Group 16
30 June 2023 10:21
Introduction:
General valence shell electronic configuration:
Common name: Chalcogens (Ore forming elements such as oxides and sulphides)
Occurrence:
Oxygen (O)
• Most abundant of all the elements on earth
• 46.6% by mass of earth’s crust
• Dry air contains 20.946% oxygen by volume
Sulphur (S)
• Earth’s crust - 0.03-0.1%
• Primarily as sulphates such as gypsum CaSO4 .2H2O, epsom salt MgSO4 .7H2O, baryte BaSO4 and sulphides such as galena PbS,
zinc blende ZnS, copper pyrites CuFeS2
• Traces of sulphur occur as hydrogen sulphide in volcanoes.
• Organic materials such as eggs, proteins, garlic, onion, mustard, hair and wool contain sulphur
Selenium (Se) and Tellurium (Te)
• Selenium and tellurium are found as metal selenides and tellurides in sulphide ores.
Polonium (Po)
• occurs in nature as a decay product of thorium and uranium minerals
Livermorium (Lv)
• Synthetic radioactive element
• very short half-life
Atomic and Ionic Radii
Ionisation Enthalpy
Ques: Group 16 have lower ionisation enthalpy values compared to those of Group15 in the corresponding periods. Why?
Sol:
Electron gain enthalpy
Order of negative electron gain enthalpy:
Electronegativity
Physical Properties:
• All these elements exhibit allotropy
• Melting and boiling points increase down the group
• Large difference between the m.p. and b.p. of oxygen and sulphur may be explained on the basis of their atomicity
Chemical Properties:
Oxidation states
Oxidation states: -2, 0, +2, +4, +6
Ques: Stability of -2 oxidation state _____________ down the group.
Reason:
Ques: Stability of + 6 oxidation state _____________ down the group and stability of + 4 oxidation state _____________ .
Reason:
Ques: Oxidation states of Oxygen in OF2 and O2F2 are ______ and ______.
Reason:
Polonium hardly shows –2 oxidation state.
Anomalous behaviour of oxygen
Reason:
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Ques: Hydrogen bonding is shown by _____________ (H2O/H2S).
Ques: Maximum covalency that can be shown by Oxygen is _______.
(Oxygen rarely exceeds its covalency to two.)
Reactivity with hydrogen
Hydride: H2E (E = O, S, Se, Te, Po)
Reaction:
Acidic Nature
Bond Dissociation energy
Thermal Stability
Reducing character
Boiling Point:
Melting Point:
All the hydrides except water possess reducing property
Reactivity with oxygen:
Type of oxides: EO2 and EO3 types where E = S, Se, Te or Po
Both types of oxides are acidic in nature.
Ques: Tendency to form EO3 type oxide _____________ down the group.
Reason:
Reactivity towards the halogens:
Type: EX6 , EX4 and EX2
Ques: Stability of halides: F– Cl– Br– I–
Ques: Give the hybridisation and shape of SF6 and SF4.
All elements except oxygen form dichloride and dibromides.
Example: S2F2 , S2Cl2 , S2Br2 , Se2Cl2 and Se2Br2
Ques: Why is H2O a liquid and H2S a gas ?
Dioxygen
Preparation:
By the thermal decomposition of the oxides of metals low in the electrochemical series and higher oxides of some metals.
Decomposition of H2O2
Large scale preparation :
Electrolysis of water leads to the release of hydrogen at the cathode and oxygen at the anode.
Gas produced at anode:
Gas produced at cathode:
Industrial preparation:
Dioxygen is obtained from air by first removing carbon dioxide and water vapour and then, the remaining gases are liquefied and
fractionally distilled to give dinitrogen and dioxygen.
Properties:
• colourless and odourless
• solubility in water is to the extent of 3.08 cm3 in 100 cm3 water at 293 K which is just sufficient for the vital support of marine and
aquatic life.
• liquefies at 90 K and freezes at 55 K.
• 3 stable isotopes: 16O, 17O and 18O.
• Magnetic behaviour:
Uses:
• Respiration and combustion processes
• It is used in oxyacetylene welding
• In the manufacture of many metals, particularly steel.
• Oxygen cylinders are widely used in hospitals, high altitude flying and in mountaineering.
