Structure and Bonding

Question 1

Van der Waals’ forces exist between all molecules.
Explain how these forces arise.

Answer 1

• electron movement in one molecule creates a temporary dipole
• that induces a dipole in another molecule
• the partial + on one molecule attracts the partial - on another molecule

Question 2

Suggest why methaneselenol (CH3SeH) has a higher boiling point than methanethiol (CH3SH).

Answer 2

-methaneselenol has a higher MR, so more electrons.
- which means the VDW forces between molecules is stronger

Question 3

The C-Br bond is polar
Explain why CBr4 is not a polar molecule.

Answer 3

• the CBr4 molecule is completely symmetrical
• so the dipoles cancel out and there is no overall dipole

Question 4

Suggest, in terms of the intermolecular forces for each compound, why CBr4 has a higher boiling point than CHBr3.

Answer 4

• CBr4 has only VDW intermolecular forces
• CHBr3 has both VDW and permanent dipole-dipole intermolecular forces
• VDW forces between CBr4 molecules are stronger than the VDW and dipole-dipole forces between CHBr3
• because CBr4 has a higher MR and so more electrons

Question 5

Context: electronegativity.

Explain how permanent dipole-dipole forces arise between hydrogen chloride molecules.

Answer 5

• Cl more electronegative than H
• so the bond is polar and there is a permanent overall dipole
• the partial + on one molecule attracts the partial - on another