1. Which of the following best explains why HF has a higher boiling point than HCl, despite HCl having a higher molecular weight?
A) HF has stronger London dispersion forces.
B) HF forms extensive hydrogen bonding, whereas HCl does not.
C) HCl is more polar than HF.
D) HF has a lower dipole moment than HCl.
Answer: B
Explanation: HF exhibits strong hydrogen bonding due to the high electronegativity of fluorine and the small size of hydrogen, leading to stronger intermolecular forces than HCl (which only has dipole-dipole interactions). London forces (A) are weaker in HF compared to HCl, but H-bonding dominates. Polarity (C) and dipole moment (D) are not the primary reasons for the boiling point difference.
2. In which of the following pairs is hydrogen bonding NOT possible?
A) H₂O and CH₃OH
B) NH₃ and CH₃NH₂
C) CH₄ and H₂
D) HF and HCOOH
Answer: C
Explanation: Hydrogen bonding requires a hydrogen atom covalently bonded to F, O, or N and a lone pair on another electronegative atom. CH₄ (methane) and H₂ lack both conditions, so no H-bonding occurs. All other options involve at least one H-bond donor (H attached to F/O/N) and acceptor (lone pair on F/O/N).
3. Why does formic acid (HCOOH) dimerize in the vapor phase, whereas acetic acid (CH₃COOH) forms longer chains?
A) Formic acid has stronger covalent bonds.
B) Steric hindrance in acetic acid prevents cyclic dimer formation.
C) Acetic acid has weaker hydrogen bonds.
D) Formic acid lacks a methyl group, allowing symmetric dimerization.
Answer: D
Explanation: Formic acid forms a cyclic dimer via two H-bonds, while acetic acid’s bulky methyl group disrupts this symmetry, leading to linear polymeric chains. Steric hindrance (B) is a factor, but the key difference is the absence of the methyl group in formic acid (D).
4. Which of the following has the highest strength of hydrogen bonding?
A) O–H···O (in water)
B) N–H···N (in ammonia)
C) F–H···F (in HF)
D) O–H···N (in water-ammonia mixtures)
Answer: C
Explanation: F–H···F is the strongest because fluorine is the most electronegative, creating the highest polarity and shortest bond length. The order of H-bond strength is typically F–H···F > O–H···O > N–H···N.
5. Ice floats on water because:
A) Hydrogen bonds in ice break upon melting.
B) Ice has a lower density due to an open hexagonal lattice stabilized by H-bonds.
C) Water molecules expand upon freezing.
D) Covalent bonds in ice are weaker than in liquid water.
Answer: B
Explanation: Ice has a hexagonal structure with H-bonds holding molecules farther apart than in liquid water, reducing density. (A) is incorrect because H-bonds persist in liquid water (but are more dynamic). (C) is partially true but not the primary reason. (D) is false—covalent bonds remain unchanged.
6. Which compound shows intramolecular hydrogen bonding?
A) Ethanol (C₂H₅OH)
B) Ortho-nitrophenol
C) Para-nitrophenol
D) Glycerol
Answer: B
Explanation: Ortho-nitrophenol forms an intramolecular H-bond between the –OH and –NO₂ groups, preventing intermolecular bonding. Para-nitrophenol (C) forms intermolecular H-bonds, while ethanol (A) and glycerol (D) only exhibit intermolecular H-bonding.
7. Why does NH₃ have a lower boiling point than H₂O despite both having hydrogen bonding?
A) NH₃ has weaker H-bonds due to lower electronegativity of N.
B) Water forms two H-bonds per molecule, while NH₃ forms only one.
C) NH₃ is lighter than H₂O.
D) Water has a bent structure, enhancing H-bonding.
Answer: A & D (Both are correct, but A is more fundamental)
Explanation: The electronegativity difference (O > N) makes O–H···O bonds stronger than N–H···N. Additionally, water’s bent structure allows for two H-bonds per molecule, whereas NH₃ forms only one. (C) is irrelevant because molecular weight is secondary to H-bond strength.
