INTERCHAPTER N Phosphorus Red and white phosphorus. White phosphorus, one of the principal allotropes of solid phosphorus, is very reactive and must be handled with care because it produces severe burns when it comes in contact with skin. The sample shown here has a yellowish cast as a result of surface reactions with air. White phosphorus is usually stored under water. Red phosphorus, on the other hand, is much less reactive than white phosphorus and does not require special handling. University Science Books, ©2011. All rights reserved. www.uscibooks.com N. PHOSPHORUS N1 Phosphorus (atomic number 15, atomic mass Figure N.1 White phosphorus 30.973 761) was the first element whose discovery consists of tetrahedral P4 could be attributed to a specific person. It was discov- molecules. ered in 1669 by the German alchemist Hennig Brandt by the unsavory process of distilling putrefied urine. Although phosphorus constitutes less than 0.1% by mass of the earth’s crust, all living organisms contain this element, and it is the sixth most abundant ele- ment in the human body. The energy requirements of essentially all biochemical reactions are supplied by phosphorus compounds. Plants require phospho- but they generally occur only at higher temperatures. rus as a nutrient, and most of the phosphorus com- For example, red phosphorus must be heated to 260°C pounds that are produced are used as fertilizers. before it burns in air. The toxicity of red phosphorus One of the most important compounds of phos- is much lower than that of white phosphorus. phorus is phosphoric acid, H3PO4(aq), a triprotic White phosphorus consists of tetrahedral P4 mol- acid. Although the formulas of the other oxyacids ecules (Figure N.1), whereas red phosphorus consists of phosphorus are usually written as H PO (aq) and 3 2 of large, random aggregates of phosphorus atoms. H PO (aq), we shall see that these acids are monopro- 3 3 The structure of red phosphorus is called amorphous, tic and diprotic, respectively. Thus, the three oxyac- which means that it has no definite shape. Butter is ids of phosphorus illustrate the idea of monoprotic another example of an amorphous substance. and polyprotic acids presented in Chapter 20. Most of the phosphorus that is produced is used to make phosphoric acid and other phosphorus com- N-1. There Are Two Principal Allotropes of pounds. Elemental phosphorus, however, is used in Solid Phosphorus the manufacture of pyrotechnics, matches, rat poi- sons, incendiary shells, smoke bombs, and tracer There are several allotropes of elemental solid phos- bullets. phorus, the most important of which are white phos- Phosphorus is not found as a free element in phorus and red phosphorus (Frontispiece). White nature. The principal sources are calcium phosphate phosphorus is a white, transparent, waxy crystal- and the apatite ores (Figure N.2): line solid that often appears pale yellow because of impurities. It is insoluble in water and alcohol but hydroxyapatite Ca10(OH)2(PO4)6(s) soluble in carbon disulfide. A characteristic property fluorapatite Ca F (PO ) (s) of white phosphorus is its high chemical reactivity. 10 2 4 6 chlorapatite Ca Cl (PO ) (s) It ignites spontaneously in air at about 25°C. White 10 2 4 6 phosphorus is very poisonous; the lethal dose is 50 to These ores collectively are called phosphate rock. 100 milligrams. White phosphorus should always be Large phosphate rock deposits occur in Russia, kept under water and handled with forceps. in Morocco, and in the United States in Florida, When white phosphorus is heated above 400°C Tennessee, and Idaho. An electric furnace is used to for several hours in the absence of air, a form called obtain phosphorus from phosphate rock. The fur- red phosphorus is produced. Red phosphorus is a red nace is charged with powdered phosphate rock, sand, to violet powder that is less reactive than white phos- SiO (s), and carbon in the form of coke. The source phorus. The chemical reactions that the red form 2 of heat is an electric current that produces tempera- undergoes are the same as those of the white form, tures of over 1000°C. A simplified version of the over- all reaction that takes place is Recall that allotropes are forms of an element with 2 Ca3(PO4)2(s) + 6 SiO2(s) +10 C(s) → different arrangements of the atoms. phosphate rock sand coke 6 CaSiO3(l ) + 10 CO(g) + P4(g) N2 GENERAL CHEMISTRY, FOURTH EDITION | McQuarrie, Rock, and Gallogly Figure N.2 The apatite minerals. Left to right: hydroxyapatite, Ca10(OH)2(PO4)6(s); fluorapatite, Ca10F2(PO4)6(s); and chlorapatite, Ca10Cl2(PO4)6(s). Liquid calcium silicate, CaSiO3(l ), called slag, is In practice, a mixture of oxides is formed in each tapped off from the bottom of the furnace, and the case, but one oxide can be greatly favored over the phosphorus vapor that is produced solidifies to white other by controlling the relative amounts of phospho- P4(s) when the mixture of CO(g) and P4(g) is passed rus and oxygen. through water (carbon monoxide does not dissolve Before the actual molecular formulas of these in water). The annual world production of elemental phosphorus oxides were known, the empirical for- phosphorus is approximately one million metric tons. mulas P2O3(s) and P2O5(s) were used. Consequently, Although some phosphate rock is used to make P4O6(s) and P4O10(s) are still commonly called phos- elemental phosphorus, most phosphate rock is used in phorus trioxide and phosphorus pentoxide. the production of fertilizers. Phosphorus is a required It is interesting to compare the molecular struc- nutrient of all plants, and phosphorus compounds tures of P4O6 and P4O10 (Figure N.3). The molecu- have long been used as fertilizer. In spite of its great lar structure of P4O6 is obtained from that of P4 by abundance, phosphate rock cannot be used as a fer- inserting an oxygen atom between each pair of adja- tilizer because, as the name implies, it is insoluble in cent phosphorus atoms; there are six edges on a tet- water. Consequently, plants are not able to assimilate rahedron, and thus a total of six oxygen atoms are the phosphorus from phosphate rock. To produce a required. The molecular structure of P4O10 is obtained water-soluble source of phosphorus, phosphate rock is from that of P4O6 by attaching an additional oxygen reacted with sulfuric acid to produce a water-soluble atom to each of the four phosphorus atoms. product called superphosphate, Ca(H2PO4)2(s), one The phosphorus oxides P4O6(s) and P4O10(s) react of the world’s most important fertilizers. with cold water to form the phosphorus oxyacids: phosphorous acid, H3PO3(aq), and phosphoric acid, H PO (aq). The respective equations are: N-2. Phosphorus Forms Several Oxyacids 3 4 White phosphorus reacts directly with oxygen to pro- P4O6(s) + 6 H2O(l ) → 4 H3PO3(aq) duce the oxides P4O6(s), and P4O10(s). With excess P4O10(s) + 6 H2O(l ) → 4 H3PO4(aq) phosphorus present, P4O6(s) is formed: The reaction of P4O10(s) with cold water is quite vigor- P4(s) + 3 O2(g) → P4O6(s) ous and can be explosive. excess Phosphorus pentoxide, P4O10(s), is a powerful dehy- drating agent capable of removing water from concen- With excess oxygen present, P4O10(s) is formed: trated sulfuric acid, which is itself a strong dehydrating agent as we saw in Interchapter J. In a similar reaction, N O (s) can be obtained by reacting P O (s) with P4(s) + 5 O2(g) → P4O10(s) 2 5 4 10 excess nitric acid. The equations for the two reactions are University Science Books, ©2011. All rights reserved. www.uscibooks.com Title General Chemistry - 4th ed Author McQuarrie/Gallogy Artist George Kelvin Figure # N-3 (936) Date 12/21/09 Check if revision Approved N. PHOSPHORUS N3 P O Figure N.3 Molecular structure of P4O6 and P4O10. (a) The P4O6 molecule can be viewed as arising from the tetrahedral P4 molecule when an oxygen atom is inserted between each pair of adjacent phosphorus atoms. (b) The P4O10 molecule can be viewed as arising from P4O6 when an oxygen atom is attached to each of P O P O the four phosphorus atoms. Notice that there 4 6 4 10 are no phosphorus–phosphorus bonds in (a) (b) either P4O6 or P4O10. P4O10(s) + 6 H2SO4(l ) → 6 SO3(g) + 4 H3PO4(l ) proton in solution is called a monoprotic acid. Thus, phosphoric acid, H3PO4(aq), is triprotic; phosphorous P O (s) + 12 HNO (l ) → 6 N O (s) + 4 H PO (l ) 4 10 3 2 5 3 4 acid, H2(HPO3)(aq), is diprotic; and hypophosphorous acid, H(H2PO2)(aq), is monoprotic. As noted previ- Phosphorus pentoxide is used as a drying agent in ously, the latter two formulas are generally written as desiccators and dry boxes to remove water vapor. H3PO3(aq) and H3PO2(aq), respectively. The structures Hypophosphorous acid, H3PO2(aq), is prepared of these three acids are shown in Figure N.4. by reacting P4(g) with a warm aqueous solution of Almost 11.5 million metric tons of phosphoric acid NaOH(aq), followed by addition of a strong acid: are produced annually in the United States alone. It is produced industrially by the reaction of phosphate P(g) + 3 OH–(aq) + 3 H O(l ) → 3 H PO–(aq) + PH (g) 4 2 2 2 3 rock and sulfuric acid. Commercial phosphoric acid – + is sold as an 85% by mass (85 g of H3PO4 to 15 g of H2PO2(aq) + H (aq) → H3PO2(aq) H2O) solution, equivalent to 15 M.