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.
Oxygen occurs as two non-metallic
forms — dioxygen O2 and ozone O3
Ozone, O3, can be easily liquified as compared to
O2, It is a dark blue, unstable liquid.
Sulphur occurs as sulphide ores and as sulphates can
be obtained from volcanoes. Sulphur occurs as octaatomic molecule S8 ring form
Selenium and tellurium
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.
Selenium conducts electricity when exposed
to light, so it is used in solar cells, light
meters, and photographic materials.
Its most important use is as the lightsensitive component in photocopy machines.
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
Sulphide ores are widely distributed, including galena(PbS).
Cinnabar(HgS), pyrite(FeS2) and sphalerite(ZnS).
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.
3
H2S  O2  SO2  H2O
2
3
Fe2O3 , 673 K
2H2S(g)  SO2 
S8 (g)  2H2O
8
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.
Most important are Rhombic
or octahedral or a form
Monoclinic or prismatic or b
form
Rhombic and monoclinic forms undergo reversible
changes at 368.5 K (transition temperature)
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
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.
Oxygen is a powerful oxidising agent and H2Se
and H2Te are reducing agents.
S, Se and Te are mostly exists in positive
oxidation states. +4 in SF4, SO2, H2SO3 and
+ 6 in SF6, SO3, H2SO4.
Hydrides
H2O is liquid, others are colourless foul smelling toxic gases.
Bond length increases down the group.
Bond energy decreases down the group. Volatile nature decreases,
Acid strength: H2O < H2S < H2Se < H2Te < H2Po
Reducing nature: H2O < H2S < H2Se < H2Te < H2Po
Thermal stability: H2O > H2S > H2Se > H2Te > H2Po
Halides
General formula of halides M2X2, MX2, MX4 and MX6, sp3d2 hybridisation
Hexahalides: e.g SF6, SeF6, TeF6
Chemical activity increases from SF6 to TeF6. SF6 is inert. It does
not undergo hydrolysis, others undergo hydrolysis
Tetrahalides: Except O, all elements form tetrahalides of
chalcogen sp3d hybridisation distorted trigonal bipyramidal
shape.On hydrolysis give ‘ous’ acid.
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 like
and 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 atom
It 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.
SO2  H2O  H2SO3
SO2  NaOH  NaSO3  H2O
SO3  H2O  H2SO4
SO3  NaOH  Na2SO4  H2O
TeO3  H2O  H2TeO4
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
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.
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
Sulfuric Acid
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
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.
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.
C  H2SO4  CO2  2SO2  2H2O
Cu  2H2SO4  CuSO4  SO2  2H2O
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 :
S  3F2  SF6

SC 2  NaF SF4
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