IIT-JEE Via Group Group Elements Elements
VIA GROUP ELEMENTS Introduction 2
4
Oxygen, sulphur, selenium, Tellurium and polonium are the elements of VIA group. They have ns np outer electron configuration. They belong to ‘p’ block in the long form of the periodic per iodic table. The first four elements are collectively known as Chalcogens (means mineral forming) since many metals occur as oxides or sulphides. Polonium is a radioactive element and short lived. It was first isolated by Madam Curie from an ore of Uranium called Pitchblende. Curie named n amed this element as Polonium in honour of her native land, Poland. The elements of this group have six electrons in their outer shell. These electrons are described as valence electrons. In these six electrons two electrons electrons are in ‘s’ orbital and four electrons are in ‘p’ orbitals. In these four electrons two two are unpaired. These unpaired electrons generally take part in chemical bonding. All these elements require two electrons to attain a stable inert gas configuration.
Electron Configuration of VIA Group Elements Element Oxygen Sulphur Selenium Tellurium Polonium
Symbol O S Se Te Po
Atomic Number 8 16 34 52 84
Electron Configuration 2 4 [He] 2s sp 2 4 [Ne] 3s 3p 10 2 4 [Ar] 3d 4s 4p 10 2 4 [Kr] 4d 5s 5p 14 10 2 4 [Xe] 4f 5d 6s 6p
General Characteristics: Characteristics: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
Physical state: Oxygen is a gas at room temperature while other members of this group are solids. Atomic radius: As the atomic number increases, the atomic radius increases from oxygen to tellurium. Ionisation energy: Ionisation energy decreases gradually on descending the group from oxygen to polonium. Electronegativity: Electronegativity decreases gradually on descending the group from oxygen to polonium. The gradual decrease in I.P. and E.N. is explained by the increase in size and screening screening or shielding ef fect. Metallic character: As the atomic size increases, the metallic character increases from oxygen to polonium. O, S, Se and Te are non-metals. Density : The density increase gradually from oxygen to tellurium. Melting point : The melting point increases gradually from oxygen to tellurium. Boiling point : The boiling point increases from oxygen to tellurium. The gradation in density, M.P and B.P is explained by the intermolecular attractive forces from oxygen to tellurium. Atomicity : Oxygen molecule is diatomic sulphur, selenium and tellurium are octa atomic molecules. In oxygen molecule the two atoms are joined j oined by a double bond. Sulphur molecule has a puckered ring structure. Oxidation states: All the elements of this group except oxygen exhibit the principal oxidation states –2, +2, +4 and +6. The normal oxidation state of oxygen is –2. The very high electronegativity of oxygen suggests that it forms the compounds of –2 state.
The electronegativities of other elements are, however low. If the chalcogen atom is more electronegative, in the molecule it shows – 2 oxidation state. If it is less electronegative it shows +2, +4, +6 oxidation states. Oxygen cannot form more than two bonds because of the non-availability of ‘d’ orbitals in its outershell. Much energy is required to excite the electrons to higher levels. The elements S, Se, Te and Po have vacant orbitals in the valence shell and can form four or six bonds in the excited state. 2, 4 or 6 bonds of sulphur can be explained as follows.
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Ground State 3s
3p
3d 3
Two unpaired electrons can form two bonds. Due to sp hybridisation, the central atom has a tetrahedral geometry with two positions occupied by lone pairs First Excited State 3s
3p
3d 3
Four unpaired electrons can form four bonds. Due to sp d hybridization, the atom acquires trigonal bipyramidal geometry with one position occupied by b y a lone pair. Second Excited State 3s
3p
3d 3 2
Six unpaired electrons can to form six bonds. Due to sp d hybridisation, the atom acquires octahedral geometry. The higher oxidation state becomes less stable on descending the group. The +4 state shows both oxidizing and reducing properties while +6 state shows only oxidizing properties. These compounds are volatile because they are covalent.
Physical Properties of the VIA Group Elements Property Atomic weight Atomic volume Atomic radius (A) Ionisation energy (K.Cal/mole) Electronegativity Density (g/c.c) Melting point (K) Boiling point (K) Oxidation states
Oxygen 15.9 14.00 0.73 314.00
Sulphur 32.06 15.50 1.09 253.90
Selenium 78.96 16.50 1.16 231.00
2.50 2.40 2.07 4.79 392.00 490.00 717.60 958.00 –2, +2, +4, –2, +2, +4, +6 +6 Allotropy: All these elements exhibit allotropy. Oxygen ozone (O 3 ).
11.
