Phosphorus Chlorides

Introduction

Phosphorus forms both PCl₃ and PCl₅. The formulas demonstrate +3 and +5 formal oxidation states, while their shapes show how different numbers of bonding regions alter molecular geometry. Both react with water, but their products and degree of hydrolysis depend on how much water is present and on reaction conditions.

Core explanation

In PCl₃, three chlorines assigned −1 require phosphorus +3. A simple Lewis structure has three P–Cl bonds and one lone pair on phosphorus. Four electron regions give a roughly tetrahedral electron-pair arrangement, while the atoms make a trigonal pyramid. The lone pair affects shape and allows PCl₃ to participate in donor chemistry. It is a molecular covalent compound under ordinary dry conditions.

In gaseous PCl₅, five P–Cl bonds give a trigonal bipyramidal molecular geometry with three equatorial and two axial chlorines. Phosphorus is formally +5. School diagrams often describe ten electrons around phosphorus; advanced bonding models avoid treating five identical localized two-electron bonds as the only possible explanation. In the solid, PCl₅ can have ionic character represented by [PCl₄]⁺ and [PCl₆]⁻, so the gas-phase molecular shape should not be imposed unchanged on the solid crystal.

Phosphorus trichloride undergoes hydrolysis with sufficient water: PCl₃ + 3H₂O → H₃PO₃ + 3HCl. H₃PO₃ is phosphorous acid, whose structural formula is HPO(OH)₂ in a common representation. One H is bonded to phosphorus, not oxygen, so the acid has two readily ionizable P–OH protons. Formula H₃PO₃ alone can conceal this distinction.

Complete hydrolysis of PCl₅ with sufficient water can be represented PCl₅ + 4H₂O → H₃PO₄ + 5HCl. Phosphoric acid has the structural pattern OP(OH)₃, with three P–OH groups. With limited water, phosphoryl chloride POCl₃ can be an intermediate or product: PCl₅ + H₂O → POCl₃ + 2HCl. The equations are not competing universal claims; the available water and conditions determine the path and extent.

Both chlorides must be handled under dry conditions when used as reagents because moisture generates acidic HCl and phosphorus oxyacids. Their reactivity is an application of P–Cl bond hydrolysis, but an exact mechanism can involve several steps and transient species. For exam purposes, distinguish partial and complete hydrolysis, balance the formulas and identify phosphorus oxidation states.

There is no redox change in the simple complete hydrolysis equations: P remains +3 from PCl₃ to H₃PO₃, or +5 from PCl₅ to H₃PO₄. Chlorine remains −1 in HCl. Water supplies oxygen and hydrogen while P–Cl bonds are replaced through hydrolysis.

Step-by-step reasoning

1. Assign chlorine −1 to obtain P +3 or +5. 2. Draw three bonds plus lone pair for PCl₃ and five bonding positions for gaseous PCl₅. 3. State physical phase before describing PCl₅'s structure. 4. Select limited-water or sufficient-water hydrolysis equation. 5. Check atoms and oxidation states to distinguish hydrolysis from redox.

Visual explanation

Sketch a trigonal pyramid for PCl₃ with one lone pair above P. Beside it draw a trigonal bipyramid for gaseous PCl₅, with three equatorial and two axial Cl atoms. Add two water arrows from PCl₅: one to POCl₃ and another, with more water, to H₃PO₄.

Real-world analogy

A connector with three attached parts and one empty direction has a different shape from one holding five parts. Exposing either assembly to water can replace components, but the number of water portions available controls how far replacement proceeds.

Real-world example

PCl₃ and PCl₅ are used as chlorinating reagents in chemical synthesis. Their moisture sensitivity is practical: an open humid container can produce HCl fumes and change the reagent composition.

Why?

Why do the two chlorides yield different phosphorus acids on complete hydrolysis? PCl₃ starts with phosphorus +3 and gives phosphorous acid; PCl₅ starts at +5 and gives phosphoric acid, while replacing chlorine-related bonds through water reaction.

Common misconception

“Solid PCl₅ must be made of trigonal bipyramidal PCl₅ molecules because the gas is.” Solid-state interactions can yield [PCl₄]⁺ and [PCl₆]⁻ descriptions. State the phase.

Worked example

Balance PCl₅ + H₂O → H₃PO₄ + HCl for complete hydrolysis. Five chlorine atoms require 5HCl. Four oxygens in H₃PO₄ require 4H₂O. The left then has eight hydrogens, matching three in H₃PO₄ and five in HCl: PCl₅ + 4H₂O → H₃PO₄ + 5HCl. P stays +5, so this is hydrolysis rather than redox.

Quick check

1. What is the molecular shape of gaseous PCl₅? Answer: Trigonal bipyramidal.

Exam focus

Contrast shape and oxidation state; give balanced hydrolysis equations; mention POCl₃ when water is limited; and avoid treating gas-phase PCl₅ structure as universal for the solid.

Advanced insight

Hypervalent bonding is better understood with delocalized molecular-orbital descriptions than by assuming phosphorus simply places five equivalent electron pairs in five old-fashioned hybrids. The structure still supports reliable geometry and formula predictions.

Summary

PCl₃ is trigonal pyramidal with phosphorus +3; gaseous PCl₅ is trigonal bipyramidal with phosphorus +5. Both hydrolyze, yielding different oxyacids under sufficient water. Phase and water quantity matter.

Practice questions

1. What is phosphorus's oxidation state in PCl₃? Answer: +3. 2. Give the complete hydrolysis equation for PCl₃. Answer: PCl₃ + 3H₂O → H₃PO₃ + 3HCl. 3. What can form when PCl₅ meets only limited water? Answer: POCl₃ and HCl, represented by PCl₅ + H₂O → POCl₃ + 2HCl.