Question

Aleluile 2. (4 pts) Rationalize the differences observed for the following substances in terms of the intermolecular forces present: a. NH3 has a boiling point of 240 K, and PH3 has a boiling point of 185 K. b. At room temperature propane (C3H8) exists as a gas, and pentane (C5H12) exists as a liquid. c. SF4 boils at -38 °C, while SF6 boils at -64 °C. d. The viscosity of methyl benzoate is three-times greater than benzene. Methyl Benzoate Benzene SO

Answer 1

2. Boiling point is the temperature at which a liquid gets converted to gaseous form at atmospheric pressure. It depends upon how strong are the intermolecular forces of attraction because to convert a liquid to gas these intermolecular forces of attraction need to be overcome.

Also in a solid, the intermolecular forces of attraction are strongest, than in liquids than in gases.

a. Both Nitrogen (N) and Phosphorus (P) are group 15 elements. As we go down the group, electronegativity decreases. N being first element of group 15 is more electronegative than P (second element of group 15), so NH₃ has hydrogen bonds (due to high difference in electronegativities of N and H) whereas PH₃ doesn't form hydrogen bonds. Thus, the intermolecular forces of attraction are higher in NH₃ than in PH₃ resulting in a higher boiling point for NH₃.

b. Both C₃H₈ and C₅H₁₂ are non-polar due to negligible difference in electronegativities of C and H. The intermolecular forces of attraction in both are London dispersion forces which depend upon the size and mass of the molecule. Larger the molecular size and mass, more are the points of contact between the molecules resulting in larger magnitude of London dispersion forces. As C₅H₁₂ has more carbons and hydrogens than C₃H₈, so the magnitude of London dispersion forces is more in former than latter. Stronger intermolecular forces of attraction in C₅H₁₂ make it a liquid at room temperature and weaker intermolecular forces of attraction C₃H₈ a gas at room temperature.

c. SF₆ has an octahedral geometry, and because of the symmetrical shape the individual S-F bond dipoles cancel out. In SF₄, the shape of the molecule is see-saw as there is a lone pair of electrons on S, resulting in the molecule to be polar. Thus the dipole-dipole forces in SF₄ cause it to have a higher boiling point than SF₆ which being non-polar will have only London dispersion forces between molecules.

d. The resistance of a fluid to flow is known as its viscosity. Larger the size of the molecule, larger is its viscosity. So methyl benzoate being larger than benzene has a higher viscosity than benzene.