Four compounds are listed: CH₄, HCl, NH₃, and C₁₀H₂₂ (decane). Which pair would you expect to have the highest and lowest boiling points, respectively?
ANH₃ highest, CH₄ lowest
BC₁₀H₂₂ highest, CH₄ lowest
CNH₃ highest, HCl lowest
DHCl highest, NH₃ lowest
C₁₀H₂₂ (decane, a large nonpolar alkane) has only London dispersion forces, but with 10 carbons and a large polarizable electron cloud, its total dispersion forces are very strong — giving it a boiling point of 174°C. NH₃ has hydrogen bonding (H bonded to N) but is a small molecule, so it boils at -33°C. CH₄ is the smallest nonpolar molecule with the weakest dispersion forces, boiling at -161°C. This example illustrates that London forces are not always weak.
Question 2 True / False
A hydrogen bond is a type of covalent bond that forms between a hydrogen atom and an electronegative atom in the same molecule.
TTrue
FFalse
Answer: False
Hydrogen bonds are intermolecular (or intramolecular) attractive forces, not covalent bonds. A hydrogen bond forms between a hydrogen atom that is covalently bonded to N, O, or F (which makes the H highly δ+) and a lone pair on a neighboring N, O, or F atom (which is δ−). This is an electrostatic attraction, far weaker than a covalent bond — roughly 5-30 kJ/mol vs. 200-1000 kJ/mol for covalent bonds.
Question 3 Short Answer
Explain why water (H₂O, MW = 18 g/mol) has a much higher boiling point (100°C) than hydrogen sulfide (H₂S, MW = 34 g/mol), even though H₂S is a heavier molecule.
Think about your answer, then reveal below.
Model answer: Water forms hydrogen bonds (H bonded to O, a highly electronegative atom), which are much stronger than the dipole-dipole forces in H₂S. To vaporize water, these strong hydrogen bonds must be broken, requiring substantially more energy than breaking the weaker IMFs in H₂S. Despite H₂S being heavier and having larger London dispersion forces, its dominant IMF (dipole-dipole) is much weaker than hydrogen bonding.
This comparison is a classic demonstration that molecular weight alone does not predict boiling point — IMF type matters more. Oxygen is electronegative enough to create a strong δ+ on H in water, enabling true hydrogen bonding. Sulfur is less electronegative, so H₂S has only dipole-dipole and London forces, which are collectively much weaker. The boiling point difference (100°C vs. -60°C) reflects the energy difference in overcoming these forces.