Question

Given below are two statements: Statement–I: HX bond length is higher in HCl than HF. Statement–II: The lowest boiling point among hydrides of group 15 elements is for the hydride having covalency 4. Choose the correct option.

• A. Both Statement–I and Statement–II are correct
• B. Both Statement–I and Statement–II are incorrect
• C. Statement–I is correct but Statement–II is incorrect
• D. Statement–I is incorrect but Statement–II is correct

Hint:

Always remember:
Bond length increases with atomic size
Abnormal boiling points (like NH\(_3\)) are due to hydrogen bonding
Group 15 hydrides have covalency 3, not 4

Answer 1

The Correct Option is C

Concept: Bond length depends on the size of the bonded atoms, while boiling point of hydrides depends on intermolecular forces such as hydrogen bonding and van der Waals forces.
Step 1: Analyze Statement–I. Hydrogen halides follow the trend: \[ \text{HF}<\text{HCl}<\text{HBr}<\text{HI} \quad \text{(bond length)} \] Fluorine is the smallest halogen atom, whereas chlorine is larger. Hence, the H–Cl bond length is greater than the H–F bond length. \[ \Rightarrow Statement–I is correct \]
Step 2: Analyze Statement–II. Group 15 hydrides are: \[ \text{NH}_3,\ \text{PH}_3,\ \text{AsH}_3,\ \text{SbH}_3,\ \text{BiH}_3 \] All these hydrides have covalency 3, not 4. Boiling point trend: \[ \text{PH}_3<\text{AsH}_3<\text{SbH}_3<\text{BiH}_3 \] with \(\text{NH}_3\) having a higher boiling point due to hydrogen bonding. Thus:
Lowest boiling point is \(\text{PH}_3\)
Its covalency is 3, not 4 \[ \Rightarrow Statement–II is incorrect \]
Step 3: Final conclusion.
Statement–I is correct
Statement–II is incorrect Therefore, the correct option is (3).