p Block Elements 3

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Chemistry


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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


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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


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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 canbe obtained from volcanoes. Sulphur occurs as octa-atomic molecule S8 ring form

Ozone, O3, can be easily liquified as compared toO2, It is a dark blue, unstable liquid.

Oxygen occurs as two non-metallic

forms dioxygen O2 and ozone O3


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

Selenium and tellurium

Selenium conducts electricity when exposedto light, so it is used in solar cells, light

meters, and photographic materials.

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

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


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Physical properties of group16 elements

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

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

Ionisation potential, electronegativity and electronaffinity decrease down the group.

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


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Extraction of sulfur

Sulfur is recovered from natural gas by first separatingH2S through absorption in monoethanolamine.

Recovered H2S is passed over a catalyst Fe2O3 in the presenceof air where H2S is partially converted into SO2 and then SO2reacts with H2S and form Sulfur.

2 3

2 2 2 2

Fe O , 673 K2 2 8 2

3H S O SO H O

2

32H S(g) SO S (g) 2H O

8

Sulphide ores are widely distributed, including galena(PbS).

Cinnabar(HgS), pyrite(FeS2) and sphalerite(ZnS).


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The Frasch Process

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


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Uses of S and its Oxides


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Uses of S, Se and Te

Most important application of sulfur is inmanufacturing 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.


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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 reversiblechanges at 368.5 K (transition temperature)

Most important are Rhombicor octahedral or a form

Monoclinic or prismatic or bform

The S2 molecule is paramagnetic and blue coloured like O2


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A piece of sulfur melts to ablood-red liquid. When burned, itemits a blue flame.


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Oxidation State

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

Oxygen is a powerful oxidising agent and H2Seand H2Te are reducing agents.

Except oxygen chalcogens exhibit 2, +2, +4 and +6

oxidation states oxygen exhibits 1 oxidation statesin peroxides, +1 in O2F2 and +2 in OF2 . Commonoxidation state of oxygen is 2.


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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


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Halides

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

Tetrahalides:Except O, all elements form tetrahalides ofchalcogen sp3d hybridisation distorted trigonal bipyramidalshape.On hydrolysis give ous acid.

Hexahalides: e.g SF6, SeF6, TeF6Chemical activity increases from SF6 to TeF6. SF6 is inert. It doesnot undergo hydrolysis, others undergo hydrolysis


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Halides

Dihalides:Except Se, all can form dihalides. SCl2 has foul smell. Theoxidation number of S in SCl

2is +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 isused in vulcanization of rubber.


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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 agood oxidising agent .Its inertness is due topresence of sterically protected sulphur atom

It is a much better insulator than air.


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Oxides

Chalcogens form dioxides and trioxides acidic nature

decreases from oxides of S to Po. Dioxides dissolve inwater 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


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Illustrative problem

Explain why H2S acts only as a reducing agent

but SO2 acts both as reducing agent as well asoxidising 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 canbe reduced +2 oxidation state or can be oxidised to +6oxidation state so it can act both as oxidising agent as well asreducing agent.On the other hand In H2S S has oxidationstate as 2 so it can only increase its oxidation state byloosing electrons and hence can act only as reducing agent.


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Oxyacids of sulphur


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Manufacture of Sulfuric Acid

Solid sulfur, S(s), is burned in air to form sulfur dioxide gas, SO2S(s) + O2(g) SO2(g)

The mixture of sulfur dioxide and air is heated to 450oC and subjectedto 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 itsproduct is aqueous sulfuric acid containing 62% to 78% H2SO4.


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Sulfuric Acid

Pure sulfuric acid is a colourless, corrosive, oily liquid thatboils(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 carefullypour the acid into water with stirring


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Dehydrating Nature

Concentrated acid (18M) is used as a dehydrating agent.

e.g., sulfuric acid dehydrate sucrose, to produce a spongy mass ofcarbon. C12H22O11(s) + 11H2SO4 12C(s) + 11H2SO4.H2O


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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.


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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 anexplosive 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.


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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 SCl2is heated with NaF. What are X and Y?

Solution :

2 6S 3F SF

2 4SC NaF SF


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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.


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Thank you

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