UNIT 9 ELElMENTS OF GROUP 16
Structure 9.1 Introdbction objectives .9.2 Occurrence, Extraction and Uses OccutraKz Extraction uses 9.3 General Characteristics Physical Properties Chemical Properties 9.4 Compounds of Group 16 Elements Hydrides Hahdes Oxwles 9.5 Oxoacids of Sulphur Sulphurous Acid Sulphunc Acid Peroxoacids of Sulphur 7hiosulphuric Acid 9.6 Anomalous Behaviour of Oxygen 9.7 Summary 9.8 Terminal Questions 4.9 Answers
9.1 INTRODUCTION - , In the previous unit, you studied the Group 15 elements. In this unit, we shall discuss Group 16 elements, viz., oxygen, sulphur, selenium, tellurium and polonium. These elements are called'chalcogens or the ore forming elements. This name was derived from the Greek npme for copper, since most of the copper ores contain members of this group. Compared to Groups 14 and 15 elements, the elements of this group are even more non- metallic in their behaviour. These also exhibit a gradual change towards metallic ;haracteristics down the group. Thus polonium, the last member of the group, is almost metallic. We will deskribe the occurrence and extraction of these elements and also their allotropic modifications. Uses of these elements and some of their compounds will also be described. We shall then go on to discuss the-general behaviour of these elements and their important compounds like hydrides. halides, oxides and oxoacids. In the next unit, you will study Group 17, which consists of halogens.
After studying this unit, you should be able to : describe the occurrence, extraction and uses of the elements of Group 16, describe the allotropic forms of all these elements, discuss the general characteristics of Group 16 elements, describe the geqeral properties of hydrides, halides and oxides of Group 16 elements, and list different oxoacids of Group 16 elements with an emphasis on the oxoacids of
9.2 OCCURRENCE, EXTRACTION AND USES
Except oxygen and sulphur, other elements of the groupare relatively scarce. While oxygen sustains all life on this planet, other elements and their compounds also find a variety of uses. We shall discuss the occurtFnce, extraction and uses of these elements in this section. 9.2.1 Occurrence Oxygen is the most abundhnt of all elements on earth. Dry air contains 20.946% oxygen by volume in the free form. Oxygen forms about 46.6% by weight df the earth's crust including the oceans and the atmosphere. Most of this combined oxygen is in the form of silicates. oxides and water. Water is one of the most abundant compounds on earth, present not only in oceans which cover three fourths of the earth's surface but also in the atmosphere as vapour and in the subterranean aquifers. Living organisms also contain large amounts of water. Water contains 88.8% of oxygen by weight. By contrast, the abundance of sulphur in the earth's crust is only 0.03-0.19. It is often found as the free element near volcanic regions in lapan. Sicily, Mexico and Luisiana in USA. Combined sulphur exists primarily in sulphates and sulphides. such as gypsum. CaSO,. 2H,O. barite, BaSO, and epsom salt. MgSO,. 7H,O. The sulphides include~galena. PbS. zinc blende. ZnS. chalcopyrite CuFeS,. iron pyrite. FeS, and cinnabar. HgS. etc It also occurs in mineral springs as H?S. Along with C. H. 3. N and P, sulphur is one of the twenty-three odd elements essential for life. It is a constituent of substances such as eggs, wool, hair and also mustard, garlic, cabbage, etc. Selenium and tellurium also occur as sulphide ores. Polonium occurs naturally as a decay product in thorium and uranium minerals. Allotropic forms All the elements of the group show allotropy. Oxygen exists in two allotropic forms. Dioxygen. 02,is a diatomic gas, paramagnetic in nature. Lewis structure of oxygen molecule with a pair of covalent bonds between two oxygen atoms, as shown in the margin is inadequate in explaining its paramagnetic nature (Fig.9. I). This structure with all paired electrons is expected to be diamagnetic rather than paramagnetic. Paramagnetism of oxygen can be explained on the basis of Molecular Orbital Theory. This theory has been explained in Selenium has been named after the goddess of moon 'Selene' detail in Unit 5 of the 'Atoms and Molecules' course. The molecular orbital configuration of 0, molecule can be represented as : Polonium was named after Marie Curie's home counlry. Poland. She ,124. rr* 2p ',. was also the discoverer of this KK, a 2s'. a* 2s2,a $7; , elernenl. ~2~:' rr* 2p! The presence of two unpaired electrons in the antibonding orbitals explaiqs the experimentally observed paramagnetic behaviour.
Ozone, 03.the other allotropic form of oxygen is a triatomic. pale blue gas. The only Fig 9.1 : Lewis struclure ol' O2 method used to make ozone commercially is to pass gaseous oxygen or air though a high molecule. voltage electric discharge called a silent electric dkcharge. Perhaps you know that the earth is covered by a layer of ozone which protects us from injurious ultraviolet rays coming from the sun. In the upper atmosphere at altitudes ranging from about 15-24 km. ozone is formed in appreciable amounts from oxygen by absorption of ultraviolet radiation from the sun. This radiation first splits O2 molecule into oxygen atoms, which react with O2 molecules to give 03.
Ozone alko absorbs ultraviolet light. This causes the 0, to decompose and form 0, again. 20, 30, The absorption of uv radiation by O3 servesa twofold purpose. It protects the inhabitants of our planet from injurious radiation and mliintains an equilibrium between the concenwations of 0, and 0,. Recently there has been a serious concern about the depletion of this layer. Nitric oxide froni emissions of supersonic jets and chlorotluorocarbons used as aerosol
propellants and as refrigerants have been identified as the main culprits. There is considerable. . international effort to save the protective ozone layer.
Sulphur displays allotropy to a remarkable degree. existing both in a variety of different molecular and physical forms. The molecular species. viz.. Sz.S,. S, and S, are in equilibriuni in gaseous sulphur. their proportions varyins with the temperature. The
commonest and the most stable allotrope of sulphur at room temperature is known as' . rhombic sulphur or a-sulphur. S,,. In rhomb~csulphur. S, rings are arranged in a way. Fig. 9.2. that gives a rhonibic crystal structure. At 369 K. rhombic sulphur gets convert.ed into monoclinic sulphur or P-sulphur. SF In monoclinic sulphur. Sx rings are arranged in a monoclinic structure. It is .;table between 369 and 392 K. At 392 K it melts to produce a liquid containing Sa molecules. S,. At about 433 K the Sa rings open up and join together Elements of Group 16 into long spiral-chain molecu~ksresulting in a thick viscous liquid, p-sulphur, S,. Liquid sulphur boils at 718 K to give gasews sulphur containing S8 mol~cules,which dissociate to S6, S4, S2 and finally to sulphur atoms at 2273 K. If liquid sulphur at 463 K is poured into cold water, plastic sulphur or y-sulphur is formed. The allotropy of sulphur as a function of temperature is summarised as follows :
1273 K 2273 K -s4-s2-s
Selenium, tellurium and polonium also exhjbit allotropy. Amorphous as well as crystalline . . .moiecule. forms of selenium and tellurium are known.
