Intermolecular forces https://www.youtube.com/watch?v=S8QsLUO_tgQ Explaining trends and anomalies in boiling points

Boiling points of halogens Fluorine (F2) Bp = -188oC Chlorine (Cl2) Bp = - 34oC Bromine (Br2) Bp = 59oC Iodine (I2) Bp = 184oC Question: What is the reason for the trend? 18e- 34e- 59e- 106e- The reason for the increase in Bp is that, as the size of the entire electron cloud increases so does the size of the instantaneous diplole moment. Stronger forces of attraction between molecules. So a greater amount of energy is needed to overcome these forces to vaporise.

Boiling points of isomers with same formula Task: Use molymods to make models of pentane and 2,2 dimethyl propane. Pentane Bp = 36oC 2,2 dimethyl propane Bp = 10oC Question: Why can two non-polar molecules with the same molecular formula have different Bp’s? Straight chain isomers are attracted more strongly to their neighbours because more atoms/electron clouds are in close enough proximity for instantaneous dipole forces to act. Therefore more energy is needed to overcome these attractive forces.

Boiling points of molecules with similar Mr Task: Draw Lewis diagrams then 3D representations of BCl3 and NCl3 BCl3 Bp = 12.6oC M= 117 g/mol NCl3 Bp = 71oC M = 120 g/mol Question: Why does NCl3 have a much higher Bp than BCl3? BCl3 is non-polar because, despite having polar bonds, it has a trigonal symmetrical trigonal planar shape where the bond dipoles cancel. NCl3 is polar because it has a non-symmetrical triangular pyramidal shape where bond dipoles do not cancel. Since polar molecules have stronger dipole-dipole attractive forces, NCl3 has higher Bp.

Boiling points of polar molecules Task: Complete the Practical 7.1 on p95-96 of Continuing Chemistry Why do HF, H20 and NH3 have anomalously high Bp’s? All three of these molecules have strong hydrogen bonding between molecules. A large amount of energy is required to break these forces of attraction so they have a higher Bp than they would if they only had ordinary dipole-dipole interactions.