule is covalenlty bonded with F, O or N and another molecule having fluorine, oxygen or nitrogen bonded with a lone pair of electrons
  Example

Question 6

Anomalous properties of water

Answer 6

• Higher melting point and boiling point than expected due to hydrogen bonds that need to be broken
• Ice floating on water
  -> Hydrogen bonds hold water molecules apart in a solid lattice, making it less dense
• Surface tension due to hydrogen bonding