9.2.2 Extraction Oxygen is separated from air on a large scale by the fractional distillation of liquid air. It is obtained as a by-prodl~clalong with hydrogen during electrolysis of water for the manufacture
heat
...... middlepipe. As it comes out from the well, sulphur has a purity of 99.5-99. 9% and contains virtually no As. Se or Te...... sulphur from sulphide ores. Selenium and tellurium are obtained in concentrated form from anode mud obtained in the electrdyticrefining of copper. Polonium is made artificially by neutron irradiation of bismuth in a nuclear reactor.
209 1 210 210 Bi, + n -> Bi ------PO $ e 83 0 83 84
9.2.3 Uses
Oxygen is essential for life. Most life processes are based on oxidative metabolism: While the terrestrial beings take up oxygen through respiration, the aquatic plants and animals survive on oxygen dissolved in water. It is an important oxidant used in various energy generation processes through combustion of wood or of fossil fuels like coal, natural gas and petroleum. Rocket fuels have liquid oxygen as the oxidant. Since oxyacetylene and oxy- hydrogen flames have very high temperatures, they are used in cuttillg metals and in welding. Many chemical industries also use oxygen as an oxidant, e.g., in manufacture of,ethylene and propylene oxides, winyl acetate for polymer industry and oxidation of ammonia for manufacture of nitric acid. Ozone which is an allotropic form of oxygen is also a powerful oxidising agent. Ozone undergoes a chatacteristic reaction with unsaturated organic compounds where it attacks a double or triple. bond. It can, therefore, be used for the detection and characterisation of the double or tripre bond. It is also used in the treatment of drinking water. p-Block Elements-11 Sulphur is used in the manufacture of sulphur dioxide, sulphuric acid, gunpowder. n~atche\. fertilisers, drugs, bleaching agents, leather and other products. Large quantities of elemental Detergents are sodium salts of alkyl sulphur are used in the vulcanisation of rubber, in certain pintments and medicines. benzene sulphonic acids where alkyl Sulphuric acid of varying concentrations is used in the manufacture of fertilisers, paints, groups art: linear. Heavy metal salts pigments, dye-stuffs, fibres, plastics, detergents and soaps. It also finds its use in refining of these benzene sulphonic acids are petroleum. In recent years nitrogen-sulphur compounds have aroused considerable interest soluble, therefore, detergents can be because of their superconducting properties. A binary polymer, (SN),, exhibits metallic used in hard water. characteristics and becomes superconducting below 0.33 K.
Uses of selenium include photocopying process of xerography, decobrisation of glasses and as a catalytic agent particularly in the isomerisation of certain petroleum products. Selenium dioxide is used as an oxidising agent in organic reactions.
Tellurium is used primarily as an additive to steel to increase its ductility. It is also used as an additive to some catalysts, used in cracking of petroleum, as a colouring material for glasses and as an additive to lead to increase itsstrength and make it corrosion resistant. Polonium - 210 is used mainly for the produbion of neutron sources; for these, polonium is alloyed with elements such as beryllium.
SAQ 1 On the basis of molecular orbital diagram given for O2 molecule. find out the number of unpaired electrons in the following:
(i) 0; (ii) 0; (iii) 0:-
9.3 GENERAL CHARACTERIS-TICS
'The valence shell electronic configuration of Group 16 elements is /is2. tip4. These elements tend to gain inert gas configuration by accepting two electrons and forming E" anions (E=O. S, Se & Te) or by sharing two electrons in forwing two covalent bonds. Let us see how their physical properties are related to the~relectronic configuration and how do they behave chemical1y.
9.3.1 Physical' Properties Similar to the groups discussed earlier. the metallic character in Group 16,increaseswith increasing atomic number. Oxygen and sulphur are entirely non-metallic ifi their chemical behaviour. Selenium and tellurium, though essentially non-metallic, assume increasing metallic character and are termed as metalloids. Polonium is most metallic in the group. Some physical properties of Group 16 elen~entsarelisted in Table 9.1. and the trends in these properties are shown graphically in Figures 9.4 - 9.7. Density, Fig. 9.4, melting and boiling points. Fig. 9.5, covalent and ionic radii, Fig 9.6 show a regular increase from oxygen to polonium. The large difference in melti,lg and boiling points between oxygen and sulphur can be explained on the basis of their structure. Oxygen exists mostly as a diatomic molecule held together by weak van der Waals forces while others exist as polyatomic molecules, e.g., SX, Sen, etc.. where the ate-ms are bonded by c6valent bonds associated with high dissociation energy. Their existence as diatomic and polyatomic molecules can be explained as follows. The bond energy of the oxygen-oxygen double bond, 0=0,is 498 kJ mol-I. This makes the 0=0bond more than three times as strong as the SObond (bond energy for 0-0 is 142 kJ mol-I). By comparison the S=S bond is less than twice as strong as the S-S single bond (bondenergy for S=S, 43tkJ mol-I; S-S, 264 kJ mol-I). This results in catenated -0--0-0- chains being unstable relative to 0=0; but catenated -S-S-S- chains being stable relative to the molecule S=S. The elements of Group 16 are characterised by high ionisation energies, Fig. 9.7, decreasing gradually from oxygen to polonium. The high values indicate reluctance of these elements to form cations. Their ele{ttonegativities decrease with increasing atomic number. Thus, in view of the fall in electronegativity, metalll,c~character'withinthe group increases with '. Tabk 9.1 : Some physical properties of Gmup 16 e[em~nts Elements of Group 16
I Property 0 S S e Te . Po I \- I Atomic Numkr 8 16 34 52 84 . I Elechnnic configuration WIG@ Covalent radius (pm) 66 Ionic mdius, ~%@m) 140 Electron affinky, E -141 (M morl) Ionisation energy 1314 1st (W mol-') Electronegativity(Am) 3.5 Density ( ldx kg m3) Melting point (K) -35 Boiling point (K) 90 Oxidation states* -2.-1 .I .2
* Oxygen shows oxidation states of +I and +2 in oxygen fluorides OF2 and
Fig. 9.4 : Densities of the oxygen family Fig. 9.5 : Melting and boiling points of the oxygen family
Fig 9.6 : Covalent and ionic radii Fig 9.7: Ionisation energies of the oxygen family ofthe oxygen family
Oxygen, the second most electronegative element, fluorine being the fmtJ1as a strong tendency to accept two electrons and give 02- ion. Thus; almost all metal oxides am ionic and contain 02- ions. Usually oxygen exhibits an oxidation state -2 in its other compounds also. It exhibits positive oxidation states only in a few compounds formed with fluorine, i.e., OF, and O,F,. The tendency for the formation of divalent anions deaeases from sulphur downwards because of the increasing s$z and decreasing electronegativity of the el-i' Sulphur, selenium and tellurium show a'tendency for covalency with formal oxidation states +2,+4 and +6 in compounds in which they are combined wit), more elec-tronega!ive elements such as oxygen and halogens. You may note that in the higher oxidation sWof +4 and +6 of these elements electrons are unpaired and promoted to vacant d orbitals. p-Block Elements-I1 Oxygen, and to a greater extent, 'sulphur differ from the other members of the group in their ability to catenate and form peroxides, H-0-0-H and polysulphides, HzSn,n=1-8, respectively. The strong tendency of catenation in sulphur is evident from the fairly high bond dissociation energy ef S-S single bond (264 kJ m~l-I).As discussed earlier it is also reflected in the high melting and boiling points of sulphur.
