Question

The correct increasing order of dipole moment of \( NF_3, H_2S, CHCl_3 \) and \( NH_3 \) is

• A. \( NF_3<H_2S<CHCl_3<NH_3 \)
• B. \( NH_3<H_2S<CHCl_3<NF_3 \)
• C. \( NF_3<CHCl_3<H_2S<NH_3 \)
• D. \( NH_3<CHCl_3<H_2S<NF_3 \)
• E. \( CHCl_3<H_2S<NF_3<NH_3 \)

Hint:

Vector cancellation reduces dipole moment (as in \( NF_3 \)).

Answer 1

The Correct Option is A

Step 1: Understanding Dipole Moment:
Dipole moment is a measure of the polarity of a molecule. It arises from the separation of positive and negative charges and is a vector quantity, meaning both magnitude and direction matter. The net dipole moment of a molecule is the vector sum of all individual bond dipoles and the contribution from lone pairs.
Step 2: Analyzing the Molecular Structures and Dipoles:
NH\(_3\) (Ammonia): It has a trigonal pyramidal shape with a lone pair on the nitrogen atom. The N-H bond dipoles point towards the more electronegative nitrogen atom. The orbital dipole of the lone pair also points away from the nitrogen atom in the same general direction. All these vectors add up, resulting in a large net dipole moment (\(\mu \approx 1.47\) D).
NF\(_3\) (Nitrogen trifluoride): It also has a trigonal pyramidal shape, similar to NH\(_3\). However, fluorine is more electronegative than nitrogen. So, the N-F bond dipoles point away from nitrogen, towards the fluorine atoms. The orbital dipole of the lone pair points in the opposite direction to the resultant of the N-F bond dipoles. This opposition leads to a significant cancellation, resulting in a very small net dipole moment (\(\mu \approx 0.23\) D).
H\(_2\)S (Hydrogen sulfide): It has a bent (V-shaped) geometry due to two bond pairs and two lone pairs on the sulfur atom. Sulfur is more electronegative than hydrogen, so the H-S bond dipoles point towards sulfur. The vector sum results in a net dipole moment (\(\mu \approx 0.97\) D).
CHCl\(_3\) (Chloroform): It has a tetrahedral geometry. The C-H bond is less polar than the C-Cl bonds. The three C-Cl bond dipoles point towards the chlorine atoms, and their resultant vector adds to the small C-H bond dipole. This results in a significant net dipole moment (\(\mu \approx 1.04\) D).
Step 3: Comparing the Dipole Moments:
Based on the approximate experimental values:
\(\mu\)(NF\(_3\)) = 0.23 D
\(\mu\)(H\(_2\)S) = 0.97 D
\(\mu\)(CHCl\(_3\)) = 1.04 D
\(\mu\)(NH\(_3\)) = 1.47 D
The increasing order is: NF\(_3\)< H\(_2\)S< CHCl\(_3\)< NH\(_3\).
Step 4: Final Answer:
The correct increasing order of dipole moment is NF\(_3\)< H\(_2\)S< CHCl\(_3\)< NH\(_3\).