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

256GROUP V

(c)Give two properties which are characteristic of odd-electron molecules.

(d)Give the name and formula of a compound in which NO^ ions are bonded to a metal ion by the donation of electron pairs.

(e)By means of equations, and stating the appropriate conditions, show how a sample of nitrogen(IV) oxide (nitrogen dioxide) may be obtained in the laboratory.

(JMB, A)

10

Group VI

(Oxygen, sulphur, selenium, tellurium, polonium)

The elements in this group have six electrons in their outer quantum level, and can thus achieve a noble gas configuration by acquiring two electrons.

PROPERTIES OF THE ELEMENTS

Some of the more important physical properties of the elements are given in Table 10.1.

Melting and boiling points increase with increasing atomic number from oxygen to tellurium, with oxygen showing the deviation typical of a group head element. The expected decrease in ionisation energy with increase in atomic number and size of the atoms should be noted.

Although the electron affinities do not change regularly with increasing atomic number, the increasing ionic radii imply decreasing lattice and hydration enthalpies. Hence, although oxygen forms a large number of wholly or partly ionic oxides with metals, containing O2 ~, sulphur forms ionic compounds only with the more electropositive elements such as sodium, and most of its compounds are partly or wholly covalent.

All the elements are able to share two electrons forming two covalent bonds. The two covalent bonds formed by oxygen can be separate bonds, for example

257

M

01

00

TiblelU

SELECTED PROPERTY OF THE ELEMENTS

A . Owler

,MOM*

/vw.,p._Mm

Ik:tron

Is!

r,

Element

tome

fi.p,

nitF

, , fl| ..lonisdtiouEktro-

t

mm

i ,

,

mm

, (K)

ofX

,'-u

(kin

fwrflf,

«fpWF

., i

nmm

(K)

ioi

]

W X - tX 1 -

IkJraol"') (Pa™g)

0

8

2s22p4

016

54

0.14690

-141+791

1310

3.5

s

16

3sV

0.104 1.90m

718

-200 +M9

W

2.5

Se

34

3d104r4|)4

0.114

0,20249

958

-213+102

941

2.4

Tc

52

4dl(Wf)4

0.132

7230.222

1260

-222

+62?

869

2.1

Po

84

5Jlofc26|)4

52?

1235

-

813

''ovaht radius.

GROUP VI 259

H\ C'V

JO, O

H C2H5X

or a double bond, for example

CH3 _

O=C=O, \C ^°

H

The covalently bonded oxygen atom still has two lone pairs of electrons and can act as an electron pair donor. It rarely donates both pairs (to achieve 4-coordination) and usually only one donor bond is formed. A water molecule, for example, can donate to a proton, forming ^O"1", and diethyl ether can donate to an acceptor such as boron trifluoride :

C2H5

0-.BF3

Sulphur in hydrogen sulphide and its derivatives is a much less effective simple electron pair donor and the other Group VI elements show this property to a very minor extent. However, compounds based on divalent sulphur (for example, dimethylsulphide (CH3)2S) are often found to be effective ligands in transition metal Complexes. Unlike oxygen, the remaining elements can increase their covalency to a maximum of six by utilising the low energy d orbitals not available to oxygen, and 6— coordinate compounds (for example SF6) are known. However, as the atomic number and size of the atoms increase from oxygen to polonium, the elements become more electropositive, the hydrides less stable and the stabilities of the higher oxidation states decrease. Only polonium can really be said to show weakly metallic properties, although tellurium oxides are amphoteric.

There are peculiarities associated with compounds containing oxygen and hydrogen where hydrogen bond formation gives rise to many properties which are not shown by the compounds of the other elements.

260 GROUP VI

THE ELEMENTS: OCCURRENCE AND EXTRACTION

OXYGEN

Oxygen occurs free in the atmosphere (21% by volume. 23% by

weight). The	proportion is constant-over the earth's surface;			it is
also constant	for many miles	upwards, because	the turbulence of
the atmosphere prevents the		tendency for the	lighter gases,	for
example helium, to increase in amount at higher altitudes.

Water contains 89% by weight of oxygen, and the outer crust of the earth contains about 47%; hence air, earth and sea together contain about 50%by weight of oxygen.