Bond Lengths and pn - d~ Bonding The bonds between S and 0 are much shorter than expected for a single bond in its oxides and therefore, may be considered as double bonds. Along with a sigma bond between S and 0,a R-bond is also formed by overlap of an oxygen p-orbital and sulphur d -orbital formiqg apn - d~ bond. Oxygen and sulphur have comparable energy and the size of p and d orbitals and, therefore, have effective px4~overlap. But going from Se to Te, weaker bonds are formed because of difference in the size and energy of the p and d orbitals. 9.3.2 Chemical Properties Oxygen is slightly less reactive than the halogens but reacts directly with nearly all the elements except the noble gases, the halogens and a few noble metals. Despite the high bond dissociation energy of 02(498 kJ mol-I), these reactions are frequently highly exothermic and, once initiated, can continue spontaneously or even explosively. For example, its reactions with carbon and hydrogen producing CO, and HzO, respectively.
Sulphur is also a very reactive element, particularly at slightly high temperatures. It reacts slowly with Hz at 390 K, more rapidly above 473 K. Hot concentrated HNO, oxidises S to H2S04.
Sulphur dissolves in hot alkali giving a mixture of sulphide and sulphite as the first products. These react with excess of S giving polysulphides of the type Na2S, and some thiosulphate, Na2S20, :
It reacts with halogens to give compounds like SF,, SF,, S2C12,S2Br,, etc. The non-metals react with sulphur mostly at elevated temperatures. Sulphur compounds exhibit humerous possible oxidation states, from -2 to +6. You will study about them in the next section. Selenium, tellurium and polonium combine directly with most elements, though less readily than do 0 and S, the most stable compounds are the selenides, tellurides and polonides (MZ-). They form compounds with electronegative elements 0,F and CI in which the oxidation states are +2, +4 and +6. SAQ 2 List the elements 0,S, Se and Te in the order of decreasing
(i) b.p. (ii) m.p. (iii) electronegativity and (iv) ionisation energy.
(i) b.p......
(ii) m.p......
' (iii) electronegativity ...... (iv) ionisation energy ~lernebtsof Group 16 ...... ?......
9.4 COMPOUNDS d~ GROUP 16 ELEMENTS
We will now discuss some compounds like the hydrides, halides and oxides of the Group 16 elements. 9.4.1 Hydrides ,All elements of this group form simple volatile binary hydrides of the type H,X (X = 0. S, etc.). The simple hydride of oxygen is H,O or water. The other known hydride of oxygen is H20,, hydrogen peroxide. Sulphur forms the most extensive series of catenated hydrides such as H2S, H2S2,H,S,, H2S,, etc. Hydrides of sulphur, other than H,S, are all yellow oils which read~lydecompose into H2S and free sulphur. Selenium, tellurium and polonium form H,Se, H,Te and H2Po, respectively. The reducing power of hydrides increases whereas their thermal stability decreases in the order HzO; H2S; H,Se; H,Te; H2Po. In fact HzSe and H2Te are better reducing agents than hydrogen. Fig 9.8: Structure of H20 Some of the physical properties of the hydrides are summarised in Table 3.2, molccule
Table 9.2: Some physical properties of b~naryhydr~des
M.P. B.P. Heat of Bond dissociation (K) (K) formation energy for E-8 *f (k~rnol-') (k~mol-'1
The hydride molecules are angular in shape. The bond angles in H,O, H,S, H,Se and H2Te being 104.5". 92". 9 lo and 89.0°, respectively. In case of H,O, oxygen is supposed to be sp7 hybridised with two lone pairs occupying two positions on the tetrahedron, Fig. 9.8. The distortion from the tetrahedral angle (109'28') is supposed to be due to stronger repulsion between, lone pairs of electrons compressing the bond angle. In the case of other hydrides, bond angles are close to 90" suggesting that almost pure p -orbitals are inv6lved in bonding to hydrogen.
Water and Heavy Water
Water, H,O, the simple hydride of oxygen is a unique compound. It has extraordinarily high melting and boiling points compared to the other hydrides of the group, Fig. 9.9. This is attributed to the high polarity of the molecule due to the large electronegativity difference between oxygen and hydrogen atoms. The high polarity of the molecule leads to very extensive association in water due to hydrogen bonding (see Unit 3). I
Polarity of water is responsible also for it being a very good solvent, especially for ionic compounds. Most reactions in nature take place in aqueous solutions, including the very complex reactions in the living systems. Ice, the solid state of water is a giant polymeric -molecule formed by tetrahedrally placed water molecules, held together by hydrogen bonds. Each oxygen atoms is surrounded by four hydrogen atoms. two covalently linked and two hydrogen bonded. In turn, each hydrogen atom is surrounded by four oxygen atoms. Because of this arrangement of water molecules, ice has a cage structure (Fig. 9.10) with lot of empty spaces. It is lighter than liquid water. Ice, in fact, floats on the surface of water. The Fig. 9.10 : Slruclure of icc 1 density of water varies with temperature and is maxlmum at 277 K. These two factors in an unique way are related with the sustenance of aquatic life. The ice once formed protects lower layers of water from further cooling, and the highest density at 277 K ensures that water at the lower depths will never get cooler than that. So, pou can see that in a way water has - lj: Melting and hiling pointl of the hydriof the elements of Group 16. made the earth aa we know it Its geographical features have been shaped by various processes in which water is an active agent The living beings also owe their existence to water. You know that, if the hydrogen atoms in water molecule are replaced by deuterium, an isotope of hjdmgen With mass number= 2, then we get wbst i$called heavy water or D20. a -, ice famed M@ting point, 276.81 K, boiling pbint, 374.42 K, density, 1,1044 x 1(Y kg m-3 and viscosity, sinks to me bottom,- dle whok 1.M centipoise, of D20hre higher than those of H20. This is a&buted to higher molecular , ocuw watld fraze w a pai~dob weight of D20, timc9 d*vsun be Watet is the only practical so- of deuterium, where it is present y the extent of able to meh than.
