Question

The correct order of boiling point in the following compounds is

• A. HF > H₂O > NH₃
• B. H₂O > HF > NH₃
• C. NH₃ > H₂O > HF
• D. NH₃ > HF > H₂O

Answer 1

The Correct Option is B

1. Identify the Compounds

The compounds are:
- HF (Hydrogen Fluoride)
- $H_2O$ (Water)
- $NH_3$ (Ammonia)

2. Recognize the Dominant Intermolecular Force
For these compounds, the dominant intermolecular force is hydrogen bonding.

3. Recall Factors Affecting Hydrogen Bond Strength
Factors that affect the strength of hydrogen bonds are:
- Electronegativity of the atom bonded to hydrogen. Higher electronegativity leads to stronger hydrogen bonds.
- Number of hydrogen bonds a molecule can form.

4. Consider Electronegativity
The electronegativity of F > O > N. Thus, if electronegativity was the only factor, the order of boiling points would be HF > $H_2O$ > $NH_3$.

5. Consider the Number of Hydrogen Bonds in Water
Water ($H_2O$) can form up to four hydrogen bonds per molecule because it has two H atoms and two lone pairs of electrons on the oxygen.

6. Consider the Number of Hydrogen Bonds in Hydrogen Fluoride
Hydrogen Fluoride (HF) can form hydrogen bonds. However, due to steric factors, and the fact that there is only one H, the hydrogen bonding is less extensive than that in water. It forms chains or rings, but not a 3D network.

7. Consider the Number of Hydrogen Bonds in Ammonia
Ammonia ($NH_3$) can form hydrogen bonds, though weaker than those in HF and water, and the network is less extensive than in water due to having only three hydrogen atoms and one lone pair.

8. Compare Water and Hydrogen Fluoride
$H_2O$ has a more extensive, three-dimensional hydrogen bonding network compared to HF. Although F is more electronegative than O, the extent of hydrogen bonding in water is higher. This higher extent of hydrogen bonding makes water have a higher boiling point than HF.

Final Answer:
(B) $H_2O$ > HF > $NH_3$

Answer 2

To compare the boiling points of HF, H₂O, and NH₃, we consider their intermolecular forces, especially hydrogen bonding:

HF (Hydrofluoric acid): HF exhibits strong hydrogen bonding due to the high electronegativity of fluorine. This leads to a relatively high boiling point.
H₂O (Water): Water also exhibits hydrogen bonding, though slightly weaker than HF. However, the extensive hydrogen bonding in water still gives it a high boiling point.
NH₃ (Ammonia): NH₃ exhibits hydrogen bonding, but it is weaker than in HF and H₂O, leading to a lower boiling point.
Therefore, the correct order of boiling points is: H₂O > HF > NH₃

The correct answer is (B) : H₂O > HF > NH₃.