1.

Although fluorine ismuch more electronegative then hydrogen yet the dipole moment of NF_(3) (0-24 D) is much lower than that of NH_(3) (1-46 D). Explain.

Answer» <html><body><p></p>Solution :The dipole moment of <a href="https://interviewquestions.tuteehub.com/tag/molecules-563030" style="font-weight:bold;" target="_blank" title="Click to know more about MOLECULES">MOLECULES</a> having lone <a href="https://interviewquestions.tuteehub.com/tag/pairs-1145786" style="font-weight:bold;" target="_blank" title="Click to know more about PAIRS">PAIRS</a> andpolar bonds is the vector sum of the following two moments. <br/> (i) Dipole moment of the lone pair/s <br/> (ii) Resultant of the dipole moments of polar bonds. <br/> Now both `NH_(3)andNF_(3)` have pyramidal sturctures with FNF and HNH bond <a href="https://interviewquestions.tuteehub.com/tag/angles-378243" style="font-weight:bold;" target="_blank" title="Click to know more about ANGLES">ANGLES</a> of `102.4^(@)and107.8^(@)` respectively. The direction of the dipole moment of the lone pair is in the same direction, i.e., away from the N atom as indicated in the Fig. 11.37 <br/> Since N is more electronegative than H, therefore, the direction of the N-H bond moments is from H to N. The resultant of these three bonds adds to the moment of lone pair hence the net dipole moment of `NH_(3)` is1.46 D. <br/> In contrast, F is more electronegative than N and hence the direction of the N-F bond moments is from N to F. The reusltant of these three dipole moments opposes the dipole moment of the lone pair and thus partly <a href="https://interviewquestions.tuteehub.com/tag/cancels-908215" style="font-weight:bold;" target="_blank" title="Click to know more about CANCELS">CANCELS</a> the resultant of the dipole moments of the three N-H bonds in`NH_(3)`. In other words, the only difference if that in case of `NF_(3)`, the dipole moments of the lone pair. Consequently, `NF_(3)` has a much <a href="https://interviewquestions.tuteehub.com/tag/lower-1080637" style="font-weight:bold;" target="_blank" title="Click to know more about LOWER">LOWER</a> dipole moment (0.24 D) than that of `NH_(3)` (1.46 D). <br/> <img src="https://d10lpgp6xz60nq.cloudfront.net/physics_images/PR_CHE_02_XI_C11B_E04_012_S01.png" width="80%"/></body></html>


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