Determining the Dipole Moment of PH? and PCl?: A Comparative Analysis

The determination of the dipole moment between PH? (phosphine) and PCl? (phosphorus trichloride) involves a multifaceted approach considering molecular geometry and electronegativity. This article aims to provide a comprehensive understanding of these factors and thereby determine which molecule exhibits a greater dipole moment.

Electronegativity and Bond Polarity

The electronegativity difference between elements is a crucial factor in determining bond polarity and, consequently, the dipole moment of a molecule. For this comparison, we consider the electronegativity values of the atoms involved:

Electronegativity Values - Cl: 3.16 - H: 2.20 - P: 2.19 (for both PH? and PCl?)

Chlorine (Cl) is more electronegative than hydrogen (H), contributing to a higher polarity in PCl? due to stronger P-Cl bonds compared to the P-H bonds in PH?.

Molecular Geometry

The molecular geometry of both PH? and PCl? is trigonal pyramidal, with a lone pair of electrons on phosphorus. This geometry does not allow the bond dipoles to cancel each other out completely, leading to a net dipole moment:

PH? (Phosphine): The lone pair on phosphorus causes the hydrogen atoms to be slightly shifted, resulting in a net dipole moment from hydrogen to phosphorus in the same direction as the lone pair dipole. PCl? (Phosphorus Trichloride): The lone pair on phosphorus similarly causes the chlorine atoms to be slightly shifted. However, the higher electronegativity of chlorine leads to stronger P-Cl bond dipoles, which, when added to the lone pair dipole, contribute to a greater overall dipole moment.

Dipole Moment and Its Determinants

The dipole moment ((mu)) is a vector quantity that depends on both the magnitude of the bond dipoles and their spatial orientation. The dipole moment in PCl? will be greater than in PH? due to:

1. Stronger Bond Dipoles in PCl?: The higher electronegativity of chlorine compared to hydrogen results in stronger and more directional bond dipoles, contributing to a larger net dipole moment.

2. Net Directional Dipole in PCl?: The bond dipoles in PCl? align more effectively, contributing to a larger net dipole moment. In contrast, the dipole moment in PH? faces in the opposite direction of the lone pair dipole, reducing its effective moment.

Conclusion

Based on the analysis of electronegativity and molecular geometry, it is clear that PCl? possesses a greater dipole moment than PH?. The higher electronegativity of chlorine results in stronger and more directed bond dipoles in PCl?, leading to a more significant net dipole moment.

Additional Information

Phosphorus Trichloride (PCl?): This inorganic compound, represented by the chemical formula PCl?, is a colorless liquid when pure. It is an important industrial chemical used in the manufacture of phosphites and other organophosphorus compounds. PCl? is toxic and reacts violently with water, releasing hydrogen chloride (HCl).

References

For further reading and verification of the information presented, refer to the following resources:

Linus Pauling, Quantitative Relation Between Structure and Bond Energy in Molecules, J. Am. Chem. Soc. 57, 2680-2689 (1935). Todd Sanderson, Chemical Principles: The Quest for Insight, 4th Ed., W. H. Freeman and Company, 2016.