3.50 1.14 54.20 90.0 –2
Tellurium 127.60 20.50 1.35 199.10
Polonium 210.0 22.7 -184.90
2.10 2.0 6.24 9.2 722.5 527.0 1262.80 - –2, +2, +4, +2, +4, +6 exists in two non non-metallic -metallic forms, oxygen (O2 ) and
o
In ozone O – O bond length length is intermediate between the single bond distance (1.48 A ) and the double bond distance o
o
(1.10 A ). It is 1.27 A . The actual structure of ozone o zone is a resonance hybrid of two canonical structures I and II. Sulphur exists in many allotropic forms. All are non-metallic, e.g: Rhombic, monoclinic and plastic sulphur. Rhombic sulphur is stable at room temperature and it has a crystalline structure. Monococlinic sulphur is stable above 368.5K and it forms needle like crystals. The temperature 368.5K at which these two allotropes exist in equilibrium state is called transition temperature. These two forms are known as enantiotrophs. When sulphur is boiled above 473 K, S 8 rings are opened and long chains are formed. When this boiling sulphur is poured into water, a rubber like mass called plastic sulphur is obtained.
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Allotrope Rhombic sulphur (α-sulphur) Monoclinic sulphur (β-sulphur) Plastic sulphur (γ-sulphur) Colloidal sulphur
ALLOTROPES OF SULPHUR Specific Gravity M.P 2.06 385.8
State Crystalline
Structure Puckered ring
Crystalline
1.98
392.2
Puckered ring
Amorphous
-
-
Colloid
-
-
Open chain S 4 molecules -
Selenium has two forms. They are red-non metallic and grey-metallic. Tellurium exists in two forms. They are non-metallic and grey-metallic. Polonium exists in two forms. They are α-polonium and β-polonium. Both are metallic. Chemical Properties: 1. Hydrides: Hydrides: Chalcogens form the hydrides of the type H 2 X (where X is chalcogen). The affinity of these elements with hydrogen decreases from oxygen to polonium. So the stability of these hydrides decreases from H 2 O to H 2 Po. Actually hydrogen polonide is formed in traces. These hydrides are formed by the action of an acid on metallic sulphides, selenides, tellurides. FeS + H 2 SO 4 → FeSO 4 + H 2 S → 2AlCl 3 + 3H 2 Se Al 2 Se 3 + 6HCl
Al 2 Te 3 + 3H 2 SO 4 → Al 2 Cl 3 + 3H 2 Te Hydrogen polonide is formed by the action of dilute acid on magnesium-polonium alloy. i. All these hydrides except water are gases at room temperature. Water is a liquid. ii. All these hydrides except water are volatile, poisonous and foul smelling gases, iii. iv. v. vi.
water has an abnormally high melting and boiling points compared This is due to inter molecular hydrogen bonding in water. The volatility of these compounds decreases from H2 S to H 2 Te. The reducing property of these hydrides increases from H2 O to H 2 Po. These hydrides dissolve in water to give weak acids. + – H 2 X + H 2 O → H 3 O + HX +
vii.
to
the
other
hydrides.
–
H 2 S + H 2 O → H 3 O + HS In solution, the acidic property increases from H 2 S to H 2 Te. All the molecules of these hydrides have bent structure. The bond angle decreases from H 2 O to H 2 Te.
O
S
H
104°31’
H
H
92° H 3 In the formation of water molecule, oxygen atom undergoes sp hybridisation. Because of the repulsion or nonbonding electron pairs, the tetrahedral bond angle is decreased. In other hydrides, the bond angle is close to 90 °. It shows that in other chalcogen atoms only pure valence p orbitals involve in the bond formation.
