3F.5

[Amy Luu 2G]

Suggest, giving reasons, which substance in each of the following pairs is likely to have the higher normal melting point: CHI3 or CHF3. The solution says CHI3 will have the higher melting point since it has larger london forces. However, I am unsure why. How do you determine which has the stronger london force? I drew the lewis structures and they were both tetrahedral. Do you have to look at the electronegativity?

[sarahsalama2E]

CHI3 has a higher boiling point because it has stronger/more london dispersion forces. Although F is more electronegative than I, the strength of london disperson forces takes precedence when comparing boiling, melting, freezing points, and etc. Because I is a bigger atom than F, it has more surface area therefore, it has a larger electron could and is more polarizable. Because it is more polarizable, it has stronger london dispersion forces, so it then has a higher boiling point.

[Brian_Ho_2B]

Suggest, giving reasons, which substance in each of the following pairs is likely to have the higher normal melting point: CHI3 or CHF3. The solution says CHI3 will have the higher melting point since it has larger london forces. However, I am unsure why. How do you determine which has the stronger london force? I drew the lewis structures and they were both tetrahedral. Do you have to look at the electronegativity?

To determine relative strengths of london dispersion forces, look at the polarizability of the atoms in the molecule. Because the strength of LDF is proportional to the polarizability, we can use that as a basis for comparison. Atoms that are larger have more electrons, and more electrons means higher polarizability. Iodine is bigger than chlorine, thus has more electrons, thus has more polarizability, thus has larger LDF.