On the industrial scale oxygen is obtained by the fractional distillation of air. A common laboratory method for the preparation of oxygen is by the decomposition of hydrogen peroxide, H2O2, a reaction catalysed by manganese(IV) oxide:

2H2O2 — 2H2O + O2T

A similar decomposition of the chlorate(I) (hypochlorite) ion, OC1~, catalysed by both light and cobalt(II) ions, is less commonly used:

2C1CT -»2Cr 4- O2T

Oxygen can also be prepared by the thermal decomposition of certain solid compounds containing it. These include oxides of the more noble metals, for example of mercury or silver:

2HgO -> 2Hg + O2T

certain higher oxides, for example of lead(IV) and manganese(IV):

2PbO2 ^ 2PbO + O2T peroxides, for example of barium:

2BaO2 ^ 2BaO + O2T

and certain oxosalts, notably the nitrates, chlorates(V), iodates(V) and manganates(VII) of alkali metals.

Pure oxygen is conveniently prepared by the thermal decomposition of potassium manganate(VII):

2KMnO4 -» K2MnO4 + MnO2 + O2T

Oxygen can be produced by certain reactions in solution, forexample the oxidation of hydrogen peroxide by potassium manganate(VII) acidified with sulphuric acid:

2MnO4~ + 5H2O2 + 6H3O+ -» 2Mn2 + + 14H2O 4- 5O2t

GROUP VI 261

SULPHUR

Large deposits of free sulphur occur in America, Sicily and Japan. Combined sulphur occurs as sulphides, for example galena, PbS, zinc blende, ZnS, and iron pyrites, FeS2, and as sulphates, notably as gypsum or anhydrite, CaSO4.

In America, the sulphur deposits (mostly in Louisiana and Texas) are dome-shaped layers about 30cm thick, between limestone above and anhydrite below. From these, the sulphur is extracted

by the Frasch process. A metal		tube, about 15cm diameter			and
containing two	concentric inner	tubes (Figure	10.1) is	sunk	into
the top of the	deposit. Water,	superheated to	450 K,	is forced

Air

* Molten

sulphur

Superheated-

water

Water

Molten

sulphur

Figure 10.1. The Frasch pump

under pressure down the outer tube, and enters the sulphur layer through perforations. The sulphur melts (m.p. 388 K) and enters the inner pipe at the bottom, up which it flows for some distance. Compressed air is forced down the innermost pipe; this emulsifies the water and molten sulphur mixture, so lowering its density, and the emulsion rises to the top of the pipe, where it is run off into vats to solidify. The purity is usually 99.8 %.

Large quantities of sulphur are recovered from petroleum and natural gas. Naturally occurring hydrogen sulphide, H2S, and that produced in the cracking and catalytic hydrogenation of petroleum is first removed by absorption and the regenerated gas is converted to sulphur by partial combustion with air, the overall reaction being,

6H2S + 3O2 -» 6H2O + 6S

262 GROUP V!

SELENIUM AND TELLURIUM

Selenium and tellurium occur naturally in sulphide ores, usually as an impurity in the sulphide of a heavy metal. They are recovered from the flue dust produced when the heavymetal sulphide is roasted.

POLONIUM

This is a radioactive element. It occurs in minute traces in barium and thorium minerals, but it can be produced by irradiation of bismuth in a nuclear reactor. (The study of its chemistry presents great difficulty because of its intense a radiation).

ALLOTROPES

Oxygen, sulphur and selenium are known to exist in more than one allotropic form.

OXYGEN

This exists in two allotropic forms, oxygen, O2 and ozone, O3. Oxygen is a colourless gas which condenses to a pale blueliquid,

b.p. 90 K, which is markedly paramagnetic indicating the presence of unpaired electrons (p. 229). Simple valence bond theory (as used in this book) would indicate the structure

:'p: q: i.e. 0=0

which accounts for the high oxygen-oxygen bond strength (bond dissociation energy, 49 kJ mol"1). but does not explain the paramagnetism. The molecular orbital theory of bonding, however, suggests not only a doubly bonded structure but also two molecular orbitals (i.e. orbitals of the complete O2 molecule) of equal energy each containing one electron, and this satisfactorily explains both the high bond strength and paramagnetism.

Oxygen, like nitrogen oxide, NO, shows little tendency to dimerise although the presence of the unstable, weakly bonded species, tetratomic oxygen O4, has been reported as a constituent of liquid oxygen.

Ozone, O3, is found in trace quantities in the upper atmosphere where it is believed to be formed by the photochemical dissociation of oxygen molecules by the intense ultra-violet light from the sun;

GROUP V! 263

absorption of this light in the process prevents it from reaching the earth where it would destroy all living matter very rapidly.

Small quantities of ozone are produced when oxygen and air are subjected to an electrical discharge and it is, therefore, found in the neighbourhood of working electrical machines. Probably a small quantity of atomic oxygen is initially produced; most of this reeombines quickly to give oxygen, O2, but a few atoms react to form ozone:

O2 + O-»O3

The ozone molecules also decompose by reaction with atomic oxygen, so that the actual concentration of ozone is small.

