Question

Explain in terms of forces between structural units why (a) HI has a higher boiling point than HBr. (b) \(\mathrm{GeH}_{4}\) has a higher boiling point than \(\mathrm{SiH}_{4}\). (c) \(\mathrm{H}_{2} \mathrm{O}_{2}\) has a higher melting point than \(\mathrm{C}_{3} \mathrm{H}_{8}\). (d) \(\mathrm{NaCl}\) has a higher boiling point than \(\mathrm{CH}_{3} \mathrm{OH}\).

Short answer

Question: Explain the differences in boiling and melting points for the following molecule comparisons based on the forces between their structural units: (a) HI vs. HBr (b) GeH4 vs. SiH4 (c) H2O2 vs. C3H8 (d) NaCl vs. CH3OH. Answer: (a) HI has a higher boiling point than HBr due to stronger London dispersion forces between its molecules, as iodine is larger and more polarizable than bromine. (b) GeH4 has a higher boiling point than SiH4 because germanium is larger and more polarizable than silicon, leading to stronger London dispersion forces between GeH4 molecules. (c) H2O2 has a higher melting point than C3H8 due to the presence of strong hydrogen bonding between its molecules, while C3H8 only experiences weaker London dispersion forces. (d) NaCl has a higher boiling point than CH3OH because of the strong ionic bonds present in its structure, while CH3OH experiences weaker hydrogen bonding among its molecules.

Step-by-step solution

(a) HI vs. HBr Boiling Point

Both HI and HBr are covalent compounds formed by a hydrogen atom and a halogen atom (iodine and bromine, respectively). The main force between their molecules is the London dispersion force, which increases with the size and polarizability of the molecules. Since iodine is larger and more polarizable than bromine, the dispersion forces between HI molecules are stronger than those between HBr molecules, leading to a higher boiling point for HI.

(b) GeH4 vs. SiH4 Boiling Point

KeH_{4}\( and SiH_{4}\) are both covalent compounds formed by a group 14 element (Germanium and Silicon, respectively) and hydrogen atoms. The primary force between their molecules are also the London dispersion forces. Since germanium is larger and more polarizable than silicon, the dispersion forces between GeH_{4}\( molecules are stronger than those between SiH_{4}\) molecules, leading to a higher boiling point for GeH_{4}.

(c) H2O2 vs. C3H8 Melting Point

H_{2}O_{2}\( (hydrogen peroxide) is a covalent compound with hydrogen bonding between its molecules, while C_{3}H_{8}\) (propane) is a non-polar covalent compound with only London dispersion forces between its molecules. Hydrogen bonding is a much stronger force compared to London dispersion forces. The hydrogen bonding between H_{2}O_{2}\( molecules is responsible for its higher melting point compared to C_{3}H_{8}\) that only experiences weaker London dispersion forces.

(d) NaCl vs. CH3OH Boiling Point

NaCl (sodium chloride) is an ionic compound with strong ionic bonds between its positive (Na+) and negative (Cl-) ions, while CH_{3}OH (methanol) is a covalent compound with hydrogen bonding between its molecules. Although hydrogen bonding is a strong force for covalent compounds, it is still weaker than ionic bonding. The strong ionic bonds in NaCl are responsible for its higher boiling point compared to CH_{3}OH, which experiences the weaker hydrogen bonding.