Occurrence, Preparation, and Properties of Phosphorus*

OpenStax-CNX module: m51172 1

Occurrence, Preparation, and

Properties of Phosphorus*

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Abstract

By the end of this section, you will be able to:

• Describe the properties, preparation, and uses of phosphorus

The industrial preparation of phosphorus is by heating calcium phosphate, obtained from phosphate rock,with sand and coke:

2Ca3(PO4)2 (s) + 6SiO2 (s) + 10C (s)∆→ 6CaSiO3 (l) + 10CO (g) + P4 (g) (1)

The phosphorus distills out of the furnace and is condensed into a solid or burned to form P4O10. Thepreparation of many other phosphorus compounds begins with P4O10. The acids and phosphates are usefulas fertilizers and in the chemical industry. Other uses are in the manufacture of special alloys such asferrophosphorus and phosphor bronze. Phosphorus is important in making pesticides, matches, and someplastics. Phosphorus is an active nonmetal. In compounds, phosphorus usually occurs in oxidation statesof 3−, 3+, and 5+. Phosphorus exhibits oxidation numbers that are unusual for a group 15 element incompounds that contain phosphorus-phosphorus bonds; examples include diphosphorus tetrahydride, H2P-PH2, and tetraphosphorus trisul�de, P4S3, illustrated in Figure 1.

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Figure 1: P4S3 is a component of the heads of strike-anywhere matches.

1 Phosphorus Oxygen Compounds

Phosphorus forms two common oxides, phosphorus(III) oxide (or tetraphosphorus hexaoxide), P4O6, andphosphorus(V) oxide (or tetraphosphorus decaoxide), P4O10, both shown in Figure 1. Phosphorus(III) oxideis a white crystalline solid with a garlic-like odor. Its vapor is very poisonous. It oxidizes slowly in air andin�ames when heated to 70 ◦C, forming P4O10. Phosphorus(III) oxide dissolves slowly in cold water to formphosphorous acid, H3PO3.



Figure 1: This image shows the molecular structures of P4O6 (left) and P4O10 (right).

Phosphorus(V) oxide, P4O10, is a white powder that is prepared by burning phosphorus in excess oxygen.Its enthalpy of formation is very high (−2984 kJ), and it is quite stable and a very poor oxidizing agent.Dropping P4O10 into water produces a hissing sound, heat, and orthophosphoric acid:

P4O10 (s) + 6H2O (l) −→ 4H3PO4 (aq) (2)

Because of its great a�nity for water, phosphorus(V) oxide is an excellent drying agent for gases andsolvents, and for removing water from many compounds.

2 Phosphorus Halogen Compounds

Phosphorus will react directly with the halogens, forming trihalides, PX3, and pentahalides, PX5. Thetrihalides are much more stable than the corresponding nitrogen trihalides; nitrogen pentahalides do notform because of nitrogen's inability to form more than four bonds.

The chlorides PCl3 and PCl5, both shown in Figure 2, are the most important halides of phosphorus.Phosphorus trichloride is a colorless liquid that is prepared by passing chlorine over molten phosphorus.Phosphorus pentachloride is an o�-white solid that is prepared by oxidizing the trichloride with excesschlorine. The pentachloride sublimes when warmed and forms an equilibrium with the trichloride andchlorine when heated.



Figure 2: This image shows the molecular structure of PCl3 (left) and PCl5 (right) in the gas phase.

Like most other nonmetal halides, both phosphorus chlorides react with an excess of water and yieldhydrogen chloride and an oxyacid: PCl3 yields phosphorous acid H3PO3 and PCl5 yields phosphoric acid,H3PO4.

The pentahalides of phosphorus are Lewis acids because of the empty valence d orbitals of phosphorus.These compounds readily react with halide ions (Lewis bases) to give the anion PX6

−. Whereas phosphoruspenta�uoride is a molecular compound in all states, X-ray studies show that solid phosphorus pentachlorideis an ionic compound, [PCl4

+] [PCl6−] , as are phosphorus pentabromide, [PBr4

+][Br−], and phosphoruspentaiodide, [PI4

+][I−].

