Chemistry

Session Objectives

1. Occurrence of group 16 elements

2. Oxidation state and trends in chemical reactivity

3. Extraction of sulfur

4. Oxoacids

5. Sulfuric acid: process and properties

Group 16 elements: Chalcogens

Oxygen O [He] 2s2 2p4

Sulphur S [Ne] 3s2 3p4

Selenium Se [Ar] 3d10 4s2 4p4

Tellurium Te [Kr] 4d10 5s2 5p4

Polonium Po [Xe] 4f14 5d10 6s26p4

Occurrence

Oxygen is the most abundant of all

elements and exists in free form as

O2. Liquid O2 is pale blue and

paramagnetic in nature.

Sulphur occurs as sulphide ores and as sulphates can be obtained from volcanoes. Sulphur occurs as octa-atomic molecule S8 ring form

Ozone, O3, can be easily liquified as compared to O2, It is a dark blue, unstable liquid.

Oxygen occurs as two non-metallic forms — dioxygen O2 and ozone O3

Its most important use is as the light-sensitive component in photocopy machines.

Selenium and tellurium

Selenium conducts electricity when exposed to light, so it is used in solar cells, light meters, and photographic materials.

Selenium and tellurium occur in sulfide ores; they are also recovered from the refining of copper.

Selenium occurs as Se8 rings. Tellurium occurs as Te8 rings. Polonium is mono atomic.

Physical properties of group16 elements

Atomic size, density, melting point and boiling point increase down the group.

Polonium has less melting point and boiling point than tellurium due to inert pair effect.

Ionisation potential, electronegativity and electron affinity decrease down the group.

Non-metallic nature decreases and metallic nature increases down the group.

Extraction of sulfur

Sulfur is recovered from natural gas by first separating H2S through absorption in monoethanolamine.

Recovered H2S is passed over a catalyst Fe2O3 in the presence of air where H2S is partially converted into SO2 and then SO2 reacts with H2S and form Sulfur.

2 3

2 2 2 2

Fe O , 673 K2 2 8 2

3H S O SO H O

23

2H S(g) SO S (g) 2H O8

Sulphide ores are widely distributed, including galena(PbS). Cinnabar(HgS), pyrite(FeS2) and sphalerite(ZnS).

The Frasch Process

Sulfur is also obtained from oil and gas deposits and is recovered in the refining process.

Uses of S and its Oxides

Uses of S, Se and Te

Most important application of sulfur is in manufacturing of H2SO4.

Se is used in Xerox machines as a photoconductor.

It is an essential element in the body.

It is an important component of enzymes.

Te is used as an additive in metallurgy.

Te is also highly toxic.

Allotropy and Polymorphism

Sulphur has more allotropic forms than any other element.These forms arise partly from the extent to which S has polymerised.

Rhombic and monoclinic forms undergo reversible changes at 368.5 K (transition temperature)

Most important are Rhombic or octahedral or form

Monoclinic or prismatic or form

The S2 molecule is paramagnetic and blue coloured like O2

A piece of sulfur melts to a blood-red liquid. When burned, it emits a blue flame.

Oxidation State

S, Se and Te are mostly exists in positive oxidation states. +4 in SF4, SO2, H2SO3 and + 6 in SF6, SO3, H2SO4.

Oxygen is a powerful oxidising agent and H2Se and H2Te are reducing agents.

Except oxygen chalcogens exhibit –2, +2, +4 and +6 oxidation states oxygen exhibits –1 oxidation states in peroxides, +1 in O2F2 and +2 in OF2 . Common oxidation state of oxygen is –2.

Hydrides

Bond energy decreases down the group. Volatile nature decreases,

Bond length increases down the group.

H2O is liquid, others are colourless foul smelling toxic gases.

Acid strength: H2O < H2S < H2Se < H2Te < H2Po

Thermal stability: H2O > H2S > H2Se > H2Te > H2Po

Reducing nature: H2O < H2S < H2Se < H2Te < H2Po

Halides

General formula of halides M2X2, MX2, MX4 and MX6, sp3d2 hybridisation

Tetrahalides: Except O, all elements form tetrahalides of chalcogen sp3d hybridisation distorted trigonal bipyramidal shape.On hydrolysis give ‘ous’ acid.

Hexahalides: e.g SF6, SeF6, TeF6 Chemical activity increases from SF6 to TeF6. SF6 is inert. It does not undergo hydrolysis, others undergo hydrolysis

Halides

Dihalides: Except Se, all can form dihalides. SCl2 has foul smell. The oxidation number of S in SCl2 is +2 and its hybridisation is sp3 and is angular shaped. SCl2 on reacting with ethylene, gives mustard gas.

