Occurrence, Preparation, and Properties of Phosphorus

LEARNING OBJECTIVES

By the end of this module, 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:

[latex]{\text{2Ca}}_{3}{\left({\text{PO}}_{4}\right)}_{2}\left(s\right)+{\text{6SiO}}_{2}\left(s\right)+\text{10C}\left(s\right)\stackrel{\phantom{\rule{0.4em}{0ex}}\Delta\phantom{\rule{0.4em}{0ex}}}{\to }{\text{6CaSiO}}_{3}\left(l\right)+\text{10CO}\left(g\right)+{\text{P}}_{4}\left(g\right)[/latex]

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

A ball-and-stick model is shown. Three orange atoms labeled “P” are single bonded together in a triangle shape. Each “P” is single bonded to yellow atoms labeled “S,” which are each single bonded to one other orange atom labeled “P.”

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

Phosphorus Oxygen Compounds

Phosphorus forms two common oxides, phosphorus(III) oxide (or tetraphosphorus hexaoxide), P4O6, and phosphorus(V) oxide (or tetraphosphorus decaoxide), P4O10, both shown in Figure 2. Phosphorus(III) oxide is a white crystalline solid with a garlic-like odor. Its vapor is very poisonous. It oxidizes slowly in air and inflames when heated to 70 °C, forming P4O10. Phosphorus(III) oxide dissolves slowly in cold water to form phosphorous acid, H3PO3.

Two ball-and-stick models are shown. In the left model, three orange atoms labeled, “P,” are single bonded to red atoms labeled, “O,” in an alternating, six-sided ring structure. Each of the orange atoms are also single bonded to another red atom, which are in turn single bonded to a single orange atom. The right model shows three orange atoms labeled, “P,” single bonded to red atoms labeled, “O,” in an alternating, six-sided ring structure. Each of the orange atoms are also single bonded to two more red atoms, one in an upward position and one facing the outside of the molecule. The upward red atoms are single bonded to a single orange atom which is single bonded to a final red atom.

Figure 2. 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:

[latex]{\text{P}}_{4}{\text{O}}_{10}\left(s\right)+{\text{6H}}_{2}\text{O}\left(l\right)\rightarrow{\text{4H}}_{3}{\text{PO}}_{4}\left(aq\right)[/latex]

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

Phosphorus Halogen Compounds

Phosphorus will react directly with the halogens, forming trihalides, PX3, and pentahalides, PX5. The trihalides are much more stable than the corresponding nitrogen trihalides; nitrogen pentahalides do not form because of nitrogen’s inability to form more than four bonds.

The chlorides PCl3 and PCl5, both shown in Figure 3, 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 off-white solid that is prepared by oxidizing the trichloride with excess chlorine. The pentachloride sublimes when warmed and forms an equilibrium with the trichloride and chlorine when heated.

Two ball-and-stick models are shown. In the left model, an orange atom labeled, “P,” is single bonded to three green atoms labeled, “C l.” The right model shows an orange atom labeled, “P,” single bonded to five green atoms labeled, “C l.”

Figure 3. 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 yield hydrogen 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 [latex]{\text{PX}}_{6}{}^{-}.[/latex] Whereas phosphorus pentafluoride is a molecular compound in all states, X-ray studies show that solid phosphorus pentachloride is an ionic compound, [latex]\left[\text{PCl}_{4}{}^{+}\right]\left[\text{PCl}_{6}{}^{-}\right][/latex], as are phosphorus pentabromide, [latex]\left[\text{PBr}_{4}{}^{+}\right][/latex] [Br–], and phosphorus pentaiodide, [latex]\left[\text{PI}_{4}{}^{+}\right][/latex] [I–].

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 are phosphorus(V) oxide, P4O10, and phosphorus(III) oxide, P4O6. The two common methods for preparing orthophosphoric acid, H3PO4, are either the reaction of a phosphate with sulfuric acid or the reaction of water with phosphorus(V) oxide. Orthophosphoric acid is a triprotic acid that forms three types of salts.