Hydrogen Peroxide Hydrogen peroxide is the other important hydride of mygen. The oldest method of its preparation is by the action of dilute sulphuric acid on barium peroxide: BaQ + H$04 BaSO, + H202 On an industrial scale, it is alrhost exclusively prepared by auto-oxidation of 2-ethyl anthraquinol as shown below:
H202when pure, is an almostoolouriess liquid, freezing at 272.1 K. k is ksvolatile 0.p. 423 K) than water and somewbatmore dense and viscous. It is miscible witb water in all pmportions. The amcentration of aqueons solutions of H202is expressed in terms of volumes of o48en that will be liberated by a unit volume of Hf12 sample. The two largely used cobhtrations am 6 volume and 30 volume. ktus calculake the pkq9tage of H2& in a 6 volume sample. W202 2H20+ 02 fl.068 kg 22.4 dm3 (at ,STP) Since the strength of the sample is 6 volume, 1 dm3 of H202will produce 6 dm3 of 0, at Elements of Group 16 STP. According to the above relationship 6 dm3 of O2 would be produced by,
Because of its oxidising property, H202is used in Restoring Thus, I dm3 of a 6 volume sample of H202will have 0.0182 kg of H2Q2.Hence, i,ts pai;~tings'. It converts the black PbS. formed from PbO in the original concentration will be 0.0182 x 100 = 1.82 kg per 106 dm3 of sample = 1.82 % wlv. paint, to white PbS04. Pure liquid H202is highly unstable. It decomposes easily, even a speck of dust can initiate explosive decomposition of concentrated solutions. H202can act both as an oxidising and a reducing agent. For example, it oxidises ferrous sulphate and lead sulphide to ferric sulphate and lead sulphate, respectively : 2FeS0, + H2S04+ HzOzhFe2(S04), + 2H20
It reduces chlorine to HCI as shown below: CL2 + H20 HOCl + HCI
Hi02 + HOCI A HCI + H20 + 02 In industry, HzO, is used mainly for bleaching cotton, wood pulp and other fibers. In the I household it finds application as a mild antiseptic and as a bleaching agent. . 'J The hydrogen peroxide molecule contains an 0-0bond and has got a skewed configuration Fig. 9.11.: Structure of hydrogen peroxide. as shown in Fig. 9.1 1. Hydrogen Sulphide Hydrogen sulphide is the only thermodynamically stable sulphane. It is a foul smelling and poisonous gas. , H2S is prepared in the laboratory by treating FeS with dilute HCI in Kipp's apparatus. Some of its physical properties are given in Table 9.2. In aqueous solution H,S is a weak acid., At 293 K, it dissociates as shown below : H2S (vq) -d Ht (aq) + SH- (aq) ; pKa, = 6.88 f 0.02 SH- (aq) -d Ht (aq) + SL (aq) ; p~a,= 14.15 f 0.05 You must have used H2S)in qualitative identification and separation of cations. This separation is based on selective precipitation of the sulphides in acidic and alkaline medium: In the acidic medium, the ionisation of H2S\,which is a weak acid, is further suppressed and the solubility product of sulphides of relatively less soluble cations of Group 11, namely. Pb2+,Hg?, Bi3+, Cu?+, Cd2+. As3+,Sb3+, Sn?+ is exceeded :
HCI H+ + CI-
In Group IV, in the presence of ammonia, a higher concentration of S?- ions is obtained leading to the precipitation of the more soluble sulphides of Zn, Mn, Ni and Co.
SAQ 3 a) Explain briefly why is H2S a gas and water a liquid. I p-I!IOC~ k.lrment\-Il h) Expre\\ the \trength ot a 9% M,/I, \ample of H,O, In volume. (Hint:Look up the problem; given in Sec 9.4.1, for finding out the percentage of a 6 volume sample of H20,.) ......
9.4.2 Halides The elements of this group form a npmber of compounds with halogens. Some of them are listed in Table. 9.3.
Table 9.3: Binary halides of Group 16 elements
Element Fluorides chlorides* ~romides* ~odides*
Pb - P0C12, Pel4 PoBr2, PoBr4 pob
* In case of oxygen, oxides. /
The halogen compounds of S. Se, Te and Po are called as halides. For oxygen, only the fl"oro compounds &e called as fluorides while the chloro, bromo and iodd compounds are designated as oxides. This is because of the high electronegativity of oxygen which is exceeded only by fluorine. You will study the oxides of halogeis in the following unit. All binary compounds of oxygen with halogens are covalent. They areunstable except OF, and I,Os. All, except fluorides of oxygen, are formally acid anhydrides or mixed acid anhydrides. Many of the halogen oxides react with water to give the corresponding oxoacid or a mixture of oxoacids. They also react with bases to give the salt ofa mixtlirk of salts of the corresponding oxoacids : Cl,O+H,O - > 2~~~1 2C10, + 2NaOH NaC 103+ NaC 10, 4 H,O
As shown in Table 9.3. S, Se. Te and Po compounds with all the halogens are know3 except iodides of S and Se and fluorides of Po. The salient features of these and structures of some important compounds are given here. Monohalides: Monohalides of the type M,X, (wh&e X = $, C1 and Br) are formed only by 0. S and Se. Oxygen monofluoride decomposes above 25K. These monohalides have got structures similar to H,O, having E-E bonds.
Monohalides of Te and Po are not formed because Te-Te and Po-Po bonds are not stable. The monohalides are slowly hydrolysed to give the oxoacid,
4 Sulphur can readily be dissolved into sulphur monochloride. S,C I ,, to give chlorosulphanp,s, such as. S3CI ,, S,C 1, and so on upto S ,,,, C I ,.
Dihalides: Dichlorides and dibromides are foinred by the lower members of the group, viz., Se, Te and Po. These are formed mainly by dissociation of the corresponding tetrahalides. Sulphur dichlor~decan also be formed by chlorination of S2CI?, preferably, in Elements of Group 16 presence of a trace of a catalyst such as FeC1,. However, due to the decomposition of SCI, into S,Cl,, it is rather unstable. While the dihalides of Se are gaseous, those of Te and Po are solids. These dihalides are hydrolysed by HzOaccording to following equation for TeBr,:
Dissolution of chlorine in sulphur monochloride, S,CI gives sulphur dichloride, 9C I?, an unstable liquid which dissociates back.