3 Key Concepts and Summary

Phosphorus (group 15) commonly exhibits oxidation states of 3− with active metals and of 3+ and 5+with more electronegative nonmetals. The halogens and oxygen will oxidize phosphorus. The oxides arephosphorus(V) oxide, P4O10, and phosphorus(III) oxide, P4O6. The two common methods for preparingorthophosphoric acid, H3PO4, are either the reaction of a phosphate with sulfuric acid or the reaction ofwater with phosphorus(V) oxide. Orthophosphoric acid is a triprotic acid that forms three types of salts.

4 Chemistry End of Chapter Exercises

Exercise 1 (Solution on p. 7.)

Write the Lewis structure for each of the following. You may wish to review the chapter on chemicalbonding and molecular geometry.

(a) PH3

(b) PH4+

(c) P2H4

(d) PO43−

(e) PF5



Exercise 2Describe the molecular structure of each of the following molecules or ions listed. You may wishto review the chapter on chemical bonding and molecular geometry.

(a) PH3

(b) PH4+

(c) P2H4

(d) PO43−


Complete and balance each of the following chemical equations. (In some cases, there may be morethan one correct answer.)

(a) P4 +Al −→(b) P4 +Na −→(c) P4 + F2 −→(d) P4 + Cl2 −→(e) P4 +O2 −→(f) P4O6 +O2 −→

Exercise 4Describe the hybridization of phosphorus in each of the following compounds: P4O10, P4O6, PH4I(an ionic compound), PBr3, H3PO4, H3PO3, PH3, and P2H4. You may wish to review the chapteron advanced theories of covalent bonding.


What volume of 0.200 M NaOH is necessary to neutralize the solution produced by dissolving 2.00g of PCl3 is an excess of water? Note that when H3PO3 is titrated under these conditions, onlyone proton of the acid molecule reacts.

Exercise 6How much POCl3 can form from 25.0 g of PCl5 and the appropriate amount of H2O?


How many tons of Ca3(PO4)2 are necessary to prepare 5.0 tons of phosphorus if the yield is 90%?

Exercise 8Write equations showing the stepwise ionization of phosphorous acid.


Draw the Lewis structures and describe the geometry for the following:(a) PF4

+

(b) PF5(c) PF6

−

(d) POF3

Exercise 10Why does phosphorous acid form only two series of salts, even though the molecule contains threehydrogen atoms?


Assign an oxidation state to phosphorus in each of the following:(a) NaH2PO3

(b) PF5(c) P4O6

(d) K3PO4

(e) Na3P(f) Na4P2O7



Exercise 12Phosphoric acid, one of the acids used in some cola drinks, is produced by the reaction of phos-phorus(V) oxide, an acidic oxide, with water. Phosphorus(V) oxide is prepared by the combustionof phosphorus.

(a) Write the empirical formula of phosphorus(V) oxide.(b) What is the molecular formula of phosphorus(V) oxide if the molar mass is about 280.(c) Write balanced equations for the production of phosphorus(V) oxide and phosphoric acid.(d) Determine the mass of phosphorus required to make 1.00 × 104 kg of phosphoric acid,

assuming a yield of 98.85%.



Solutions to Exercises in this Module

Solution to Exercise (p. 4)(a)

;(b)

;(c)

;(d)

;(e)



Solution to Exercise (p. 5)(a) P4 (s)+4Al (s) −→ 4AlP (s) ; (b) P4 (s)+12Na (s) −→ 4Na3P (s) ; (c) P4 (s)+10F2 (g) −→ 4PF5 (l) ;(d) P4 (s) + 6Cl2 (g) −→ 4PCl3 (l) or P4 (s) + 10Cl2 (g) −→ 4PCl5 (l) ; (e) P4 (s) + 3O2 (g) −→ P4O6 (s)or P4 (s) + 5O2 (g) −→ P4O10 (s) ; (f) P4O6 (s) + 2O2 (g) −→ P4O10 (s)Solution to Exercise (p. 5)291 mLSolution to Exercise (p. 5)28 tonsSolution to Exercise (p. 5)(a)

;(b)



;(c)

;(d)



Solution to Exercise (p. 5)(a) P = 3+; (b) P = 5+; (c) P = 3+; (d) P = 5+; (e) P = 3−; (f) P = 5+


Occurrence, Preparation, and Properties of Phosphorus*

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