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Oxygen exhibits catenation to a small extent while sulphur exhibits to a greater extent. So they form polyoxides and polysulphides which are less stable than normal salts. e.g.: H 2 O 2 : H – O – O – H; H 2 S 2 : H – S – S – H; H2S3 : H – S – S – S – H When ice cold acid is added to a peroxide or persulphide, hydrogen peroxide or hydrogen persuilphide is formed. BaO 2 + 2HCl → H 2 O 2 + BaCl 2 Na 2 S 2 + 2HCl → H 2 S 2 + 2NaCl HYDRIDES VIA GROUP ELEMENTS Formula Bond Angle Boiling Point H2O HOH = 105 373 K H2S HSH = 92 213 K H 2 Se HSeH = 91 231 K H 2 Te HTeH = 90 270 K H 2 Po HPoH = 90 Unstable
Name of Hydride Hydrogen oxide Hydrogen sulphide Hydrogen selenide Hydrogen telluride Hydrogen polonide
2. Halides: Halides: The general formulae of VIA group halides are M 2 X 2 , MX 2 , MX 4 and MX 6 (M = S, Se and Te; X = halogen) in which the oxidation states of the chalcogen are +1, +2, +4 and +6 respectively. i. Oxygen combines with chlorine to form a number of oxides. These are chlorine monoxide (Cl 2 O), chlorine dioxide(ClO 2 ), chlorine hexoxide (Cl 2 O 6 ) and chlorine heptoxide (Cl 2 O 7 ) in which the oxidation states of chlorine are +1, +4, +6 and +7 respectively. ii. Other elements of this family combine with chlorine to form tetra chlorides. E.g: SCl 4 , SeCl 4 , TeCl 4 and PoCl 4 , SCl 4 is very unstable liquid. The other tetra chlorides are stable solids. All these tetra chlorides undergo hydrolysis. MCl 4 + H 2 O
M(OH) 4 + 4 HCl H 2 MO 3 + H 2 O
iii. iv. v.
vi.
Structure of these tetra halides is distorted trigonal bipyramid (trigonal bipyramid with one orbital occupied by a lone pair of electrons). The higher oxidation state of +6 is exhibited by the elements S, Se, and Te with fluorine. They form SF 6 , SeF 6 and TeF 6 . All these fluorides are formed by the direct combination of these elements with fluorine. These halides undergo hydrolysis and the degree of hydrolysis increases with the increasing atomic number. Thus TeF 6 hydrides TeF 6 + 6H 2 O → H 6 TeO 6 + 6HF 3 2 Structure of these fluorides is octahedron due to sp d hybridisation of the central atom. Halides of VIA group elements are listed in the table.
Element
M2X2
Oxygen
O 2F 2
HALIDES OF VIA GROUP ELEMENTS MX 2 MX 4 MX 6 XM 2 Other Compounds F2O Br 2 O Cl 2 O
-
-
4
ClO 2 BrO 2
O 3 F 2 , O 4 F 2 , ClO 6 Cl 2 O 7 , BrO 3 , I 2 O 4 , I4O 9, I 2O5
IIT-JEE Via Group Group Elements Elements
Sulphur
S 2F 2 S 2 Cl 2
SF 2 SCl 2
SF 4 SCl 2
SF 6
-
-
Selenium
Se 2 Cl 2 Se 2 Br 2
SeCl 2
SeF 4 SeCl 4 SeBr 4
SeF 6
-
-
Tellurium
-
TeCl 2 TeBr 2
TeF 4 TeCl 4 TeBr 4
TeF 6
-
-
Polonium
-
PoCl 2 PoBr 2
PoCl 4 PoBr 4 Pol 4
-
-
-
3. i.
ii. iii. iv.
v.
4. i.
Oxides All these elements combine with oxygen to form dioxides. MO 2 and trioxides MO 3 (M = S, Se, Te & Po) eg: sulphur forms sulphur dioxide, SO 2 and sulphur trioxide SO 3 . The dioxide is prepared by heating the element in air. S + O 2 → SO 2 As the metallic character increases the acidic nature of the oxides gradually decreases. Reducing power gradually decreases and oxidizing power increases. To prepare trioxides special methods are used. S, Se and Te form trioxides. SO 3 , SeO 3 and TeO 3 . Sulphur trioxide is prepared by heating a m ixture of SO 2 and O 2 over a catalyst. V2O5 2SO 2 + O 2 → 2SO 3 The acidic nature of these oxides is in the order of SO 3 > SeO 3 > TeO 3 Tellurium and polonium give monoxides (TeO, PoO). Oxyacids The dioxides of VIA group elements dissolve in water to form ‘ous’ acids of the type H2 MO 3 . The solubility of dioxides decreases from SO 2 to TeO 2 . MO 2 + H 2 O → H 2 MO 3 SO 2 + H 2 O → H 2 SO 3 (sulphurous acid) SeO 2 + H 2 O → H 2 SeO 3 (selenious acid)
ii. iii. iv.
TeO 2 + H 2 O → H 2 TeO 3 (tellurous acid) PoO 2 is insoluble in water. It is soluble onlyin o nlyin acids. The strength of the oxyacids is in the order of H 2 SO 3 > H 2 SeO 3 > Hs 2 TeO 3 In these acids, the oxidation state of VIA group element is +4. The trioxides dissolve in water to form ‘ic’ acids of the type H 2 MO 4 . MO 3 + H 2 O → H 2 MO 4 SO 3 + H 2 O → H 2 SO 4 (sulphuric acid) SeO 3 + H 2 O → H 2 SeO 4 (selenic acid)
v.
