Question

For each of the following pairs, choose the member with the lower boiling point. Explain your reason in each case. (a) \(\mathrm{NaCl}\) or \(\mathrm{PCl}_{3}\) (b) \(\mathrm{NH}_{3}\) or \(\mathrm{AsH}_{3}\) (c) \(\mathrm{C}_{3} \mathrm{H}_{7} \mathrm{OH}\) or \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OCH}_{3}\) (d) \(\mathrm{HI}(g)\) or \(\mathrm{HCl}(g)\)

Short answer

a) Sodium Chloride (NaCl) or Phosphorus Trichloride (PCl3) b) Ammonia (NH3) or Arsenic Trihydride (AsH3) c) Propanol (C3H7OH) or Methoxyethane (C2H5OCH3) d) Hydrogen Iodide (HI) or Hydrogen Chloride (HCl) Answer: a) PCl3 has a lower boiling point. b) AsH3 has a lower boiling point. c) C2H5OCH3 (Methoxyethane) has a lower boiling point. d) HCl has a lower boiling point.

Step-by-step solution

(a) Sodium Chloride (NaCl) or Phosphorus Trichloride (PCl3) Boiling Point Comparison

Sodium chloride (NaCl) is an ionic compound with a strong electrostatic attraction between positively charged sodium (Na+) and negatively charged chloride (Cl-) ions. Phosphorus trichloride (PCl3) is a covalently bonded molecule with polar covalent bonds due to the difference in electronegativity between phosphorus and chlorine atoms. Among ionic and covalent compounds, ionic compounds generally have higher boiling points than covalently bonded compounds due to their stronger electrostatic interactions. Hence, \(\mathrm{NaCl}\) should have a higher boiling point than \(\mathrm{PCl}_{3}\). Therefore, \(\mathrm{PCl}_{3}\) has the lower boiling point of the two.

(b) Ammonia (NH3) or Arsenic Trihydride (AsH3) Boiling Point Comparison

Ammonia (NH3) and arsenic trihydride (AsH3) are both covalent compounds. However, ammonia has hydrogen bonding between its molecules due to the presence of a highly electronegative nitrogen atom bonded to hydrogen atoms. In contrast, arsenic trihydride does not have hydrogen bonding and exhibits only weaker London dispersion forces. Generally, compounds with hydrogen bonding have higher boiling points than those with only London dispersion forces because hydrogen bonds are stronger. Hence, \(\mathrm{NH}_{3}\) should have a higher boiling point than \(\mathrm{AsH}_{3}\). Therefore, \(\mathrm{AsH}_{3}\) has the lower boiling point of the two.

(c) Propanol (C3H7OH) or Methoxyethane (C2H5OCH3) Boiling Point Comparison

Propanol (\(\mathrm{C}_{3} \mathrm{H}_{7} \mathrm{OH}\)) is an alcohol that has hydrogen bonding between its molecules due to the presence of an oxygen atom bonded to a hydrogen atom. Methoxyethane (\(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OCH}_{3}\)) is an ether and does not exhibit hydrogen bonding, but it does have dipole-dipole interactions. Compounds with hydrogen bonding usually have higher boiling points than those with dipole-dipole interactions because hydrogen bonds are stronger. Hence, \(\mathrm{C}_{3} \mathrm{H}_{7} \mathrm{OH}\) should have a higher boiling point than \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OCH}_{3}\). Therefore, \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OCH}_{3}\) has the lower boiling point of the two.

(d) Hydrogen Iodide (HI) or Hydrogen Chloride (HCl) Boiling Point Comparison

Hydrogen iodide (HI) and hydrogen chloride (HCl) are both covalent compounds with polar covalent bonds, so they exhibit dipole-dipole interactions between their molecules. However, HI also exhibits more significant London dispersion forces than HCl due to its larger molecular size and greater electron cloud distortion. Generally, compounds with stronger London dispersion forces (as well as other intermolecular forces) have higher boiling points. Therefore, \(\mathrm{HI}(g)\) should have a higher boiling point than \(\mathrm{HCl}(g)\). As a result, \(\mathrm{HCl}(g)\) has the lower boiling point of the two.