Chemistry End of Chapter Exercises

  1. Write the Lewis structure for each of the following. You may wish to review the chapter on chemical bonding and molecular geometry.
    1. PH3
    2. [latex]{\text{PH}}_{4}{}^{+}[/latex]
    3. P2H4
    4. [latex]{\text{PO}}_{4}{}^{3-}[/latex]
    5. PF5
  2. Describe the molecular structure of each of the following molecules or ions listed. You may wish to review the chapter on chemical bonding and molecular geometry.
    1. PH3
    2. [latex]{\text{PH}}_{4}{}^{+}[/latex]
    3. P2H4
    4. [latex]{\text{PO}}_{4}{}^{3-}[/latex]
  3. Complete and balance each of the following chemical equations. (In some cases, there may be more than one correct answer.)
    1. [latex]{\text{P}}_{4}+\text{Al}\rightarrow[/latex]
    2. [latex]{\text{P}}_{4}+\text{Na}\rightarrow[/latex]
    3. [latex]{\text{P}}_{4}+{\text{F}}_{2}\rightarrow[/latex]
    4. [latex]{\text{P}}_{4}+{\text{Cl}}_{2}\rightarrow[/latex]
    5. [latex]{\text{P}}_{4}+{\text{O}}_{2}\rightarrow[/latex]
    6. [latex]{\text{P}}_{4}{\text{O}}_{6}+{\text{O}}_{2}\rightarrow[/latex]
  4. Describe 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 chapter on advanced theories of covalent bonding.
  5. What volume of 0.200 M NaOH is necessary to neutralize the solution produced by dissolving 2.00 g of PCl3 is an excess of water? Note that when H3PO3 is titrated under these conditions, only one proton of the acid molecule reacts.
  6. How much POCl3 can form from 25.0 g of PCl5 and the appropriate amount of H2O?
  7. How many tons of Ca3(PO4)2 are necessary to prepare 5.0 tons of phosphorus if the yield is 90%?
  8. Write equations showing the stepwise ionization of phosphorous acid.
  9. Draw the Lewis structures and describe the geometry for the following:
    1. [latex]{\text{PF}}_{4}{}^{+}[/latex]
    2. PF5
    3. [latex]{\text{PF}}_{6}{}^{-}[/latex]
    4. POF3
  10. Why does phosphorous acid form only two series of salts, even though the molecule contains three hydrogen atoms?
  11. Assign an oxidation state to phosphorus in each of the following:
    1. NaH2PO3
    2. PF5
    3. P4O6
    4. K3PO4
    5. Na3P
    6. Na4P2O7
  12. Phosphoric acid, one of the acids used in some cola drinks, is produced by the reaction of phosphorus(V) oxide, an acidic oxide, with water. Phosphorus(V) oxide is prepared by the combustion of phosphorus.
    1. Write the empirical formula of phosphorus(V) oxide.
    2. What is the molecular formula of phosphorus(V) oxide if the molar mass is about 280.
    3. Write balanced equations for the production of phosphorus(V) oxide and phosphoric acid.
    4. Determine the mass of phosphorus required to make 1.00 × 104 kg of phosphoric acid, assuming a yield of 98.85%.

Selected Answers

1. (a)
This Lewis structure shows a phosphorus atom with a lone pair of electrons single bonded to three hydrogen atoms.

(b)
This Lewis structure shows a phosphorus atom single bonded to four hydrogen atoms. The structure is surrounded by brackets and has a superscript positive sign outside the brackets.

(c)
This Lewis structure shows two phosphorus atoms, each with a lone pair of electrons, single bonded to one another. Each phosphorus atom is also single bonded to two hydrogen atoms.

(d)
This Lewis structure shows a phosphorus atom single bonded to four oxygen atoms, each with three lone pairs of electrons. The structure is surrounded by brackets and has a superscript 3 negative sign outside the brackets.

(e)
This Lewis structure shows a phosphorus atom single bonded to five fluorine atoms, each with three lone pairs of electrons.

3. (a) [latex]{\text{P}}_{4}\left(s\right)+\text{4Al}\left(s\right)\rightarrow\text{4AlP}\left(s\right)[/latex]

(b) [latex]{\text{P}}_{4}\left(s\right)+\text{12Na}\left(s\right)\rightarrow{\text{4Na}}_{3}\text{P}\left(s\right)[/latex]

(c) [latex]{\text{P}}_{4}\left(s\right)+{\text{10F}}_{2}\left(g\right)\rightarrow{\text{4PF}}_{5}\left(l\right)[/latex]

(d) [latex]{\text{P}}_{4}\left(s\right)+{\text{6Cl}}_{2}\left(g\right)\rightarrow{\text{4PCl}}_{3}\left(l\right)[/latex] or [latex]{\text{P}}_{4}\left(s\right)+{\text{10Cl}}_{2}\left(g\right)\rightarrow{\text{4PCl}}_{5}\left(l\right)[/latex]

(e) [latex]{\text{P}}_{4}\left(s\right)+{\text{3O}}_{2}\left(g\right)\rightarrow{\text{P}}_{4}{\text{O}}_{6}\left(s\right)[/latex] or [latex]{\text{P}}_{4}\left(s\right)+{\text{5O}}_{2}\left(g\right)\rightarrow{\text{P}}_{4}{\text{O}}_{10}\left(s\right)[/latex]