	Platinum electrodes
Dry —	-Ozonised
oxygen	oxygen
	-Dilute sulphuric
	acid

Figure 10.2. Preparation of ozone: Brodie's apparatus

Ozone is formed in certain chemical reactions, including the action of fluorine on water (p. 323) and the thermal decomposition of iodic(VII) (periodic) acid. It is also formed when dilute (about 1M) sulphuric acid is electrolysed at high current density; at low temperatures the oxygen evolved at the anode can contain as much as 30% ozone.

Ozone is normally produced by the use of a silent electrical discharge and a number of ozonisers have been produced. Brodie's apparatus is shown in outline in Figure10.2.

Using a potential of approximately 20000 V the ozonised oxygen produced can contain up to 10% ozone and pure ozone can be obtained by liquifaction of the mixture followed by fractional distillation (O2, b.p. 90 K; O3, b.p. 161 K).

264 GROUP VI

At room temperature ozone is a slightly blue diamagnetic gas which condenses to a deep blue liquid. It has a characteristic smell, and is toxic. Ozone is a very endothermic compound :

O3 -+fO2 :A//= -142kJmor!

It decomposes exothermically to oxygen, a reaction which can be explosive. Even dilute ozone decomposes slowly at room temperature; the decomposition is catalysed by various substances (for example manganese(IV) oxide and soda-lime) and occurs more rapidly on heating.

Ozone is very much more reactive than oxygen and is a powerful oxidising agent especially in acid solution (the redox potential varies with conditions but can be as high as + 2.0 V). Some examples are:

1. the conversion of black lead(II) sulphide to white lead(II) sulphate (an example of oxidation by addition of oxygen):

PbS 4- 4O3 -> PbSO4 + 4O2T

2. the oxidation of iron(II) to iron(III) in acid solution:

2Fe2+ + O3 + 2H3O+ -> 2Fe3+ + O2t + 3H2O

The adherence of mercury to glass, i.e. tailing' in presence of ozone, is probably due to the formation of an oxide. The oxidation of the iodide ion to iodine in solution is used to determine ozone quantitatively.

H2O + O3 -> 2OH~ + I2 4- O t

The liberated iodine istitrated with standard sodium thiosulphate(VI) solution after acidification to remove the hydroxide ions.

Addition compounds called ozonides are produced when alkenes react with ozone and reductive cleavage of these compounds is used extensively in preparative and diagnostic organic chemistry.

The molecular formula of ozone was determined by comparing its rate of diffusion with that of a known gas. The geometric structure

of the molecule is angular O./°\O with two equal O—O distances, which are slightly greater than in the oxygen molecule, and an O—O—O angle of 116°.

Ozone has long been used on a small scale for water purification since it destroys viruses, and recent developments suggest that this use will increase in importance.

G R O UP VI 265

SULPHUR

The structures of sulphur in solid, liquid and gaseous phases are complicated. Rhombic sulphur is the solid allotrope stable at room temperature. It is yellow, readily soluble in carbon disulphide, from which it can be crystallised, and has a density of 2.06 g cm"3. Above 369 K, the transition temperature, rhombic sulphur is no longer stable, slowly changing to monoelinic sulphur, and if rhombic sulphur is melted, allowed to partly solidify, and the remaining molten sulphur is poured off, there remain long needle-like crystals (almost colourless) of monoelinic sulphur, density 1.96 g cm~3. A good specimen of monoelinic sulphur can be prepared by crystallising a concentrated solution of sulphur in xylene, taking care to keep the temperature above 368 K. On standing at room temperature, monoelinic sulphur slowly changes to the rhombic form. Both these allotropes contain S8 molecules with rings of eight sulphur atoms.

, - .

When sulphur is melted viscosity changes occur as the temperature is raised. These changes are due to the formation of long-chain polymers (in very pure sulphur, chains containing about 100000 atoms may be formed). The polymeric nature of molten sulphur can be recognised if molten sulphur is poured in a thin stream into cold water, when a plastic rubbery mass known as plastic sulphur is obtained. This is only slightly soluble in carbon disulphide, but on standing it loses its plasticity and reverts to the soluble rhombic form. If certain substances, for example iodine or oxides of arsenic, are incorporated into the plastic sulphur, the rubbery character can be preserved.

Colloidal sulphur is produced by careful addition of acid to sodium thiosulphate solution.

SELENIUM

Like sulphur,selenium exists in a number of allotropic forms. These include both crystalline, rhombic and monoelinic modifications