Tetrahalides: S, Se, Te and Po form tetrahalides with all the halogens. However, tetrafluoride of polonium, tetrabromide of sulphur and tetraiodides of sulphur and selenium are not formed. The tetqahalides are formed mainly by the reaction between the corresponding elements, e.g.,
These are covalent compounds which hydrolyse to give the corresponding dioxides.
SF, + 2H,0 SO, + 4HF
As stated'above, the tetrahalides dissociate in solution or in the vapour state to the corresponding dihalides.
The tetrahalides also form complex anions of,the type EX:- in the presence of excess of the corresponding halogen acid.
There has bekn much controversy concerning the structure of tetrahalides. The presently accepted structure of SF, and SeF, is see-saw with one equatorial position occupied by an Fig. 9.12 : Structure of sulphur electron pair as shown in Fig. 9.12. tetrafluoride. Hexahalides: S, Se-and Te react with excess of fluorine to give the corresponding hexafluorides. Fluorine being the most electronegative, oxidises the other elements to their highest oxidation states. Therefore, hexahalides other than fluorides are not formed. In hexafluorides the central atom uses ~tsd-orbitals for bonding. The structure of SF, can be explained on the basis of sp38hy bridisation as shown below:
S atom in the iround state
S atom in the fl i I exc~tedstate sp3d2 hybridisation
S atom combined with 6 fluorine atoms to give SF,
The sp38hybridisation gives rise to a symmetrical structure with fluorine atoms octaheclrally arranged around the hexavalent sulphur as shown in Fig. 9.13. The other hexafluorides also have sinlilar structures.
While SF, and SeF, are quite inactive; TeF, gets slowly hydrolysed in the presence or water. Fig 9.13 : Suucture of sulphur hexatluoride.
9.4.3 Oxides As said before, oxygen reacts practically with all the elements in the periodic table, except Iigh~rnoble gases, to form binary compounds called oxides. An element can form more than one oxide of varied compositions, depending on the method of preparation, reaction conditions, etc. The oxides can be classified on the basis o_f the oxidation stateof oxygen in p-Block Elements-I1 the oxide or on the basis of their chemical nature. The first classification categorises oxides into the following five categories :
i) Normal oxides contain oxygen in its normal oxidation state of -2, e.g., MgO, O=C=O, etc. ii) Peroxides contain the peroxide ion, o?, e.g., Na-0-O-Na, Ha-0-H, etc.
iii) Suboxides involve bonds between atoms of the element in addition to bonds between the element and oxygen, e.g., O=C=C=C=O. iv) Superoxides contain 0; ion and are formed by alkali metals, e.g., KO, ,CsO, , etc. v) Mixed oxides are regarded as composed of two simpler oxides, e.g., Pb304 (2PbO . PbO,). A broader classification is based on the reactian of the oxide with water. According to this, oxides aie classified as'acidic, basic, neutral or amphoteric. Acidic oxides dissolve in water to give acidic solutions. These react with alkalis forming a salt and water, e.g., CO, , NO,, SO,, etc.
CO, + 2NaOH Na2C0, + H,,
Basic oxides dissolve in water to give ~lkalinesolutions. These react with acids forming salt and water, e.g., Na,O, CaO, MgO, etc.
Neutral oxides have neither acidic nor baSic properties and when dissolved in water, they give neutral solutions, e.g., CO, N20, etc.
Amphoteric oxides show both acidic and basic properties. They react with acids as well as with alkalis to give salt and water, e.g., A1203,ZnO, PbO, etc. ZnO + 2HCI ZnC1, + H20 ZnO + 2NaOH + Na,ZnO, + H1O A1,0, + 6HC1 2~1~1,+ ~H,O A1203+ 2NaOH + 3H,O A2NaAl(OH), Elements of Group 16 form a number of oxides. These are given in Table 9.4.
Table 9.4 : Ox~desof Group 16 elements -
oX1&s Element Mono Di Tn Others
s so s% S03 s20, S2q. s2q,so4 Se seo Seo2 Se03 se2% Te TeO Tef?2 Te“3 - RD Po0 p* - -
The important stable oxides ate the di and trioxides. Their salient features are briefly discussed here. Dioxides . These are generally obta~~edby heating the element in air. The dioxides of Se and Te can also be prepared by treating the element with conc. HNO, followed by heating the oxoacid formed,,e.g., Se + 4m03 H,SeO, &$NO, + H,O H,SeO, SeO, +'H,o SO, is a gas, SeO, is a white volatile solid while TeO, is a non-volatile white solid. Elements of Croup 16 Gaseous SO, and SeO, have discrete symmetrical molecules which are bent or angular : Fig. q.14 (a). On solidification SeO, forms long polymeric chains, Fig. 9.14(b).
slightly soluble in water and is an arnphoteric oxide. SO, +H,U ------4H,SO, (sulphurous~acid)
SO2combines reversibly with oxygen in the presence of platinised asbestos or vanadium pentoxide to give sulphur trioxide. This reaction forms the basis of the Contact Process for manufacturing sulphuric acid as you will study later. catalyst 2S0, + 0, -SO, -7 Volatile sulphur compounds, mainly SO,, are released into the atmosphere as a result of Recipitation of dissolved acidic combustion of sulphur containing fossil fuels. SO,, released in densely populated areas does oxides along rains is called
SO, is a strong reducing agent in aqueous solution. The following reactions show this : 2FeC1, + SO, + 2H20 .2FeCI, +H,S04 + 2HCI CI, + SO, + 2H20 2HCl+ H,S04 2KMn04+ 2H,O + SSO, K2S04+ 2MnS0, + 2H,S04 Presumably the bleaching effect of SO, also depends upon its reducing properties. SO, oxidises hydrogen sulphide to s provided moisture is present. At 1273 K it oxidises carbon to carbon dioxide. 2S0, + H2S H2S04+ 3s
Trioxides The selenium and tellurium trioxides are not stable, SO, is the only important trioxide in the group. Its preparation has been described above. Sulphur trioxide exists in several polymorphic forms. The solid form, m.p. 290 K has a trimejc cyclic structure, in which four oxygen atoms are arranged approximately tetrahedrally around each sulphur atom, Fig. 9.1 S(c). This form gradually changes into a linear polymerised structure in the presence of moisture. The latter has a fibrous needle-like appearance, Fig.9.1S(b). On.heating, the polymeric form dissociates into discrete SO, molecules present in the vapour. These have a triangular, symmetrical and planar structure, Fig. 9.15(a). I p-Block Clements-I1 SO, is a powerful acidic oxide, it fumes in moist air and reacts explosively with water to form sulphuric acid.
