Chambers, Holliday. Modern inorganic chemistry

GROUP V 247

which contain more than one phosphorus atom and P—O—P bonds. Structures include both ring and chain forms. Separation of these complex anions can be achieved by ion exchange and chromatography.

Two examples of condensed phosphoric(V) acids are heptaoxodiphosphoric(V) (pyrophosphoric) and polytrioxophosphoric (metaphosphoric) acids.

Heptaoxodiphosphoric acid, H4P2O7, as its old name suggests, is formed as one product when phosphoric(V) acid is heated (loss of water on heating leads to a mixture of acids). It forms two series of salts, the sodium salts, for example, have the formulae Na2H2P2O7 and Na4P2O7.

In solution, both heptaoxodiphosphoric(V) acid and the heptaoxodiphosphates(V) (pyrophosphates) are slowly converted (more rapidly on heating) to phosphoric(V) acid or its salts, for example

H4P2O7 + H2O -» 2H3PO4

Polytrioxophosphoric(V) acid, (HPO3)n? is formed as a polymeric glassy solid when phosphoric(V) acid is heated for a long period. It may also be obtained in solution by passing sodium polytrioxophosphate(V) through a cation-exchange column. It is a monobasic acid, forming only one set of salts, but the simple formula, NaPO3, for the sodium salt, is misleading since there are many polytrioxophosphates known of general formula (NaPO3)^ where n may be 3, 4 or a much larger number.

A salt originally called sodium hexametaphosphate, with n believed to be 6, is now thought to contain many much larger anion aggregates. It has the important property that it "sequesters', i.e. removes, calcium ions from solution. Hence it is much used as a water-softener.

Arsenic

THE + 3 ACIDS

Arsenic(III) (arsenious) acid, H3AsO3.—When arsenic(III) oxide is dissolved in water the corresponding acid is formed :

As4O6 + 6H2O ^ 4H3AsO3

It is an extremely weak acid but does form salts. Two kinds are known, trioxoarsenates(III), for example Na3AsO3, and dioxoarsenates(III), for example Cu(AsO2)2-

248 GROUPV

The arsenate(III) ion can be reduced by systems which generate hydrogen (forexample metal/acid) to give arsine, for example

AsO^ + 3Zn + 9H+ -> AsH3T + 3Zn2+ + 3H2O

whilst other reducing agents give either arsenic or an arsenide. Powerful oxidising agents, for example Cr2O7~ and MnO^ ions,

oxidise the arsenate(III) ion to arsenate(V). The reaction with iodine, however, is reversible depending on the conditions:

AsOf ~ + I2 + 2OH" ^ AsOr + 2I~ 4- H2O

THE + 5 ACIDS

Arsenic(V) acid, H3AsO4 (strictly, tetraoxoarsenic(V) acid) is obtained when arsenic is oxidised with concentrated nitric acid or when arsenic(V) oxide is dissolved in water. It is a moderately strong acid which, like phosphoric(V) acid, is tribasic; arsenates(V) in general resemble phosphates(V) and are often isomorphous with them.

Arsenates(V) are more powerful oxidising agents than phosphates(V) and will oxidise sulphite to sulphate, hydrogen sulphide (slowly) to sulphur and, depending on the conditions, iodide to iodine.

Antimony

No + 3 acid is known for antimony but antimonates(III) (antimonites) formed by dissolving antimony(III) oxide in alkalis are known, for example sodium dioxoantimonate(III), NaSbO2.

The + 5 acid is known in solution and antimonates(V) can be obtained by dissolving antimony(V) oxide in alkalis. These salts contain the hexahydroxoantimonate(V) ion,[Sb(OH)6]~,

Bismuth

Bismuth(HI) oxide is basic. If, however, a suspension of bismuth(III) hydroxide is oxidised with a strong oxidising agent such as the peroxodisulphate ion (p. 304)the hexahydroxobismuthate(V) ion [Biv(OH)6]~ is formed. Evaporation of, for example, the sodium salt, gives the trioxobismuthate(V), NaBiO3. Bismuthates(V) are extremely powerful oxidising agents and will oxidise, for example, the manganese(II) ion to manganate(VII),

G R O U P V 249 HALOGEN COMPOUNDS OF GROUP V ELEMENTS

Nitrogen trifluoride and trichloride can both be prepared as pure substances by the action of excess halogen on ammonia, a copper catalyst being necessary for the formation of nitrogen trifluoride.

