Q4. Arrange H2O, H2S and H2Se in order of boiling point and explain the position of H2O.
H2S (213 K) < H2Se (232 K) < H2O (373 K). From H2S to H2Se the boiling point rises with molecular size (stronger van der Waals forces). H2O is far out of line because its molecules are joined by hydrogen bonds.
Q5. Which of these can form hydrogen bonds with water molecules: CH3CH2OH, CH3CH2CH3, CH3COCH3?
Ethanol (it has O–H and O) and propanone (its C=O oxygen accepts a hydrogen bond from water’s O–H). Propane has no F, O or N and cannot.
Q6. In which physical state is hydrogen bonding greatest, and in which least?
Greatest in the solid state and least in the gaseous state (NCERT Section 4.9.1, page 132). In the solid each molecule is locked in the positions that give the most hydrogen bonds (four per water molecule in ice, which is why ice is open and less dense than water); in the liquid bonds keep breaking and re-forming; in the gas the molecules are far apart and few hydrogen bonds exist. Strictly, this compares intermolecular hydrogen bonding in one substance; intramolecular bonds need not follow it (reasoned).
Q7. Explain the strong hydrogen bond in KHF2. Does the F/O/N guideline imply that the hydrogen dichloride ion cannot exist?
HF2− is held together by a very strong F–H···F hydrogen bond between HF and F−. The F/O/N guideline does not forbid other hydrogen bonds: the modern IUPAC definition is broader (Wikipedia, “Hydrogen bond”). Whether a salt of HCl2− is stable depends on the cation and conditions, not on electronegativity alone (reasoned).
Q8. Glycerol, CH2(OH)CH(OH)CH2(OH), is a thick (viscous) liquid while propan-1-ol, CH3CH2CH2OH, flows easily. Explain.
Each glycerol molecule has three O–H groups and forms many hydrogen bonds with its neighbours, building a network that resists flow. Propan-1-ol has only one O–H group and forms far fewer hydrogen bonds.