JEE Chemistry 2024 Question with Solution

Given: Two statements compare the net dipole moments of $NH_3$ and $NF_3$ and the directions of bond dipoles and lone pair orbital dipole.

Find: Which statement combination is correct.

The solution states the experimental result that the net dipole moment of $NH_3$ is about $1.47 \, \text{D}$ and that of $NF_3$ is about $0.23 \, \text{D}$.

So Statement I is false, because it claims $\mu(NF_3) > \mu(NH_3)$, whereas the actual result is

$$\mu(NH_3) > \mu(NF_3)$$

For the direction analysis:

• In $NH_3$, each $N-H$ bond dipole points from $H$ to $N$. The resultant bond dipole and the lone pair orbital dipole are in the same direction, so they add.
• In $NF_3$, each $N-F$ bond dipole points from $N$ to $F$. The resultant bond dipole is opposite to the lone pair orbital dipole, so they partially cancel.

Therefore Statement II is also false, because it says exactly the reverse of the correct dipole-direction relation.

Hence, both Statement I and Statement II are false.

The correct option is C.

Given: Pyramidal molecules $NH_3$ and $NF_3$.

Find: Whether the statements about their net dipole moments and dipole directions are true or false.

Concept used: The net dipole moment is the vector sum of all bond dipoles and the dipole associated with the lone pair on the central atom.

For $NH_3$:

• Nitrogen is more electronegative than hydrogen.
• So each bond dipole is directed from $H$ to $N$.
• The resultant of the three bond dipoles is along the same general direction as the lone pair dipole.
• Hence they reinforce each other, giving a larger net dipole moment.

For $NF_3$:

• Fluorine is more electronegative than nitrogen.
• So each bond dipole is directed from $N$ to $F$.
• The resultant of the three bond dipoles is opposite to the lone pair dipole.
• Hence they oppose each other, giving a much smaller net dipole moment.

This matches the values quoted in the solution:

$$\mu(NH_3) \approx 1.47 \, \text{D}, \qquad \mu(NF_3) \approx 0.23 \, \text{D}$$

So:

1. Statement I is false.
2. Statement II is false.

Therefore, the correct option is C, that is, both statements are false.

Discrepancy note: The answer key says option (2), but the solution explicitly concludes that option C is correct. By the provided rules, the solution is treated.