TeO 3 + H 2 O → H 2 TeO 4 (telluric acid) Just as the strength of the ‘ous’ acids the strength of the ‘ic’ acids is in the order of H 2 SO 4 > H 2 SeO 4 > H 2 TeO 4
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vi.
In tyhese acids, the oxidation state of VIA Group element is +6. Among the oxyacids of VIA group elements, oxyacids of sulphur are important (shows in the table) Oxy Acids of Sulphur Name
Sulphoxylic acid
Formula
Structure
H 2 SO 2
HO – S – OH
Sulphurous acid
H 2 SO 3
Di or pyrousulphorus acid
H2 S 2 O 5
Sulphuric acid
H 2 SO 4
Thiosulphuric acid
H2 S2 O 3
Pyrosulphuric acid
H2S2O 7
Dithionic acid
H2S2O 6
Peroxy monosulphuria acid (or) permonosulphuric acid (or) Caro’s acid
H 2 SO 5
Peroxo disulphuric acid (or) perdisulphuric acid (or) Marshall’s acid
H2S2O 8
O = S – OH | OH O || HO – S – S – S – OH || || O O O || HO – S – OH || O S || HO – S – OH || O O O || || HO – S – O – S – O H || || O O O O || || HO – S – S – O H || || O O O || HO – O – S – O H || O O O || || HO – S – O – O – S – O H || || O O
6
Oxidation state +1
+4
+4
+6
+2
+6
+5
+6
+6
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Ozone, O 3 . Introduction: Ozone was first noticed by Van Marum in 1785. Schenbien named it as Ozone because of its peculiar smell. The triatomic nature was observed Soret in 1866. He considered it as an allotrope of oxygen. Ozone is largely present in the upper layer of the atmosphere. It prevents the powerful ultra-violet radiation from the Sun.
PREPARATION PREPARATION OF OZONE Ozone is generally prepared by subjecting oxygen to silent electric discharge in an apparatus called Ozonizer There are two important ozonizers. They are: 1. Siemen’s Ozonizer and 2. Brodie’s Ozonizer Siemen’s Ozonizer: Siemen’s ozonizer consists of two coaxial glass tubes sealed at one end. The inner surface of the inner tube and outer surface of the outer tube are coated with tin foil. These tin foils are connected to the terminals of a powerful induction coil. A current of pure and dry oxygen gas is passed through the annular space and is subjected to silent electric discharge. Ozonized oxygen which is a mixture of oxygen with 3 to 8% ozone comes out from fr om the other end. 2O − 68 K. Cal. 3O2 3
2. Brodie’s ozonizer: Brodie’s ozonizer consists of two concentric glass tubes. The inner tube contains dilute sulphuric acid and the outer tube is suspended in dilute sulphuric acid taken in a beaker. Two copper wires form the electrodes. One electrode is dipped in dilute sulphuric acid of the inner tube and the other in dilute sulphuric acid in the beaker. These wires are connected to a powerful induction coil. A current of pure and dr y oxygen gas passed through the annular a nnular space and subjected silent electric discharge. Ozonized oxygen comes out through the outlet.
3O2 2O3
− 68
K. Cal
MANUFACTURE OF OZONE Ozone is manufactured by Siemen – Halske’s ozonizer. It consists of about six to eight porcelain or glass cylinders. Each is surrounded by a cylinder of aluminium fitted in a metal box. These aluminium alum inium cylinders are kept in vertical position on an insulating glass plate fitted f itted into an iron tank. This tank is divided into three compartments. Through these compartments cold water is circulated to keep k eep the apparatus cool. The aluminium rods are raised to a high potential of 8000 – 10000 volts. Air is passed through the annular space around the aluminium cylinders. Then the air is subjected to the action of silent electric discharge and comes out in the form of ozonized air.
3O2 2O3 PHYSICAL PROPERTIES 1. Ozone is a pale blue gas at room temperature. 2. It has a characteristic fishy odour. 3. In liquid state ozone is blue in colour.
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− 68
K. Cal
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4. 5.
It is slightly more soluble in water than oxygen. It is soluble in turpentine oil. It acts as a respiratory irritant if present in air in larger proportions.
CHEMICAL PROPERTIES 1. Ozone slowly decomposes into oxygen at room temperature but rapidly at 473 K. The rate of decomposition is increased in presence of catalysts like silver, platinum, vanadium etc. → 3O 2 + 68 K. Cal. 2O3 2.
i.