Nitrogen trifluoride is an exothermic compound (A/ff = — 124.7 kJ moF1). It is an unreactive gas with a high thermal stability and a very low dipole moment (cf. NH3, p. 216).

In contrast the endothermic trichloride, A/ff = + 230.1 kJ moP1), is extremely reactive with a tendency to explode, being particularly unstable above its boiling point, 344 K, in light, or in the presence of organic compounds. Unlike the trifluoride it is readily hydrolysed by water to ammonia and chloric(I) acid:

NC13 4- 3H2O -> NH3 + 3HOC1

The pure tribromide and triodide are unknownbut their ammoniates have been prepared by the action of the appropriate halogen on ammonia. The tribromide, NBr36NH3, is a purple solid which decomposes explosively above 200K. The iodide, NI3.wNH3, is a black explosive crystalline solid, readily hydrolysed by water.

Phosphorus, arsenic, antimony and bismuth

With the exception of phosphorus trifluoride, these elements form their trihalides by direct combination of the elements, using an excess of the Group V element. As a series they show increasing ionic character from phosphorus to bismuth, this being indicated by their increasingly higher melting and boiling points and their increasing ability to form cations in aqueous solution. In addition to the trihalides a number of pentahalides have also been prepared. All the pentafluorides are known, together with the pentachlorides of phosphorus, and antimony. Phosphorus also forms a pentabromide. Some of the important halides are discussed in more detail below.

THE PHOSPHORUS(III) HALIDES

Phosphorus trifluoride

Phosphorus trifluoride is a colourless gas; the molecule has a shape similar to that of phosphine. Although it would not be expected to be an electron donor at all (since the electronegative

250 GROUP V

fluorine atoms will attract the lone pair electrons), it forms a compound with nickel, Ni(PF3)4, verylike nickel tetracarbonyl, Ni(CO)4, This is explained by the fact that phosphorus can expand its valency shell of electrons and so receive electrons from the nickel by a kind of 'back-donation', i.e. each nickel-phosphorus bond is Ni=PF3, not just Ni<-PF3.

Phosphorus trichloride

Phosphorus and chlorine combine directly to form either the trichloride or the pentachloride depending on the relative amounts of phosphorus and chlorine used.

The trichloride is obtained as a liquid, boiling point 349 K, when a jet of chlorine burns in phosphorus vapour. Care must be taken to exclude both air and moisture from the apparatus since phosphorus trichloride reacts with oxygen and is vigorously hydrolysed by water, fuming strongly in moist air.The hydrolysis reaction is:

PC13 + 3H2O -» H3PO3 + 3HC1 phosphonic

acid

Similar reactions occur with organic compounds which contain hydroxyl groups, thus

Hydrogen chloride is also evolved.

The reaction with oxygen converts phosphorus trichloride to phosphorus trichloride oxide (oxychloride), POC13 ; the trichloride is able to remove oxygen from some molecules, for example sulphur trioxide

PC13 + SO3 -* O= P-C1 + SO2

Phosphorus trichloride reacts with chlorine in excess to give phosphorus pentachloride, an equilibriumbeing set up:

PC13 + C12 ^ PCL

GROUP V 251

PHOSPHORUS(V) HALIDES

The properties of the phosphorus trihalides given above indicate the ability of phosphorus to increase its valency above 3. In phosphorus pentafluoride, PF5 (a gas), and the vapour of phosphorus pentachloride, PC15 (solid at ordinary temperatures), phosphorus is covalently bound to the halogen atoms by five equal bonds to give a trigonal bipyramid structure. However, the covalency can increase further to six; the acid, HPF6, and its salts, for example NaPF6, containing the octahedral PF^ ion (hexafluorophosphate) are well known and stable. Here again then, fluorine excites the maximum covalency and we can compare the ions A1F|~, SiF^"", PF^.

However, phosphorus pentachloride in the solid state has an ionic lattice built up of (PC14)+ and (PC16)~ ions and these ions are believed to exist in certain solvents. Thus under these conditions the maximum covalency is reached with chlorine. In phosphorus pentabromide, PBr5, the solid has the structure [PBr4]+ [Br]~.

