Question
Compare the hydrogen bonding in HF, H₂O, and NH₃. Arrange them in order of boiling point and explain why water has a higher boiling point than both HF and NH₃, despite HF having a more polar H–F bond.
Solution — Step by Step
Hydrogen bonding occurs when a hydrogen atom is covalently bonded to a highly electronegative atom (F, O, or N) with a lone pair. The partially positive H atom is attracted to a lone pair on an electronegative atom of a neighbouring molecule.
All three molecules qualify:
- HF: H bonded to F (most electronegative element)
- H₂O: H bonded to O
- NH₃: H bonded to N
The strength of hydrogen bonding depends on two factors: (1) electronegativity of the atom bonded to H (governs bond polarity), and (2) number of hydrogen bonds per molecule (depends on available H atoms and lone pairs).
HF: Each HF molecule has 1 H atom (can donate 1 H-bond) and 3 lone pairs on F (can accept multiple H-bonds). In practice, in liquid HF, each molecule forms about 1.8 hydrogen bonds on average — constrained by the 1 H atom available. HF forms chains and zigzag structures.
H₂O: Each water molecule has 2 H atoms (can donate 2 H-bonds) and 2 lone pairs on O (can accept 2 H-bonds). So each water molecule forms up to 4 hydrogen bonds — a complete tetrahedral network. This 3D network is the key reason water has anomalously high boiling point.
NH₃: Each NH₃ molecule has 3 H atoms (can donate 3 H-bonds) and only 1 lone pair on N (can accept only 1 H-bond). This lone pair availability limits the total H-bonding network. On average, NH₃ forms about 1.3 hydrogen bonds per molecule.
Per individual hydrogen bond:
- H–F···F: Strongest single H-bond (F is the most electronegative atom, makes the H–F bond most polar → H is most positive)
- H–O···O: Intermediate strength
- H–N···N: Weakest of the three (N is least electronegative of F, O, N)
So if we only considered the strength of individual H-bonds: HF > H₂O > NH₃.
To boil a liquid, you must overcome all the intermolecular forces — particularly the total H-bonding energy per molecule.
Total H-bonding per molecule:
- H₂O: up to 4 H-bonds (extensive 3D network)
- HF: ~1.8 H-bonds (limited by 1 H atom)
- NH₃: ~1.3 H-bonds (limited by 1 lone pair on N)
Despite HF forming stronger individual hydrogen bonds, water forms more H-bonds per molecule. The total energy needed to free a water molecule from its neighbours is greater.
Boiling points:
- HF: 19.5°C (292.5 K)
- H₂O: 100°C (373 K)
- NH₃: –33.4°C (239.6 K)
Order of boiling points: NH₃ < HF < H₂O
Why This Works
The key insight: boiling point depends on the total intermolecular cohesive energy, not just the strength of one interaction. Water wins because:
- It donates 2 H-bonds per molecule (two –OH groups)
- It accepts 2 H-bonds per molecule (two lone pairs on O)
- Result: each water molecule participates in ~4 H-bonds simultaneously — a full 3D tetrahedral network
This tetrahedral H-bonded network is why water has exceptional properties: unusually high boiling point, high surface tension, high specific heat capacity, maximum density at 4°C (not 0°C), and ice floating on water.
Alternative Method
Compare expected boiling points without H-bonding (based on molecular weight alone): H₂S (MW 34) boils at –60°C; H₂O (MW 18) would be expected to boil well below –60°C without H-bonding. The actual boiling point of +100°C represents an ~160°C elevation due to H-bonding. This “anomaly” graphically shows hydrogen bonding’s massive effect on water.
JEE Main and CBSE Class 11 Chemical Bonding both ask this comparison. The key answer points: (1) All three form H-bonds, (2) Individual H-bond strength: HF > H₂O > NH₃, (3) Number of H-bonds per molecule: H₂O (4) > HF (2) > NH₃ (1), (4) Total energy per molecule determines boiling point, so H₂O > HF > NH₃.
Common Mistake
Students often reason: “HF has the strongest H-bond (F is most electronegative), so HF should have the highest boiling point.” This is wrong because it ignores the number of H-bonds per molecule. A weaker bond formed multiple times can require more total energy to break than a single stronger bond. Water forms 4 H-bonds per molecule vs. HF’s ~2 — the greater number more than compensates for the lower individual strength.