Question

In each pair decide which would have a lower boiling point. Explain why. a) He or Ne b) CH _4 or CCl _4​

c) CF _4 or CF _3 H d) NH _3 or PH _3

Answer 1

The compound with the lower boiling point can be determined based on the intermolecular forces present. He would have a lower boiling point than Ne, CH4 would have a lower boiling point than CCl4, CF4 would have a higher boiling point than CF3H, and NH3 would have a higher boiling point than PH3.

Step-by-step explanation:

For each pair, the compound with the lower boiling point can be determined based on the intermolecular forces present. Intermolecular forces are forces of attraction between molecules that determine their physical properties, such as boiling point. The weaker the intermolecular forces, the lower the boiling point.

a) He or Ne: Both helium (He) and neon (Ne) are noble gases, which have very weak intermolecular forces. However, helium is smaller in size and has fewer electrons, so its dispersion forces are weaker than those of neon. Therefore, helium would have the lower boiling point compared to neon.

b) CH4 or CCl4: Methane (CH4) is a nonpolar molecule, while carbon tetrachloride (CCl4) is a polar molecule. Nonpolar molecules experience weaker intermolecular forces than polar molecules. Methane has only dispersion forces, which are weaker than the dipole-dipole forces present in carbon tetrachloride. Therefore, methane would have the lower boiling point compared to carbon tetrachloride.

c) CF4 or CF3H: Both carbon tetrafluoride (CF4) and trifluoromethane (CF3H) are polar molecules. However, carbon tetrafluoride has a symmetrical tetrahedral structure, which leads to more effective dipole-dipole interactions and stronger intermolecular forces than the more asymmetrical trifluoromethane. Therefore, carbon tetrafluoride would have the higher boiling point compared to trifluoromethane.

d) NH3 or PH3: Ammonia (NH3) and phosphine (PH3) are both polar molecules. However, ammonia has stronger intermolecular forces than phosphine due to its greater polarity and the presence of hydrogen bonding. Therefore, ammonia would have the higher boiling point compared to phosphine.

Answer 2

a) He would have a lower boiling point than Ne because Ne has stronger dispersion forces due to its larger size. b) CH4 would have a lower boiling point than CCl4 because CCl4 has stronger intermolecular forces. c) CF4 would have a lower boiling point than CF3H because CF4 has stronger dispersion forces. d) PH3 would have a lower boiling point than NH3 because NH3 can form hydrogen bonds.

Step-by-step explanation:

a) He would have a lower boiling point than Ne. Both He and Ne are noble gases, but Ne is larger in size and has more electrons, resulting in stronger dispersion forces. Stronger dispersion forces require more energy to break, leading to a higher boiling point. Therefore, He, being smaller, will have a lower boiling point compared to Ne.

b) CH4 would have a lower boiling point than CCl4. CH4 is a nonpolar molecule and experiences only weak dispersion forces. On the other hand, CCl4 is a polar molecule and experiences stronger dipole-dipole forces in addition to dispersion forces. Stronger intermolecular forces in CCl4 result in a higher boiling point compared to CH4.

c) CF4 would have a lower boiling point than CF3H. Both CF4 and CF3H are nonpolar molecules and experience only dispersion forces. However, CF4 is a larger molecule than CF3H, and larger molecules have more electrons and therefore stronger dispersion forces. Stronger dispersion forces in CF4 require more energy to break, resulting in a higher boiling point for CF4 compared to CF3H.

d) PH3 would have a lower boiling point than NH3. NH3 has a higher boiling point compared to PH3 because NH3 molecules can form hydrogen bonds due to the presence of a hydrogen atom bonded to a highly electronegative nitrogen atom. Hydrogen bonds are stronger intermolecular forces compared to the weak dipole-dipole forces present in PH3. Therefore, NH3 has a higher boiling point than PH3.