Mustard gas; S(CH2CH2Cl )2

Monohalides: S2Cl2 is toxic yellow liquid. Its structure is open book likeand has an angle of 104°. The oxidation number of S in S2Cl2 is +1. It is used in vulcanization of rubber.

Halides

Sulfur reacts readily with all the halogens except iodine.

It ignites sponteneously in flourine and burns brightly to give hexaflouride.

SF6 is colourless, tasteless, odourless, nontoxic, thermally

stable, and insoluble.

Despite the +6 oxidation state of S, it is not a good oxidising agent .Its inertness is due to presence of sterically protected sulphur atomIt is a much better insulator than air.

Oxides

Chalcogens form dioxides and trioxides — acidic nature decreases from oxides of S to Po. Dioxides dissolve in water to form ous acids trioxides form ‘ic’ acids.

2 2 2 3SO H O H SO

2 3 2SO NaOH NaSO H O

3 2 2 4SO H O H SO

3 2 4 2SO NaOH Na SO H O

3 2 2 4TeO H O H TeO

Illustrative problem

Explain why H2S acts only as a reducing agent but SO2 acts both as reducing agent as well as oxidising agent

Solution:

The minimum oxidation state of S is -2 and its maximum oxidation State is+6.In SO2 the oxidation state is +4 so it can be reduced +2 oxidation state or can be oxidised to +6 oxidation state so it can act both as oxidising agent as well as reducing agent.On the other hand In H2S S has oxidation state as –2 so it can only increase its oxidation state by loosing electrons and hence can act only as reducing agent.

Oxyacids of sulphur

Manufacture of Sulfuric Acid

Solid sulfur, S(s), is burned in air to form sulfur dioxide gas, SO2

S(s) + O2(g) SO2(g)

The mixture of sulfur dioxide and air is heated to 450oC and subjected to a pressure of 1 -2 atm in the presence of a catalyst (vanadium (V) oxide) to produce sulfur trioxide, 2SO2(g) + O2(g) 2SO3(g)

SO3 is absorbed in 98% H2SO4 to form oleum . SO3(g) + H2SO4 H2S2O7

There are two major processes used in the production of H2SO4, lead chamber process and contact process.

The lead-chamber process is the older of the two processes, and its product is aqueous sulfuric acid containing 62% to 78% H2SO4.

Sulfuric Acid

Pure sulfuric acid is a colourless, corrosive, oily liquid that boils(and decomposes) at about 300oC.

It has three very important properties: strong acid, dehydratig agent, oxidizing agent.

Oleum on dilution with water gives desired concentration of HSO4H2S2O7(l) + H2O(l) 2H2SO4(l)

The best way to dilute sulphuric acid is to carefully pour the acid into water with stirring

Dehydrating Nature

Concentrated acid (18M) is used as a dehydrating agent.e.g., sulfuric acid dehydrate sucrose, to produce a spongy mass of carbon. C12H22O11(s) + 11H2SO4 12C(s) + 11H2SO4.H2O

Oxidising Nature

Oxidising power is intermediate between phosphoric and nitric acids.

Both metals and non-metals are oxidised by

concentrated sulfuric acid, which is reduced to SO2.

2 4 2 2 2C H SO CO 2SO 2H O

2 4 4 2 2Cu 2H SO CuSO SO 2H O

With Zn, the reduction of sulfuric acid goes further to give sulfur or H2S.

Uses of Sulfuric Acid

Used in lead acid batteries, production of fertilizers (NH4)2SO4,

superphosphate, Ca(H2PO4)2, to remove oxides from iron and steel

before galvanising or electroplating, petrochemicals, dyestuffs, and detergents..

Used in the production of nitroglycerine, which is used as an explosive but can also be used as a vasodilator, a substance that dilates blood vessels and can be used in the treatment of certain types of heart disease.

Illustrative Problem

A yellow powder is burnt in a stream of F2 to obtain a colorless gas X which is thermally stable and has octahedral geometry. Another gas with same atoms as that of Y is obtained when SCl2 is heated with NaF. What are X and Y?

Solution :

2 6S 3F SF

2 4SC NaF SF

Environmental Issues

• Smog consists mainly of particulate (ash and smoke), SO2 and H2SO4 mist.

– Main contributor is the emission of SO2.

– Acid rain.

– Levels of SO2 and H2SO4 above 0.10 ppm are considered potentially harmful.

• Especially to respiratory tract.

Thank you