Explain why NH3 has a much larger dipole moment, 1.46D than NF3, 0.24D.
Answer (2)
Draw Lewis structures and determine molecular geometry: Both N H 3 and N F 3 are trigonal pyramidal.
Determine bond dipoles: N-H bonds point towards N, while N-F bonds point towards F.
Analyze vector sum of dipoles: In N H 3 , bond dipoles and lone pair dipole reinforce each other.
Explain the difference: In N F 3 , bond dipoles and lone pair dipole partially cancel, resulting in a smaller dipole moment. Thus, N H 3 has a much larger dipole moment than N F 3 .
Explanation
Understanding the Problem We want to explain why N H 3 has a much larger dipole moment (1.46 D) than N F 3 (0.24 D). To do this, we need to consider the molecular geometry and the bond dipoles in each molecule.
Lewis Structures and Molecular Geometry First, let's draw the Lewis structures for N H 3 and N F 3 . Both molecules have a central nitrogen atom bonded to three other atoms (hydrogen or fluorine) and a lone pair of electrons. According to VSEPR theory, both molecules have a trigonal pyramidal geometry.
Electronegativity Next, we need to consider the bond dipoles. Electronegativity is the ability of an atom to attract electrons in a chemical bond. Nitrogen (N) has an electronegativity of 3.04. Hydrogen (H) has an electronegativity of 2.20. Fluorine (F) has an electronegativity of 3.98.
Bond Dipoles in NH3 In N H 3 , the N-H bond is polar because nitrogen is more electronegative than hydrogen. The bond dipole points from the hydrogen atom towards the nitrogen atom. Since there are three N-H bonds, there are three bond dipoles pointing towards the nitrogen atom.
Bond Dipoles in NF3 In N F 3 , the N-F bond is also polar because fluorine is more electronegative than nitrogen. The bond dipole points from the nitrogen atom towards the fluorine atom. Since there are three N-F bonds, there are three bond dipoles pointing away from the nitrogen atom.
Overall Dipole Moment of NH3 Now, let's consider the overall dipole moment of each molecule. The dipole moment is a vector sum of all the bond dipoles and the lone pair dipole. In N H 3 , the three N-H bond dipoles point towards the nitrogen atom, and the lone pair dipole also points in the same direction. Therefore, the bond dipoles and the lone pair dipole reinforce each other, resulting in a large overall dipole moment.
Overall Dipole Moment of NF3 In N F 3 , the three N-F bond dipoles point away from the nitrogen atom, and the lone pair dipole points in the opposite direction. Therefore, the bond dipoles and the lone pair dipole partially cancel each other, resulting in a small overall dipole moment.
Conclusion In summary, N H 3 has a much larger dipole moment than N F 3 because the bond dipoles and the lone pair dipole reinforce each other in N H 3 , while they partially cancel each other in N F 3 .
Examples
Understanding dipole moments is crucial in predicting the physical and chemical properties of molecules. For instance, the higher dipole moment of water ( H 2 O ) compared to carbon dioxide ( C O 2 ) explains why water is a much better solvent for polar substances. In drug design, understanding dipole moments helps predict how a drug molecule will interact with its target receptor, which is essential for developing effective medications. Similarly, in materials science, dipole moments influence the arrangement of molecules in a material, affecting its macroscopic properties like dielectric constant and refractive index.
NH₃ has a larger dipole moment (1.46 D) than NF₃ (0.24 D) due to the arrangement of their bond dipoles and lone pair. In NH₃, the bond dipoles reinforce each other, while in NF₃, they partially cancel out. This leads to a stronger overall dipole moment in NH₃ as compared to NF₃.
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