(f) [latex]{\text{P}}_{4}{\text{O}}_{6}\left(s\right)+{\text{2O}}_{2}\left(g\right)\rightarrow{\text{P}}_{4}{\text{O}}_{10}\left(s\right)[/latex]

5. [latex]\begin{array}{l}{\text{PCl}}_{3}\left(g\right)+{\text{3H}}_{2}\text{O}\left(l\right)\rightarrow{\text{H}}_{3}{\text{PO}}_{3}\left(aq\right)+\text{3HCl}\left(aq\right)\\ {\text{H}}_{3}{\text{PO}}_{3}\left(aq\right)+\text{NaOH}\left(aq\right)\rightarrow{\text{NaH}}_{2}{\text{PO}}_{4}\left(aq\right)+{\text{H}}_{2}\text{O}\left(l\right)\\ \text{HCl}\left(aq\right)+\text{NaOH}\left(aq\right)\rightarrow\text{NaCl}\left(aq\right)+{\text{H}}_{2}\text{O}\left(l\right)\end{array}[/latex]

From the first equation there are 4 moles of acid (one from H3PO3 and three from HCl) generated from each mole of PCl3:

[latex]\frac{2.00\cancel{\text{g}}{\text{PCl}}_{3}}{137.33\cancel{\text{g}}{\text{mol}}^{-1}}=\text{0.0145635 mol}{\text{PCl}}_{3}[/latex]

mol [latex]{\text{H}}_{3}{\text{O}}^{+}[/latex] = 4(0.0145635 mol PCl3) = 0.05825 mol (HCl + H3PO3) = 0.05825 mol [latex]{\text{H}}_{3}{\text{O}}^{+}[/latex]

[latex]V\text{(0.2 M NaOH)}=\text{0.05825 mol}\times \frac{\text{1 L}}{\text{0.200 mol}}=\text{0.291 L}=\text{291 mL}[/latex]

7. The equation is:

[latex]\begin{array}{l}{\text{2Ca}}_{3}{\left({\text{PO}}_{4}\right)}_{2}+{\text{6SiO}}_{2}+\text{10C}\rightarrow{\text{6CaSiO}}_{3}+\text{10CO}\left(g\right)+{\text{P}}_{4}\left(g\right)\\ \\ \text{theoretical mass}{\text{Ca}}_{3}{\left({\text{PO}}_{4}\right)}_{2}=\text{5.0 tons}{\text{P}}_{4}\times \frac{1\cancel{\text{mol}{\text{P}}_{4}}}{123.89504\cancel{\text{g}}}\times \frac{2\cancel{\text{mol}{\text{Ca}}_{3}{\left({\text{PO}}_{4}\right)}_{2}}}{1\cancel{\text{mol}{\text{P}}_{4}}}\times \frac{310.18\cancel{\text{g}}}{1\cancel{\text{mol}{\text{Ca}}_{3}{\left({\text{PO}}_{4}\right)}_{2}}}=\text{25.0 tons}{\text{Ca}}_{3}{\left({\text{PO}}_{4}\right)}_{2}\end{array}[/latex] At 90 percent yield, [latex]\frac{\text{25.0 tons}}{0.90}=\text{28 tons}{\text{Ca}}_{3}{\left({\text{PO}}_{4}\right)}_{2}[/latex] is required.

9. (a)
This Lewis structure shows a phosphorus atom single bonded to four fluorine atoms, each with three lone pairs of electrons. The structure is surrounded by brackets and has a superscript positive sign outside the brackets. The label, “Tetrahedral,” is written under the structure.

(b)
This Lewis structure shows a phosphorus atom single bonded to five fluorine atoms, each with three lone pairs of electrons. The label, “Trigonal bipyramidal,” is written under the structure.

(c)
A Lewis structure shows a phosphorus atom single bonded to six fluorine atoms, each with three lone pairs of electrons. The structure is surrounded by brackets and has a superscript negative sign outside the brackets. The label, “Octahedral,” is written under the structure.

(d)
This Lewis structure shows a phosphorus atom single bonded to three fluorine atoms, each with three lone pairs of electrons. The phosphorus atom is also double bonded to an oxygen atom with two lone pairs of electrons. The label, “Tetrahedral,” is written under the structure.<

11. (a) P = 3+; (b) P = 5+; (c) P = 3+; (d) P = 5+; (e) P = 3-; (f) P = 5+