2 vol 3 vol Oxidising reactions: Ozone acts as a powerful oxidizing agent. Usually only one atoms of oxygen from each molecule is available for bringing about oxidation. Thus the principal equation is : O3 → O2 + O Ozone oxidizes lead sulphide (black) to lead sulphate (white) O3 → O2 + O × 4 → PbSO 4 PbS + 4O PbS + 4O3 → PbSO 4 + 4O 2
ii.
It oxidizes hydrohalic acids to halogens. O3 → O2
+O
2HCl + O → P bS O 4 2HCl + O3
→ H2 O + Cl2 + O2
iii. It oxidizes potassium maganate solution (green) to to potassium permanganate permanganate solution (pink). O3 → O2
+O
2K 2 Mn MnO 4
+ H2O + O → 2KMnO 4 + 2KOH
2K 2MnO 4
+ H2O + O 3 → 2KMnO 4 + 2KOH + O 2
iv. it oxidizes oxidizes potassium ferrocyanide solution solution to potassium ferricyanide ferricyanide solution solution.. O3 → O2 + O
v.
2K 4Fe F e(CN)6
F e(CN)6 + + H2 O + O → 2K3Fe
2K 4Fe Fe(CN)6
Fe(CN)6 + + H2 O + O3 → 2K 3Fe
Ozone liberates iodine from moist potassium iodide. O3 → O2
+
2KOH 2KOH + O2
O
2KI + H2 O + O → 2KOH + I2 2KI + H2 O + O3 → 2KOH + I2 + O2 vi.
Ozone directly oxidizes moist iodine, sulphur, phosphorus, arsenic, antimony etc. to their corresponding oxyacids with highest oxidation state. I 2 + H 2 O + 5O 2 → 2HIO 3 + 5O 2 S + H 2 O + 3O 3 → H 2 SO 4 + 3O 2 P 4 + 6H 2 O + 10O 2 → 4H 3 PO 4 + 10O 2 2As + 3H 2 O + 5O 3 → 2H 3 AsO 4 + 5O 2 2Sb + 3H 2 O + 5O 3 → 2H 3 SbO 4 + 5O 2
vii.
Tailing of mercury: Mercury reacts with ozone to form mercurous oxide 2Hg + O 2 → Hg 2 O + O 2
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The mercurous oxide produced dissolves in mercury m ercury which loses its mensiscus and sticks to the walls of the glass. This phenomenon is known as tailing of mercury. When this mercury is washed with water, it restores its original properties. viii. Exceptional oxidation reactions: In few cases, all the three atoms of ozone molecules are used up for oxidation. 3SO 2 + O 3 → 3SO 3 3SnCl 2 + 6HCl + O 3 → 3SnCl 4 + 3H 2 O 3. Reducing properties: In some reactions ozone acts as a reducing agent. Ozone takes up an oxygen atom from the substance for bringing about ab out reduction. Thus, the principal equation is: O 3 + O → 2O 2 i. Ozone reduces barium peroxide to barium oxide. BaO 2 + O 3 → BaO + 2O 2 ii.
It reduces hydrogen peroxide to water. H 2 O 2 + O 3 → H 2 O + 2O 2
iii.
It reduces silver oxide to metallic silver. Ag 2 O + O 3 → 2Ag + 2O 2
4.
Bleaching action Ozone combines with unsaturated hydrocarbons to form very unstable compounds called ozonides. These products are hydrolysed to carbonyl compounds and hydrogen peroxide. This process is known as ozonolysis. It is used to locate the position of carbon-carbon multiple in the original unsaturated compound. E.g: O CH 2 = CH 2 + O 3 → Ethylene
H2C
CH2 O ethyleneozonide
O O H2C
CH2 + H2O
→ 2HCHO + H 2 O 2 formaldehyde
O
O O CHO
ii.
HC = CH + O 3 →
HC O
CH
+H2O
+ H2O2 CHO glyoxal
O
Uses of Ozone Ozone is used 1. as a germicide and disinfectant for purification of air and sterilization of water. 2. as a mild bleaching agent for ivory, flour, starch, oils, wax etc. 3. to detect double and triple bonds in organic compounds. 4. to purify air in underground railways, zoos, mines, cinema halls etc.
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5.
a mixture of ozone and cyanogens (C2 N 2 ) is used as rocket fuel.