Phosphorus pentachloride, PC15

Phosphorus pentachloride is prepared by the action of chlorine on phosphorus trichloride. To push the equilibrium over to the right, the temperature must be kept low and excess chlorine must be present. Hence the liquid phosphorus trichloride is run dropwise into a flask cooled in ice through which a steady stream of dry chlorine is passed: the solid pentachloride deposits at the bottom of the flask.

Phosphorus pentachloride sublimes and then dissociates on heating, dissociation being complete at 600 K. It is attacked by water, yielding first phosphorus trichloride oxide, thus:

The replacement of the —OH group by a chlorine atom (reaction 9.4) is a very general reaction of phosphorus pentachloride. For example, if concentrated sulphuric acid is written as (HO)2SO2 then its reaction with phosphorus pentachloride may be written:

Cl (HO)2SO2 + 2PC15 -» 2O=PC13 + 2HC1 + XSO2

Q\

sulphur dichloride dioxide

254 GROUP V

ammonium molybdate and nitric acid. Specific tests for various oxophosphates are known.

Arsenic

Because of its toxicity, it is often necessary to be able to detect arsenic when present only in small amounts in other substances.

Arsenic present only in traces (in any form) can be detected by reducing it to arsine and then applying tests for the latter. In Marsh's test, dilute sulphuric acid is added dropwise through a thistle funnel to some arsenic-free zinc in a flask; hydrogen is evolved and led out of the flask by a horizontal delivery tube. The arseniccontaining compound is then added to the zinc-acid solution, and the delivery tubeheated in the middle.If arsenicis present, it isreduced to arsine by the zinc-acid reaction, for example:

AsOl~ + 4Zn + 11H+ -» AsH3 + 4Zn2+ + 4H2O

The evolved arsine is decomposed to arsenic and hydrogen at the heated zone of the delivery tube; hence arsenic deposits as a shiny black mirror beyond the heated zone.

Antimony andbismuth

As can be expected, antimony compounds resemble those of arsenic. In the Marsh test, antimony compounds again give a black deposit which, unlike that formed by arsenic compounds, is insoluble in sodium chlorate(I) solution.

Solutions of many antimony and bismuth salts hydrolyse when diluted; the cationic species then present will usually form a precipitate with any anion present. Addition of the appropriate acid suppresses the hydrolysis, reverses the reaction and the precipitate dissolves. This reaction indicates the presence of a bismuth or an antimony salt.

When hydrogen sulphide is bubbled into an acidic solution of an antimony or a bismuth salt an orange precipitate, Sb2S3, or a brown precipitate, Bi2S3, is obtained. Bismuth(III) sulphide, unlike antimony(III) sulphide, is insoluble in lithium hydroxide.

QUESTIONS

1. Give an account of the oxides and the chlorides of arsenic, antimony and bismuth, including an explanation of any major

G R O UP V 255

differences. Showhow the increasing metalliccharacter ofthe element is reflected in the chemical behaviour of these compounds. Suggest a reason for the non-existence of AsCl5 and BiQ5.

(C,S)

2. Outline the laboratory preparation of a sample of dinitrogen tetroxide. Describe and explain what happens when it is heated from 290 K to 900 K. Suggest electronic structuresfor dinitrogen tetroxide and the other nitrogen-containing molecules formed from it on heating to 900 K. Point out any unusual structural features.

(C,A)

3. For either Group IV or Group V:

(a)point out two general trends in the physical properties of the elements, and explain, as far as you can, why these trends occur;

(b)give examples of the way in which the most stable oxidation number of the elements in their compounds tends to decrease by two towards the bottom of the group, and describe how this tendency is related to their oxidising and reducing properties:

(c)describe in outline how, starting from the element, you would prepare a pure sample of either an oxide or chloride of an element in the group, and state how you would, in principle, try to establish its empirical formula.

(N, Phys. Sci. A)

4. Compare and contrast the following pairs of compounds as regards (a) methods of preparation, (b) important properties including hydrolysis, (c) thermal stability:

(i)NCl3andPCl3;

(ii)NH3 andPH3 ;

(iii)N2O5andP4O10.

As far as possible account for different behaviour in terms of the structures of the compounds and the nature of bonding present.

(L,S)

5. (a) What is meant by the statement that 'nitrogen dioxide, NO2, is an odd-electron molecule'?

(b)When NO2 dimerises to form N2O4, the product is not an odd-electron molecule. What explanation can you offer for this fact?