SO3+ H,O H2S04
With excess SO,, H,S04 gives pyrosulphuric acid or oleum
H,S04 + SO, H,S207(oleum)
In some reactions it acts as an oxidising agent, e.g., it oxidises HBr to free bromine.
2HBr + SO, H20 + Br, + SO,
SAQ 4
Find out the oxidation states of sulphur in the following :
s20, s20, , so, , so,.
......
9.5 OXOACIDS OF SULPHUR
As you know, oxoacid is an acid in which the ionisable hydrogen atom is bonded, through an oxygen atom to the central nonmetal atom, e.g., E-OH.
S, Se and Te all form oxoacids. The oxoacids of sulphur are by far the most numerous and impoftant as-compared to Se and Te. Many of the oxoacids do not exist as such but their salts are known. Oxoacids of sulphur are given in Table 9.5. It is evident from Table 9.5 that sulphur forms oxoacids in which'its oxidation number varies from -2 to +6. We will now discuss sulphurous, sulphuric, peroxpsulphuric and thiosulphuric acids and their salts in brief.
9.5.1 Sulphurous Acid
Sdphurous acid, H,SO,, is known only in solution. It exists mainly as a hydrate of sulphur
contairiing SO:- ions are yell known.
Sulphurous acid possesses both oxidising as well as reducing properties. It owes its reducing nature to the ease with which it can get oxidised to sulphuric acid. It reduces I, to HI, KMnO, to MnSO, and K,Cr,07 to Cr,(SO,),:
H?SO, + H,O + I, H,SO, + 2HI
2KMn0, + 5S02+ 2H20 K2S0, + 2MnS0, + 2H,SO,
K,Cr,07 + 3S0, + H2S0, K,SO, + Cr2(SO,),+ H,O
In the presence of strong reducing agents like H2S,it can behave as an oxidising agent.
2H,S + H2S0, 3H20+ 3s . . Sodium and calcium bisulphite solutions are used in making paper from,wood because they dissolve fibrous material lignin and facilitate pulping of celluLose. Table 9.5: Oxoacids of sulphur Elements of Croup 16
Formula Name Oxidation Schematic Salt states Structure 0 Sulphuric VI I Sulphate, SO:- , H-sulfate, HOSO;
I Pymsulphuric Pymsulphate, 0~~0~02' %'z07 C OH i 1 Y2O3 Thiosulphuric VI. -11 S-OH Thidsulphate, SSO;- 0" \OH
Y Pemxomonosulphuric VI 04S%p Pemrornonosulphate, 00~0:
0 0 I %S208 Pemxodisulphuric VI Pemxodisulphate, 0~~00~0:- B 51 S p296 Dithionic* v S,- -OH Dithionate, ~SSO Ob OH '0
0 Sulphite, SO?, H2S03 SUI~~U~US* IV /ST OH O OH H-sulphite, HOSO;
p 0 Dithionite, O,SSO; %s204 Dithionous* m /'\' scOH O 011 g
* Unstable oxoacids
9.5.2 Sulphuric Acid
Sulphuric acid is one of the most important chemicals, both in industry as well as in the laboratory. We shall discuss its preparation and properties in detail now.
Two methods are industrially used for the manufacture of H,SO,. These are : i) lead chamber process and ii) contact process. Both these processes involve three basic steps namely,
i) Production of SO,
ii) Oxidati6n of SO, to SO,
iii) Conversion of SO, to H,$O,
We shall discuss as to how these steps are camed out m the two industrial processes.
a) Lead chamber process, is the older of the two processes. In this process SO, is produced by burning S or roasting pyrites.
s+o,- SO, p-Block Elements-11 The oxidation of SO, to SO, is carried out by using oxides of nitrogen. These are produced by oxidation of NH, using platinum as catalyst.
A mixture of air, SO, and the nitrogen oxides is passed into a set of chambers lined with lead sheets from the top of which water is sprayed. The exact nature of the reactions which take place in the lead chambers is not fully understood. The following scheme of reactions seems to be operative:
NO, + NO + 2S0, + H,O + 0, 2(NO-HSO,)
2 (NO.HS0,) + H,O 2H,S04 + NO, + NO
Nitrosyl hydrogen sulphate formed as an intermediate reacts wjth H,O to give H2S0, and the nitrogen oxides. The oxides play the role of a catalyst.
Though chamber process involves relatively simpler steps, yet it is not the method of choice where high purity of the acid is required. The chamber acid contains As,O, (from pyrites), PbSO, (from chamber) and some oxides of nitrogen as impurities. This acid finds use in the preparation of fertilisers where removal of impurities is not necessary. The second drawback with chamber method is that it gives dilute acid (60-78%). since the process of concentration of the acid is expensive-
Contact process
The fact that SO, can be oxidised to SO, by aii in the presence of platinum catalyst is the basis of the contact process which is used the world over for the manufacture of H,SO,. SO, is obtained in a manner similar to the chamber process. But before oxidation it requires thorough purification in order to avoid the poisoning of the catalyst, particularly by the arsenic impurities. The purified SO? is then oxidised directly according to the following reaction :
SO2 + 1/20? SO, ; M=-98 k~ rnol-l
It is a reversible and exothermic reaction proceeding with a reduction in the volume. According to Le Chatelier principle, the yield of SO, would increase under the following conditions :
i) high pressure
ii) high concentration of 0,
iii) low temperature
iv) continuous removal of SO, from the reaction
Theoretical considerations require that the reaction be carried out at low temperature and high pressure. However, at low temperature, the reaction is too slow and high pressure makes the cost of plant uneconomical. The yields of S0,are optimised,by using a suitable catalyst, an excess of 0, and removing SO,formed, the latter two shiftkhe equilibrium to the right. Optimally the reaction is carried out at 700 - 725 R and one atmosphere pressure using vanadium pentoxide as catalyst and an excess of oxygen.
The purified SO, is mixed with air (15) and is passed through a four stage catalytic converter operating between a range of temperatures, Fig. 9.16. After conversion of SQ? to SO, to the extent of 99.5% or so, the gases come out of the converter. Elements of Croup 16
Feed WS 10 % SO,, I I % O2 1
to oleum or intermediate absorber
6 ' absorber
Fig. 9.16: Schematic diagram of converter
Reaction of SO3 with H,O is violent and gives a fog of dilute sylphuric acid. Therefore, SO3 The concentration of oleum is expressed in terms of the pcentage is passed through concentrated H,S04 to give oleum. From oldurn, H,S04 of any required of free SO3 in it. concentration can be prepared by appropriately diluting it.