Tests of Ozone 1. It has a strong fishy like odour. 2. It turns starch iodide paper blue. 3. It turns benzidine paper brown and tetramethyl base paper violet. 4. Tailing of mercury: Mercury loses its meniscus in contact with ozone and sticks to the surface of glass. 5. A clean silver foil is blackened by ozone. Formula of Ozone i. Ozone is prepared by subjecting oxygen to silent electric discharge. It gives oxygen on heating. This shows that it is made up of oxygen atoms only. ii.
Soret’s experiment: Soret took two graduated flasks of exactly same capacity. These flasks were filled with exactly the same sample of ozone. These T hese flasks were inverted in a trough of water.
In one of the flasks (A) he introduced turpentine oil and heated the other (B). A decrease in volume was noticed in the flask A due to the absorption of ozone. An increase in volume was noticed in the flask B due to the decomposition of ozone to oxygen. The decrease in volume was found to be double the increase i.e. If the increase in volume is one, the decrease is two volumes. The decrease in volume (2 volumes) is the volume of ozone present in ozonised air. This increases by one volume on heating i.e. when 2 volumes of ozone decomposes, 3 volumes of oxygen is obtained. Ozone → Oxygen 2 vol 3 vol
Applying Avogadro’s law 2n molecules of ozone yield 3n molecules of oxygen. Two molecules of ozone yield 3 molecules of oxygen. One molecule of ozone yields 3/2 molecules m olecules or 3 atoms of oxygen. Hence formula of ozone should be O 3 .
Structure of Ozone: o
Ozone is diamagnetic diamagnetic and and has an an angular angular structure. structure. Both Both the oxygen oxygen to oxygen oxygen bonds bonds have the same length length (1.27 A ) o
o
which is intermediate between the double bond distance (1.10 A ) and a single bond distance (1.48 A ). The molecule may be represented as a resonance hybrid of the following structures. .. .. +O O+ .. :O
– .. : O:
– :O .. :
:O .. Spectroscopic studies suggest a non-linear structure as shown below. O O
117°
O
Manufacture of Sulphuric Acid, H 2 SO 4 : It is prepared on a large scale by two processes. 1. Lead chamber process and 2. Contact process
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1. Lead chamber process: It is an older method for the manufacture of sulphuric acid. In this process the chambers made up of lead are used. So this is called Lead chamber process. The metal lead is used because it is cheaper and has no action with sulphuric acid. Principle: In this process SO 2 is oxidized catalytically by means of oxides of nitrogen in the presence of water to form nitroso sulphuric acid. N 2 O 3 + O 2 + H 2 O + 2SO 2 → 2 NOHSO 4 When nitroso sulphuric acid is hydrolysed sulphuric acid is formed. 2NOHSO 4 + H 2 O → 2H 2 SO 4 + NO + NO 2 The oxides of nitrogen simply act as carriers of oxygen from air to sulphur dioxide. This method involves homogenous catalysis. Conditions: The above reactions take place at 673 to 733 K and 1.5 – 2 atmospheres pressure. 2. Contact Process: Principle: Purified SO 2 is oxidized to SO 3 catalytically. SO 3 vapour is absorbed in concentrated H 2 SO 4 to form pyrosulphuric acid or Oleum, H 2 S 2 O 7 . Water is added to oleum to form sulphuric acid of the desired concentration. 2 SO 2 + O 2 → 2 SO 3 SO 3 + H 2 SO 4 → H 2S 2 O7 → 2 H 2 SO 4 H 2 S 2 O 7 + H 2 O
Conditions: Oxidation of sulphur dioxide is reversible, exothermic and it proceeds with decrease in volume. According Le Chatelier’s principle, the optimum conditions for the th e better yield are: i.
Low temperature: The oxidation of sulphur dioxide to sulphur trioxide is exothermic. So this is favoured by low temperature. The reaction is too slow at low temperature. Hence the oxidation is carried out at about a bout 723 K. ii. High pressure: High pressure favours the oxidation. However, at high pressure, the material of the plant corrodes. So the reaction is carried out at a pressure slightly higher than one atmosphere by using excess of oxygen. iii. The presence of catalyst in contact process lowers the activation energy of the reacting gases. It also increases the number of fruitful collisions. These factors favour the formation of sulphurtrioxide. Powdered state of the catalyst is more effective. Vanadium pentoxide is used as a catalyst and a compound of potassium is uses as a promoter. The plant used for the manufacture of sulphuric acid by contact process is mainly consists of four units. 1. Sulphur or pyrites burners: In pyrites burners sulphur dioxide is obtained either by burning sulphur in air or by roasting iron pyrites. S + O 2 → SO 2
2. i. ii. iii. iv.
