Чамберс К., Холлидей А.К. Современная неорганическая химия, 1975

GROUPV 207

Bi(OH)3 is insoluble in alkali but readily soluble in acids to form salts.

The outer quantum level of the Group V elements contains five electrons, but there is no tendency for the elements at the top of the group to lose these and form positive ions. Nitrogen and phosphorus are, in fact, typical non-metals, having acidic oxides which react with alkalis to give saks. Nitrogen, the head element, shows many notable differences from the other Group V elements, the distinction arising from the inability of nitrogen to expand the number of electrons in its outer quantum level beyond eight. (The other Group V elements are able to use d orbitals in their outer quantum level for further expansion.) The nitrogen atom can (a) share three electrons to give a covalency of three, leaving a lone pair of electrons on the nitrogen atom, (b) share three electrons and donate the unshared pair to an acceptor atom or molecule, as in NH^,

//°

H3N-»A1C13 and nitric acid H—O—N when nitrogen achieves

XO

its maximum covalency of four, (c) acquire three electrons when

combining with very electropositive elements to form the nitride ion, N3 ~.

The other Group V elements can behave in a similar manner but their atoms have an increasing reluctance to accept electrons, and to donate the lone pair. These atoms can, however, increase their covalency to five, for example in the vapour of phosphorus pentachloride, or even to six, for example in the ions [PF6]~, [PC16]~. Hence phosphorus, arsenic, antimony and bismuth are able to form both trivalent and pentavalent compounds but as we go from phosphorus to bismuth it becomes increasingly more difficult to achieve a pentavalent state—thus phosphorus(V) oxide, P4O10, is readily obtained by burning phosphorus in excess air, but the corresponding oxides of antimony and bismuth require the action of strong oxidising agents for their preparation and bismuth(V) oxide is particularly unstable.

THE ELEMENTS: THEIR OCCURRENCE AND EXTRACTION

NITROGEN

Nitrogen is an essential constituent of all living matter, being one of the elements present in proteins. Proteins are synthesised by

GROUPV 209

These gases leave the furnace at about 600 K. pass through electrostatic precipitators to remove dust, and the phosphorus is then condensed out.

ARSENIC, ANTIMONY AND BISMUTH

Each of these elements occurs naturally as a sulphide ore: arsenic as realgar As4S4, orpiment As4S6 and arsenical pyrites with approximate formula FeAsS; antimony as stibnite Sb2S3; and bismuth as

Bi2S3.

The method of extraction is similar for each element involving first the roasting of the sulphide ore when the oxide is produced, for example

Sb2S3 + 5O2 -> Sb2O4 + 3SO2

followed by reduction of the oxide with carbon, for example

As4O6 + 6C -> As4 + 6COt

PROPERTIES OF THE ELEMENTS

The main physical properties of these elements have been given in

Table 9.1.

ALLOTROPES

Solid phosphorus, arsenic and antimony exist in well known allotropic modifications. Phosphorus has three main allotropic forms, white, red and black. White phosphorus is a wax-like solid made up of tetrahedral P4 molecules with a strained P—P—P angle of 60°; these also occur in liquid phosphorus. The reactivity of white phosphorus is attributed largely to this strained structure. The rather less reactive red allotrope can be made by heating white phosphorus at 670K for several hours; at slightly higher temperatures, -690 K, red phosphorus sublimes, the vapour condensing to reform white phosphorus. If, however, red phosphorus is heated in a vacuum and the vapour rapidly condensed, apparently another modification, violetphosphorus, is obtained. It is probable that violet phosphorus is a polymer of high molecular weight which on heating breaks down into P2 molecules. These on cooling normally dimerise to form P4 molecules, i.e. white phosphorus, but in vacua link up

210 GROUPV

again to give the polymerised violet allotrope. Red phosphorus may have a structure intermediate between that of violet phosphorus and white phosphorus, or it may be essentially similar to the violet species.

Black phosphorus is formed when white phosphorus is heated under very high pressure (12000 atmospheres). Black phosphorus has a well-established corrugated sheet structure with each phos phorus atom bonded to three neighbours. The bonding forces between layers are weak and give rise to flaky crystals which conduct electricity, properties similar to those ol graphite. It is less reactive than either white or red phosphorus.

Arsenic and antimony resemble phosphorus in having several allotropic modifications. Both have an unstable yellow allotrope. These allotropes can be obtained by rapid condensation of the vapours which presumably, like phosphorus vapour, contain As4 and Sb4 molecules respectively. No such yellow allotrope is known for bismuth. The ordinary form of arsenic, stable at room temperature, is a grey metallic-looking brittle solid which has some power to conduct*. Under ordinary conditions antimony and bismuth are silvery white and reddish white metallic elements respectively.

