Phosphorus pentachloride is the chemical compound with the chemical formula PCl5. It exists as colorless crystals at room temperature. It is a common chlorine agent used in various reactions. It is sensitive to moisture and water. Many students may have questions about whether PCl5 is polar or not. In this article, I will answer that question as well as explore the issues surrounding it.
So, Is PCl5 Polar or Nonpolar? PCl5 is nonpolar in nature because it has a symmetrical geometric structure, which causes the polarity of the P-Cl bonds to cancel each other out. As a result, the net dipole moment of PCl5 becomes zero.
PCl5 is a colorless crystal at room temperature. It is one of the most common chlorinating reagents.
It was first discovered in 1808 by the famous British chemist Humphrey Davy.
The chemical compound PCl5 has a pungent odor. The IUPAC name for PCl5 is phosphorus pentachloride.
Samples used commercially in the laboratory may be yellowish in color due to the presence of HCl.
If we talk about the chemical composition of PCl5, it consists of 5 chlorine atoms and 1 phosphorus atom connected by covalent bonds.
Phosphorus has 5 valence electrons and chlorine has 7.
A chlorine atom requires 1 electron to complete its octet. Therefore, the 5 chlorine atoms share 1 electron from the phosphorus atom and leave no lone pair.
The PCl5 molecule becomes neutral with this electron configuration.
The electronegativity of phosphorus is 2.19 and that of chlorine is 3.16. The difference between the electronegativity of phosphorus and chlorine creates polarity in the P-Cl bond.
The electronegativity of an atom is its ability to attract a bonded pair of electrons. The more electronegative atom attracts the bonded pair of electrons a little more towards itself.
Similarly, the chlorine atom in the P-Cl bond pulls the bonded electron closer to itself. This gives the chlorine atom a partial negative charge and the phosphorus atom a partial positive charge.
However, due to the symmetrical geometric shape of PCl5, that is, trigonal-bipyramidal, the polarity of the P-Cl bonds cancel each other out and therefore PCl5 is a polar molecule as a whole.
What are Polar and Nonpolar Molecules?
Molecules are held together by bonding forces, which can be ionic, metallic covalent, and hydrogen bonds. Covalent bonds can be polar or non-polar depending on various factors such as electronegativity, geometric shape and dipole moment.
Polar Molecules: These are molecules having a non-zero net dipole moment. A covalent bond between two atoms is said to be polar if the two atoms have an unequal charge distribution.
These two atoms have unequal electronegativity and therefore have a non-zero dipole moment.
Examples of such connections are H2O, OF2, etc. You can check the reason for the polarity of H2O.
Non-polar Molecules: These are molecules which have no dipole moment. Two atoms that are covalently bonded form a non-polar bond when the atoms share the same fraction of charge.
These atoms also have the same value of electronegativity and therefore form a zero dipole moment.
Examples of such molecules are CO2, O2, PCl5 etc. See CCl4 non-polarity reasons.
Note: It is possible that polar bonds exist in non-polar molecules. Due to the polarity of these bonds cancel each other due to the symmetrical geometric structure of the molecule.
Why PCl5 is a nonpolar molecule?
Phosphorus pentachloride is naturally non-polar because it has a symmetrical shape. It has a triangular bipyramidal geometric shape.
Three of the P-Cl bonds lie in the plane held at the corners of an equilateral triangle, and the other two bonds lie axial to the plane of the three bonds.
The PCl5 molecule has chlorine and phosphorus atoms with an electronegativity difference of 0.97 D, which determines the polarity of the P-Cl bond.
The chlorine atom is more electronegative than phosphorus. This causes the chlorine atoms to share more of the charge, attracting slightly more bond pairs and gaining a partial negative charge.
On the other hand, the phosphorus atom acquires a partial positive charge.
The P-CL bond gives some non-zero dipole moments, but due to the symmetrical geometric structure, the polarity of the P-Cl bond is canceled out by other P-Cl bonds.
Therefore, it is very important to understand that although PCl5 contains polar bonds, it is essentially non-polar due to its symmetrical shape.
The Geometrical Structure of PCl5
Five chlorine atoms are bonded to phosphorus and all bonds are covalent.
The three bonds are in a plane such that the three P-Cl bonds form an angle of 120 degrees with each other and are in the corners of an equilateral triangle.
And the remaining two P-Cl bonds are at either end of the axis that passes through the plane of the three P-Cl bonds.
The two axial molecular bonds are perpendicular to the plane of the other three P-Cl bonds
The bond length of the two P-CL bonds located axially on the molecule is 240 pm, the length of the P-CL bonds located in different planes is 202 pm.
The form of PCl5 is trigonal-bipyramidal.
Above is a picture of the molecular structure of phosphorus pentachloride.
Factors affecting polarity of a molecule
Electronegativity: The electronegativity of an atom determines the attraction of the electrons bound to it. When there is a difference in the electronegativity of the two atoms forming a covalent bond, the bond is said to be polar.
The polarity of a molecule is directly proportional to the difference in their electronegativity.
Dipole moment: It is the measure of the polarity of a molecule. Greater the dipole moment of a molecule, more is its polarity.
Therefore, the polarity of a molecule is also directly proportional to its dipole moment. Mathematically, the dipole moment of a molecule is the product of the charge and the bond length.
Geometrical Structure: The molecules that are symmetrical in nature tend to be nonpolar. These molecules are overall a nonpolar in nature even if it contains a polar bond within it.
Because due to the symmetrical structure, the polarity in such a molecule gets canceled by each other.
Properties of PCl5
- Phosphorus pentachloride exists as a solid crystal with a pungent odor.
- The molecular mass of this compound is 208.2 g/mol.
- Exposure to a high concentration of PCl5 can lead to serious health problems.
- The density of PCl5 is 2.1 g/cm3.
- It readily gets decomposed in water to form phosphoric acid.
- The melting point of this chemical compound is 160.5 °C or 320.9 °F.
- The Boiling point of PCl5 is around 166.8 °C or 332.2 °F.
- At a temperature of 80 °C, the vapor pressure of PCl5 is 1.11 kPa.
- It is non-flammable in nature.
Uses of PCl5
- It is widely used as a chlorinating agent used in the preparation of various products in the industry.
- It is also used as a dehydrating agent.
- It has its great use as an intermediate compound in the manufacturing of pesticides and water treatment.
- It is used for the improvement of the grain structure of metals.
- It also helps to produce a chemical compound ie; lithium hexafluorophosphate that is an electrolyte used in lithium-ion batteries
Phosphorus pentachloride is inherently non-polar due to its geometric structure. It is symmetrical, i.e. trigonal bipyramidal. As a result, the polarities of the P-CL bonds cancel each other out.