4FeS 2 + 11O 2 → 2Fe 2 O 3 + 8SO 2 The gases contain impurities like arsenic trioxide, sulphur or pyrite dust and sulphuric acid fog. Thy must be removed to prevent the poisoning of the catalyst. Purification unit: It consists fo the following parts Dusting tower or precipitator: In this tower steam is blown in, to make the dust particles settle down. Sometimes dust particles are precipitated mechanically or electrically in precipitator. Cooling pipe: The gases are cooled by passing them through pipe, cooled by air. The gases moving upwards are washed by a stream of water flowing down. Drying tower: The moist gases are dried in drying tower by a spray of concentrated sulphuric acid. Arsenic purifier: The dried gases are passed through arsenic purifier to be free from arsenic impurit ies. It contains gelatinous ferric hydroxide.
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v.
Testing box: The gases are passed into a testing box to find whether thy are free from impurities or not. In the box, a strong beam of light is passed against the gases. If any particles are present, they get illuminated due to Tyndall effect.
3.
Contact chamber: Pure gases are passed through a preheater and is heated to about 723K. These hot gases are passed into the contact chamber. In contact chamber sulphur dioxide and oxygen combine in presence of a catalyst vanadium pentoxide to form sulphur trioxide. 2SO 2 + O 2
2SO 3 + 45.2K.Cal
The reaction is exothermic and the heat produced raises the temperature of the catalyst to about 723 K. Once the reaction starts, the gases need not be heated in heat exchanger. 4.
Absorption tower: The gases containing mainly sulphur trioxide and nitrogen enter the absorption tower. Here sulphur trioxide is absorbed by 98% sulphuric acid to give oleum. SO 3 + H 2 SO 4 → H 2S 2 O7 Sulphuric acid of desired strength can be obtained by diluting the oleum with water. H 2 S 2 O 7 + H 2 O → 2H 2 SO 4
Properties of sulphuric acid Physical properties 1. Pure sulphuric acid is a colourless syrupy liquid. 2. Its density is 1.84 g/ml. 3. Sulphuric acid forms hydrates with explosive violence. So the affinity for water is very high. Chemcial Properties 1. Acidic nature: Sulphuric acid is a very strong dibasic acid. It reacts with alkalies forming two types of salts, bisulphates and sulphates. NaOH + H 2 SO 4 → NaHSO 4 + H 2 O 2 NaOH + H 2 SO 4 → Na 2 SO 4 + 2H 2 O 2.
Action on non-metals: Sulphuric acid oxidizes carbon to carbondioxide, sulphur to sulphurdioxide and phosphorus to phosphoric acid. C + 2H 2 SO 4 → 2H 2 O + 2SO 2 + CO 2 S + 2H 2 SO 4 → 2H 2 O + 3SO 2
3.
2P + 5H 2 SO 4 → 2H 3 PO 4 + 2H 2 O + 5SO 2 Action on metals: Metals like copper, silver, mercury, antimony which lie below hydrogen in electrochemical series do not evolve hydrogen with dilute sulphuric acid. These metals react in presence of air or oxygen to form sulphates and water 2Cu + 2H 2 SO 4 + O 2 → 2CuSO 4 + 2H 2 O Concentrated sulphuric acid reacts with these metals on boiling and liberate sulphur dioxide. Cu + 2 H 2 SO 4 → CuSO 4 + 2H 2 O + SO 2 2Ag + 2 H 2 SO 4 → Ag 2 SO 4 + 2H 2 O + SO 2 Metals like Zn, Mg, Na, K, Al, Pb, Sn etc., which lie above hydrogen in electrochemical series react with dilute H 2 SO 4 to liberate H 2 . Zn + H 2 SO 4 → ZnSO 4 + H 2 Mg + H 2 SO 4 → MgSO 4 + H 2
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These metals react with concentrated sulphuric acid and liberate sulphurdioxide. Zn + H 2 SO 4 → ZnSO 4 + 2H 2 O + SO 2 2Al + 6H 2 SO 4 → Al 2 (SO 4 ) 3 + 6H 2 O + 3SO 2 Metals like lead, gold, platinum do not react with sulphuric acid. 4.
Action with halides: Concentrated sulphuric acid reacts with fluorides and chlorides to form hydrogen fluoride and hydrogen chloride respectively. CaF 2 + H 2 SO 4 → CaSO 4 + 2HF NaCl + H 2 SO 4 → NaHSO 4 + HCl But hydrogen bromide and hydrogen iodide which are formed from bromides and iodides are oxidized to bromine and iodine respectively. 2KBr + H 2 SO 4 → K 2 SO 4 + 2HBr 2HBr + H 2 SO 4 → 2H 2 O + SO 2 + Br 2 2KI + H 2 SO 4 → K 2 SO 4 + 2HI 2HI + H 2 SO 4 → 2H 2 O + SO 2 + I 2 These reactions are used for the detection of halides.