SO3+ HSO, H,S,O, (oleum)
Concentrated stllphuric acid is a colourless, thick, oily liquid having sp.gr. 1.838 at 288 K, m.p. 283.4 K and b.p. 613 K. At its boiling point, it gives dense white fumes of SO3. H,SO, is a strong acid which is'almost completely ionised in aqueous solution. It behaves as adibasic acid, forming two distinct series of salts, namely, hydrogen sulphates, containing HSO: and sulphates, containing SO:. The dilute acid attacks many metals forming sulphates and hydrogen, but it does not react with lead, copper, mercury and silver :
concentrated H,SO, dissolves many metals giving sulphur dioxide. But there is no action on gold, platinum. glass, silica and silicon-steel, the latter is used for making ,distillation vessels.
Cu + 2H,S04 CuSO, + 2H,O + SO,
Hot concentrated.H2S04is a good oxidising agent. It oxidises Hz, C, P, S, H1;HBr to H20, CO, &PO,, SO,, I, a'nd~r,respectively. It is also a good sulphonating agent.
C + H,S04 ------+ CO + SO, + H,O
C6H,SO3H H,O Ca6+ H&Q4 > + It is due to the dehydrating power of . - Benzene sulphonic acid sulphuric acid that it is highly sive to shin.First of all, it Concentrated H,S04 has a great affinity for water and is used as a dehydrating agent. This water and the heat produced property of concentrated sulphuric acid is used in drying gases like O,, N,, SO,, etc. H,S04 =uses funher damage. chars organic substances, by removing elements of water, leaving black carbon behind, e.g.,
. -- -1 1H20 C12H22011 12C sucrose -conc. H,S04 p-&~ock Elements41 Below its melting point, anhydrous sulphuric acid is a white crystalline solid consisting of a 0 thrw-dimensional hydrogen-bonded network, which persists in the liquid state and makes the liquid viscous. The two hydroxyl groups and two oxygens are tetrahedrally arranged around the S atom in H2S0, assshown in Fig. 9.17 with S-OH and S-O bond lengths being 154 pm and 147 pm, respectively. This shortening of S-O bond shows pn dxbonding between sulphur and oxygen. The bond, as a result, has appreciable double bond character. 1n SO:- ion. allthe four S-0 bond lengths are 149 pm. This indicates extensive .- delocalisation of bonding electrons.
I Fig. 9.17 : Structure of sulphuric acid 0- 0- 0 2 -
t3 C----) ' .'. 0 0
9.5.3 Peroxoacids of Sulphur There.are two peroxoacids of sulphur, viz., peroxomonosulphuric acid, H2S0, and peroxodisulphuric acid, H2S20,. These acids are known also as Caro's acid and Marshall's acid, respectively, after the names of their discoverers. These acids can be considered as derived from hydrogen peroxide by the replacement of one or both the hydrogen atoms by sulphonic group, -SO,OH:
HMH H0-GSO2OH HO-S02-O-O-SO2-OH Hydrogen peroxide 'Peroxomonosulphuric acid Peroxodisulphuric acid
Peroxomonosulphuric Acid
It is also known as perrnofiosulphuric acid. It is obta~nedby the action of chlorosulphuric acid on cold anhydrous hydrogen peroxide : H202+ C1S02. OH H-0-0-SO2. OH + HCI
It may also be prepared by the action of concentrated sulphuric acid on 5% hydrogen peroxide:
H202+ H2S04 H-O-O-SO2. OH + H20 Peroxomonosulphuric acid may also be obtained by hydrolysis of peroxodisulphuric acid at 273 K or by grinding potassium peroxodisulphate crystals with sulphuric acid in a freezing mixture.
H2S20s+ H20 A H2S05+ H2S04 K2S2O8+ HZS04 HzSOJ+ KzS,O,
Peroxomonosulphuric acidis a white crystalline solid, m.p. 318 K. However, the salts of this acid are unstable in the solid state. The acid is a strong oxidising agent. It oxidises SOz to SO,, sulphites to sulphates, ferrous salts to ferric salts and liberates iodine immediately from a solution of potassium iodide :
2KI + H2S05 A K2S0, + H20+ I? 2FeS04 + H2S05 . Fe2 fSO4), + H20 Peroxodisulphuric Acid
It is also known as perdisulphuric acid or simply persulphuric acid. The acid can be obtained by the action of chlorosulphonic acid on Hz02:
22 HzOz + 2C1S020H H2S20,+ 2HC1 This acid is obtained by the electrolysis of 50% sulphuric acid in cold using high current Elements of Group 16 density and a platinum anode. The following reactions are supposed to take place during the process :
H,SOi H+ + HSO ;
2HSO; H,S,O, + 2e at anode
2H++ 2e H, at cathode
The peroxodisulphuric acid is a colourless solid, m.p. 338 K. The acid is highly soluble in water. In aqueous solution it slowly changes to peroxomonosulphuric acid.
Potassium and ammonium peroxodisulphate are the most important salts of this acid. These are freely soluble in water. ~hesesalts are, in fact, easier to prepare than the acid and both are made on an industrial scale by anodic oxidation of the corresponding sulphates under controlled conditions.
Peroxodisulphuric acid and its salts are powerful oxidising agents. They liberate iodine from Kl slowly. This distinguishes H,S208 from H,SO,, which liberates iodine immediately.
2KI + K2S20s 2K2S04+ I2
Both fie acids are distinguished from H,02 by their failore to react with KMnO,. The peroxodisulphates also oxidise ferrous salts to ferric, manganous salts to permanganates and chromic salts to diehromate in presence of a trace of silver nitrate :
2MnS04 + 5K2S20,+ 8H20 2KMn0, +4K,SO, + 8HzS04
Cr, (SO,), + K,S,08 + 7H20 Kzdr,0, + 7H,SO,
Some metals, e.g., copper and zinc dissolve in an aqueous solution of persulphates giving metal sulphates :
+ CuSO, + K2S04 Zn + K2S208 ZnSO, + K2S0,
9.5.4 Thiosulphuric Acid Thiosulphuric acid, H2S,0,, has never beepisolated but its salts are well known which contain the thiosulphate ion, S?Oy.The%est'known thiosulphate is the sodium salt, Na,S,O, . SH20.1t is obtained by boiling aSolution of sodium sulphite with sulphur followed by filtration and crystallisation.
