Homework 3F.5 Part C

[Ellis Song 4I]

Suggest, giving reasons, which substance in each of the following pairs is likely to have the higher normal melting point: c) CHI3 or CHF3

The solutions manual says CHI3 would have a higher melting point because it has stronger London Dispersion forces but how is the strength of London Dispersion determined?

[Andrew Pfeiffer 2E]

It is determined that triiodomethane will have stronger London dispersion forces on the basis of polarizability. Triiodomethane is bigger than trifluoromethane, and its electrons are held more loosely (and can be distorted more easily). Thus, triiodomethane undergoes stronger London dispersion forces. Hope this helped!

[Shail Avasthi 2C]

CHI3 is a much larger molecule than CHF3, because the size of Iodine's electron cloud is much larger than the size of Fluorine's. The larger the molecule (by molar mass and by size), the greater its polarizability and therefore the greater its LDFs (induced dipole/induced dipole)