5.
Affinity for water: It has great affinity for water. Therefore it is used for drying all gases except ammonia and hydrogen sulphide. It is also used as a strong dehydrating agent.
a.
Paper, starch, wood etc, are all charred by concentrated sulphuric acid due the removal of water. It is corrosive towards skin and causes painful blisters. Sugar is charred to give charcoal, Charring of sugar is due to dehydration. conc. conc.H H2SO4 C 2 H 22 O 11 → 12C + 11H 2 O It removes water from oxalic acid and formic acid.
b. c.
COOH conc. conc.H H2SO4 | → H2 O + CO + CO2 COOH conc. conc.H H SO
2 4 → H2 O + C HCOOH CO O
d.
Conc.H 2 SO 4 removes water from alcohol to give ethylene. o
170 C C2H2 OH → C2H4 + H2 O
Uses of Sulphuric Acid: It is used 1. as a reagent in the laboratory. 2. as a dehydrating and drying agetnt. 3. in fertilizer industry for the manufacture of ammonium sulphate and superphosphate of lime. 4. in refining petroleum and in coaltar industry. 5. in paper, textile, leather and rubber industries. 6. in the manufacture of dyes, drugs and disinfectants. 7. in the manufacture of paints and pigments.
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IIT-JEE Via Group Group Elements Elements
8. 9. 10. 11.
in the manufacture of important chemicals like hydrochloric acid, nitric acid, phosphoric acid, alums, sulphates, ether etc. in the manufacture of explosives like T.N.T dynamite, nitroglycerine etc. in storage batteries. in metallurgy.
Sodium Thiosulphate (Na 2 S 2 O 3 .5H 2 O) Sodium thiosulphate is also known as Hypo. Preparation: Sodium thiosulphate is prepared by boiling sodium sulphite solution with sulphur. Na 2 SO 3 + S → Na 2 S 2 O 3 The excess of sulphur is filtered off and the filtrate is evapourated to form sodium thiosulphate crystals, Na 2 S 2 O 3 . 5H 2 O Properties: 1. It is a colourless crystalline solid. 2. It is highly soluble in water. 3. Action of heat: On heating thiosulphate loses water of crystallization at 488 K. Na 2 S 2 O 3 .5H 2 O → Na 2 S 2 O 3 + 5H 2 O It decomposes when heated above 493 K to form sodium sulphate and sodium penta sulphide. 4Na 2 S 2 O 3 → 3Na 2 SO 4 + Na 2 S 5 4. Action of dilute acids: Dilute acids decompose thiosulphate with the liberation of sulphur dioxide an d precipitation of sulphur. Na 2 S 2 O 3 + 2HCl → 2NaCl + H 2 O + S + SO 2 5. Action of silver nitrate: Silver nitrate reacts with hypo to form a white precipitate which quickly changes to different colours and finally to black. Na 2 S 2 O 3 + 2AgNO 3 → Ag 2 S 2 O 3 + 2NaNO 3 White Ag 2 S 2 O 3 + H 2 O → Ag 2 S + H 2 SO 4 black 6. Action of ferric chloride: Ferric chloride reacts with hypo to form a violet colour ferric thiosulphate. 3Na 2 S 2 O 3 + FeCl 3 → Na 3 [Fe(S2 O 3 ) 3 ] + 3NaCl 7. Action with iodine solution: Sodium thiosulphate decolourises iodine solution to form tetrathionate. 2Na 2 S 2 O 3 + I 2 → 2NaI + Na 2 S 4 O 6 This reaction is used in estimation of iodine in volumetric analysis. 8. Action of chlorine: Chlorine oxidizes sodium thiosulphate solution to sodium sulphate. → Na 2 SO 4 + S + 2HCl Na 2 S 2 O 3 + H 2 O + Cl 2 Hence hypo is used as antichlor in textile text ile industry. 9. Action of silver halides: Silver halides are easily soluble in hypo solution. So hypo is used in photography as developer. AgX + 2Na 2 S 2 O 3 → Na 3 [Ag(S 2 O 3 ) 2 ] + NaX Uses of Sodium Thiosulphate: It is used 1. in photography for fixing. 2. in textile industry as an antichlor. 3. in the extraction of silver and gold. 4. in the iodometric titrations. 5. in medicine.
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IIT-JEE Via Group Group Elements Elements
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