CHEMICAL REACTIVITY

1. Reaction with air

NITROGEN

The dissociation energyofthe N=N bond isvery large. 946 kJ mol" \ and dissociation of nitrogen molecules into atoms is not readily effected until very high temperatures, being only slight even at 3000 K. It is this high bond energy coupled with the absence of bond polarity that explains the low reactivity of nitrogen, in sharp contrast to other triple bond structures such as —C=N, —C^O,

—C^C—t. Nitrogen does, however, combine with oxygen to a small extent when a mixture of the gases is subjected to high temperature or an electric discharge, the initial product being nitrogen

* The incorporation of minute amounts of arsenic in semi-conductors has been mentioned (p. 166).

•!• Certain living systems can 'fix' atmospheric nitrogen, using a metalloenzyme called nitrogenase. Attempts are being made to imitate this mode of fixation by synthesising transition metal complexes in which molecular nitrogen, N2, is present as a ligand. The problem of easy conversion of this to (for example) NH3 or NOJ remains to be solved.

GROUPV 211

Connections to induction coil

Conical flask

Platinum wire electrodes

Figure 9.1

monoxide, NO. The combination caused by an electric discharge can readily be shown in the laboratory using the simple apparatus shown in Figure 9.1.

PHOSPHORUS

White phosphorus is very reactive. It has an appreciable vapour pressure at room temperature and inflames in dry air at about 320 K or at even lower temperatures if finely divided. In air at room temperature it emits a faint green light called phosphorescence; the reaction occurring is a complex oxidation process, but this happens only at certain partial pressures of oxygen. It is necessary, therefore, to store white phosphorus under water, unlike the less reactive red and black allotropes which do not react with air at room temperature. Both red and black phosphorus burn to form oxides when heated in air, the red form igniting at temperatures exceeding 600 K,

GROUPV 213

The phosphine produced is impure and contains small quantities of diphosphane, P2H4 (p. 227).

Arsenic, unlike phosphorus, is only slightly attacked by boiling sodium hydroxide; more rapid attack takes place with the fused alkali; an arsenate(III) is obtained in both cases,

As4 4- 12OKT -

cf. aluminium (p. 144). Arsine is not formed in this reaction.

Antimony and bismuth do not react with sodium hydroxide.

4. Reaction with halogens

Nitrogen does form a number of binary compounds with the halogens but none of these can be prepared by the direct combination of the elements and they are dealt with below (p. 249). The other Group V elements all form halides by direct combination.

PHOSPHORUS

White and red phosphorus combine directly with chlorine, bromine and iodine, the red allotrope reacting in each case at a slightly higher temperature. The reactions are very vigorous and white phosphorus is spontaneously inflammable in chlorine at room temperature. Both chlorine and bromine first form a trihalide:

but this is converted to a pentahalide by excess of the halogen. No pentaiodide is known (p. 316).

ARSENIC, ANTIMONY AND BISMUTH

A complete set of trihalides for arsenic, antimony and bismuth can be prepared by the direct combination of the elements although other methods of preparation can sometimes be used. The vigour of the direct combination reaction for a given metal decreases from fluorine to iodine (except in the case of bismuth which does not react readily with fluorine) and for a given halogen, from arsenic to bismuth.

In addition to the trihalides, arsenic and antimony form pentafluorides and antimony a pentachloride; it is rather odd that arsenic pentachloride has not yet been prepared.

214 GROUPV

COMPOUNDS OF GROUP V ELEMENTS

1. HYDRIDES

All Group V elements form covalent hydrides MH3. Some physical data for these hydrides are given below in Table 9.2. The abnormal values of the melting and boiling points of ammonia are explained by hydrogen bonding (p. 52). The thermal stabilities of the hydrides decrease rapidly from ammonia to bismuthine as indicated by the mean thermochemical bond energies of the M—H bond, and both stibine, SbH3, and bismuthine, BiH3, are very unstable. All the

Table 9.2

PROPERTIES OF GROUP V HYDRIDES

Group V hydrides are reducing agents, the reducing power increasing from NH3 to BiH3, as thermal stability decreases.

These stability changes are in accordance with the change from a non-metal to a weak metal for the Group V elements nitrogen to bismuth.

Nitrogen, phosphorus and arsenic form more than one hydride. Nitrogen forms several but of these only ammonia, NH3, hydrazine, N2H4 and hydrogen azide N3H (and the ammonia derivative hydroxylamine) will be considered. Phosphorus and arsenic form the hydrides diphosphane P2H4 and diarsane As2H4 respectively, but both of these hydrides are very unstable.

Hydrides of nitrogen

AMMONIA NH3

Ammonia is manufactured by the direct combination of the elements

N2 4- 3H2 ^ 2NH3 AH - -92.0kJmor~l

The production by this method was developed originally by Haber after whom the process is now named. Since the reactionis reversible