Help with IMFs plz

Question

The electron cloud of HF is smaller than that of F2, however, HF has a much higher boiling point than F2 has. Which of the following explains how the dispersion-force model of intermolecular attraction does not account for the unusually high boiling point of HF?

a. F2 is soluble in water, whereas HF is insoluble in water.

b. The F2 molecule has a greater mass than the HF molecule has.

c. Liquid F2 has weak dispersion force attractions between its molecules, whereas liquid HF has strong ionic interactions between H+ and F- ions.

d. Liquid F2 has weak dispersion force attractions between its molecules, whereas liquid HF has both weak dispersion force attractions and hydrogen bonding interactions between its molecules.

I believe the answer is D, but can someone plz explain why B is not the answer? Thxx

Answer 1

B is not correct because in general, mass in not a determinant of boiling point, unless there are no other forces than dispersion. If dispersion forces were the only ones acting, then mass is important. In the case of F2 vs. HF we have F-F vs H-F

In F-F, this is a symmetrical molecule, with no dipole and no hydrogen bonding. So the only forces holding the F2 molecules together (intermolecular) would be dispersion.

With H-F however, there are strong hydrogen bonds among the HF molecules, in addition to dipoles and dispersion forces. Thus, HF will have a higher boiling point.

Answer 2

The answer is actually C. Fluorine is the most electronegative of all elements, but when F2 is formed, the fluorine atoms cancel each other's electronegative effects, thus producing a mainly nonpolar molecule that relies upon dispersion forces to exert electronegative effects. H, on the other hand, is less electronegative than fluorine. The HF molecule is a polar molecule: H has a partially positive charge and F has a partially negative charge. HF has a higher boiling point than F2 because more effort is needed to make H and F atoms stop being close to each other (electrostatic attraction causes close proximity).