• The combustion of fuels, e.g., hydrazines in liquid oxygen, provides tremendous thrust in rockets
Simple Oxides
Ozone
• Allotropic form of oxygen
• Too reactive to remain for long in the atmosphere at sea level
• At a height of about 20 kilometres, it is formed from atmospheric oxygen in the presence of sunlight.
• Ozone layer protects the earth’s surface from an excessive concentration of (UV) radiations
Preparation
When a slow dry stream of oxygen is passed through a silent electrical discharge, conversion of oxygen to ozone (10%) occurs.
If concentrations of ozone greater than 10 per cent are required, a battery of ozonisers can be used, and pure ozone (b.p. 101.1K) can be
condensed in a vessel surrounded by liquid oxygen.
Structure:
Properties:
• Pale blue gas, dark blue liquid and violet-black solid
• Has a characteristic smell and in small concentrations it is harmless
• If conc. rises above about 100 ppm, breathing becomes uncomfortable resulting in headache and nausea
• Thermodynamically unstable with respect to oxygen
• it acts as a powerful oxidising agent
• Quantitative method for estimating O3 gas
Uses:
• It is used as a germicide, disinfectant and for sterilising water.
• It is also used for bleaching oils, ivory, flour, starch, etc.
• It acts as an oxidising agent in the manufacture of potassium permanganate.
Sulphur — Allotropic Forms
Two Types: α-sulphur and β-sulphur
The stable form at room temperature is rhombic sulphur, which transforms to monoclinic sulphur when heated above 369 K.
Rhombic sulphur (α-sulphur)
• yellow in colour, m.p. 385.8 K and specific gravity 2.06
• Rhombic sulphur crystals are formed on evaporating the solution of roll sulphur in CS2.
• It is insoluble in water but dissolves to some extent in benzene, alcohol and ether.
• It is readily soluble in CS2 .
β-sulphur
• Its m.p. is 393 K and specific gravity 1.98.
• It is soluble in CS2.
• It is stable above 369 K and transforms into α-sulphur below it.
• Conversely, α-sulphur is stable below 369 K and transforms into β-sulphur above this.
• At 369 K both the forms are stable.
• This temperature is called transition temperature.
• Both rhombic and monoclinic sulphur have S8 molecules.
• These S8 molecules are packed to give different crystal structures.
• The S8 ring in both the forms is puckered and has a crown shape.
Several other modifications of sulphur containing 6-20 sulphur atoms per ring have been synthesised. In cyclo-S6 , the ring adopts the
chair form.
At elevated temperatures (~1000 K), S2 is the dominant species and is paramagnetic like O2 .
Sulphur Dioxide
Preparation
Preparation
Properties:
Uses:
• in refining petroleum and sugar
• in bleaching wool and silk
• as an anti-chlor, disinfectant and preservative.
• Sulphuric acid, sodium hydrogen sulphite and calcium hydrogen sulphite (industrial chemicals) are manufactured from sulphur
dioxide. Liquid SO2 is used as a solvent to dissolve a number of organic and inorganic chemicals.
Oxoacids of Sulphur
Sulphuric Acid
Preparation:
Contact Process
(i) burning of sulphur or sulphide ores in air to generate SO2 .
(ii) conversion of SO2 to SO3 by the reaction with oxygen in the presence of a catalyst (V2O5 )
(iii) absorption of SO3 in H2SO4 to give Oleum (H2S2O7).
The sulphuric acid obtained by Contact process is 96-98% pure
Properties
colourless, dense, oily liquid with a specific gravity of 1.84 at 298 K
acid freezes at 283 K and boils at 611 K
dissolves in water with the evolution of a large quantity of heat
chemical reactions of sulphuric acid are as a result of the following characteristics:
(a) low volatility (b) strong acidic character (c) strong affinity for water and (d) ability to act as an oxidising agent
Concentrated sulphuric acid is a strong dehydrating agent
Hot concentrated sulphuric acid is a moderately strong oxidising agent.
Uses:
• The bulk of sulphuric acid produced is used in the manufacture of fertilisers (e.g., ammonium sulphate, superphosphate).
• petroleum refining
• manufacture of pigments, paints and dyestuff intermediates
• detergent industry
• metallurgical applications (e.g., cleansing metals before enameling, electroplating and galvanising
• storage batteries
• in the manufacture of nitrocellulose products
• as a laboratory reagent.
