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 primarily depends on the atomic sizes of the bonded atoms, whereas the boiling points of hydrides depend on intermolecular forces such as hydrogen bonding and van der Waals interactions.
Step 1: Examine Statement–I. For hydrogen halides, the bond length follows the order: \[ \text{HF}<\text{HCl}<\text{HBr}<\text{HI} \] Fluorine is the smallest halogen, while chlorine has a larger atomic radius. Therefore, the H–Cl bond length is greater than the H–F bond length. \[ \Rightarrow Statement–I is correct \]
Step 2: Examine Statement–II. Group 15 hydrides are: \[ \text{NH}_3,\ \text{PH}_3,\ \text{AsH}_3,\ \text{SbH}_3,\ \text{BiH}_3 \] All these hydrides possess a covalency of 3, not 4. The boiling point trend is: \[ \text{PH}_3<\text{AsH}_3<\text{SbH}_3<\text{BiH}_3 \] with \(\text{NH}_3\) showing an anomalously high boiling point due to hydrogen bonding. Hence:
The hydride with the 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).