Na2S0, + S ------+ Na,S20,
Sodium thiosulphate is a colourless, crystalline solid highly soluble in water and commercially known as hypb. The thiosulphate ion is unstable in the presence of acids breaking to glve HzO, SOz and S.
Na2S20,+ 2HC1-----4- 2NaCI + SO, + S + H,O
It functions as a reducing agent, reducing chlorine to chloride. This reaction is utilised for removing excess chlorine from bleached fabrics.
Na,S,O, + 4C1? + 5H,O > ZNaHSO, + 8HC1 p-Block Elements-I1 The milder oxidising agent iodine behaves rather differently giving sodium tenathionate instead of Na2S0,, though it is also converted into iodide.
Na2S20,+ I, 2NaI + Na2S,0, sodium tetrathionate
This reaction is used in iodometric titrations for determination of copper. Sodium thiosulphate is used in photography for 'fixing' the negative. It removes the unreacted light sensitive silver bromide from photographic plates to avoid further reaction of light.
AgBr + 2Na,S20, Na,[Ag(S,O,),] + NaBr
SAQ 5 Explbn, in the space given below. how do you account for high b.p. and high viscosity of sulphuric acid.
......
9.6 ANOMALOUS BEHAVIOUR OF OXYGEN -
Oxygen differs considerably from the rest of the farnib members. Smdl size, very high electmnegativity and non-availability of d-orbitals in the valence sheU are the factors responsible for these differences. The main differences are as given &low :
Oxygen is a gas while other members are solids. Oxybcn is diatomic whereas rest of the members are polyatomic. Being highly electronegative, oxygen shows only negative oxidation states of -2 and -1 except in OF, in which its oxidation state is +2. It shows paramagnetic behaviour in gaseous, liquid and solid states. Oxygen has a tendkncy towards extensive hydrogen bond formation. H20is a liquid, whereas hydrides of the other elements of this group aFe gases.
9.7 SUMMARY
Let us recall what all we have discussed iq this unit :
Members of oxygen family show the usual gradation in properties from non-metallic oxygen and sulphur to metallic polonium, in between being selenium and tellurium, the metalloids. Oxygen is the only element in themup existing as a diatomic molecule. Rest of the members are polyatomic. However, all of them show allotropy. Oxygen differs in many ways from other elements of the group and these differences have been diacusse&
All of them form bivalent anions like Wand SZ.. Dueto the availability of d-orbitals in S, Se, Te and Po, the oxidation state of these elements increases from 2 to 4 and then to 6.
The hydrides and halides of these elements have been discussed. One hydride of oxygen. namely, H,O is very important for Life and has unique propetties. Except oxygg which forms only dihalides, rest of the members form tetra and hexavalent compounds also Elements of Group IC with halogens.
The ~rou~'l6elements form a number of oxides. Di-and trioxides being more
of sulphuric acid, a very common chemical, has been discussed in detail explaining the contact and the lead chamber processes.
1 Why does oxygen exist as a diatomic molecule and sulphur as a polyatomic molecule at normal tem~eratures?
3 Explain why ice has a lower density as compared to that of water? 4 Arrange the hydrides of Group 16 elements in the decreasing order of:
i) acidity ii) hydrogen bonding iii) stability iv) reducing power
5 Why is H2S a stronger acid as compared to H,O ? 6 Halogens form various types of compounds with oxygen. Is it justified to call them halides of oxygen? Give reason for your answq. 7 Give reasons for the following : i) Why is SO, not dissolved in water directly during the preparation of sulphuric acid ?
9.9 ANSWERS -----
Self-assessment Questions 1 Species No. of unpaired electrons i) 0; 1
ii) 0; 1
iii) 0;- 0
ii) Te > Se > s >>> 0 iii) 0 >>> Se > S > Te iv) O>>S>Se>Te
3 (a) Water molecules are highly associated with one another by hydrogen bonding because of the presence of highly electronegative oxygen atoms. Whereas the hydrogen sulphide molecules are associated with one another by weak van der waals forces. As a result water exists as a liquid and H,S exists as a gas. (b) The strength of the sample of H,Oz is 9% w/v. It means 100 dm' of HZO,sample would have 9 kg of H,Oz. Hence I dd of H,O, sample would have 0.09 kg of H,O,. From the equation, We know that 0.068 kg of H,02 gives 22.4 dm3 of 0, at STP. Hence, 0.09 kg of 22.4 x 0.09 H,02 should give dm3 of 0, at STP or 1 dm3 of H202should give 0.068 29.65 dm3 of 0, at STP.
Hence, the strength of a 9% w/v sample of H202would be 29.65 volume.
4 Oxide s20 s203 so2 so3 Oxidation state 1 3 4 6 of sulphur 5 The high boiling point and viscosity of sulphuric acid art: due to the presence of hydrogen bonding which binds a number of simple molecules into clusters.
Terminal Questions
1 Oxygen-oxygen double bond in 0, molecule has bond energy three times that of oxygen-oxygen 2ingle bond. The molecule, therefore, prefers to be in the diatomic form. While the bond energy of sulphur-sulphur double bond is less than twice the sulphur- sulphur single bond energy. The S-S bond being stable makes the element exist in polyatomic form. 2 Oxygen has a very high electronegativity. In fact, it is the second most electronegative element, as a result it does not show a positive oxidation state. 3 Ice has a caged structure because of a network of tetrahedrally placed water molecules. This structure has a lot of empty space which makes ice lighter than water.
ii) H20>> H,S > HzSe > H2Te > H,Po iii) H20>> H2S > H2Se > H,Te >> H2Po
iv) H,Po > H2Te > H,Se > H2S > H,O 5 Due to high electronegativity of oxygen, the 0-H bond in H,O is stronger-than the SSH bond of H2S.Therefore, it is relatively easy to take out a proton from H2S than from H,O. Hence, H,S is more acidic.
6 You know that in naming a covalent compound containing two elements, as a rule, the more metallic element's name appears first followed by non or less metallic one. Of the various halogens, only fluorine is more non-metallic and more electronegative than oxygen thereby the compounds of oxygen and fluorine are termed as fluorides. Since oxygen is more non-metallic and more electronegative than all other halogens, its compounds with C1, Brand I are termed as oxides.
7 (i) In the preparation of H2SO4.SO3 is not dissolved directly in HzO as it reacts violently with water forming a dense white fog of dilute sulphuric acid. (ii) The contact process is preferred over lead chamber process for two reasons. (a) Contact process gives pure H2S04while chamber acld has got impurities of arsenic and lead. (b) Sulphuric acid of any concentration can be obtained by contact process while the acid obtained from lead chamber proceys is quite dilute (60 - 78%).