P block elements

This ppt covers a part of the ncert class 12 chap-7 p-block elements.It covers the group-16 elements, oxygen, sulphur. If you like please like, share.

1. Group-16 elements P-BLOCK ELEMENTS

2. Simple oxides Group-16 Elements

3. Simple oxides Oxide-binary compound of oxygen with another element. Oxygen reacts with most of the elements, one element may form two or more oxides. Oxides can be simple (e.g., MgO , Al 2 O 3 ) or mixed ( Pb 3 O 4 , Fe 3 O 4 ).

4. Classification Acidic Oxide that combines with water to give an acid is termed acidic oxide. (e.g., SO 2 , Cl 2 O 7 , CO 2 , N 2 O 5 ,Mn 2 O 7 , CrO 3 , V2O 5 ) Basic Oxide which give a base with water known as basic oxides. (e.g., Na 2 O, CaO , BaO ) Amphoteric Some metallic oxides exhibit a dual behavior of reacting with both acids and bases. (e.g., Al 2 O 3 )

5. Classification Acidic Oxide SO 2 + H 2 O → H 2 SO 3 Basic Oxide CaO + H 2 O → Ca(OH) 2 Amphoteric Oxide Al 2 O 3 (s) + 6HCl (aq) + 9H 2 O(l) → 2[Al(H 2 O) 6 ] 3+ (aq) + 6Cl - (aq) Al 2 O 3 (s) + 6NaOH(aq) + 3H 2 O(l ) → 2Na 3 [Al(OH) 6 ](aq)

6. oZONE Group-16 Elements

7. oZONE Ozone is an allotropic form of oxygen . Too reactive to remain for long in the atmosphere at sea level At a height of about 20 km, it is formed from atmospheric oxygen in the presence of sunlight. This ozone layer protects the earth’s surface from an excessive concentration of ultraviolet (UV) radiations.

8. preparation Ozone

9. Preparation Slow dry stream of oxygen is passed through a silent electrical discharge, oxygen converted to ozone (10%) . product - ozonised oxygen. 3O 2 → 2O 3 H (298 K) = +142 kJ mol–1 Endothermic process - necessary to use a silent electrical discharge – to prevent its decomposition. If conc. of ozone >10 % 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.

10. Properties Ozone

11. Physical properties Pure ozone is a pale blue gas Liquid - dark blue Solid - violet-black Ozone has a characteristic smell In small concentrations – harmless If conc. above about 100 ppm breathing – uncomfortable - headache & nausea.

12. Properties Ozone is thermodynamically unstable with respect to oxygen. Its decomposition into oxygen results in the liberation of heat ( H is negative) and an increase in entropy ( S is positive). These two effects reinforce each other, resulting in large negative Gibbs energy change ( G) for its conversion into oxygen . It is not really surprising, therefore, high concentrations of ozone can be dangerously explosive.

13. Due to the ease with which it liberates atoms of nascent oxygen (O 3 → O 2 + O) , it acts as a powerful oxidising agent. Oxidises lead sulphide to lead sulphate and iodide ions to iodine. PbS (s) + 4O 3 (g) → PbSO 4 (s) + 4O 2 (g) 2I – (aq) + H 2 O(l) + O 3 (g) → 2OH – (aq) + I 2 (s) + O 2 (g) Properties

14. Ozone estimation When ozone reacts with an excess of potassium iodide solution buffered with a borate buffer (pH 9.2), iodine is liberated which can be titrated against a standard solution of sodium thiosulphate . This is quantitative method for estimating O 3 gas.

15. Ozone depletion Ozone

16. Ozone depletion Nitrogen oxides (particularly nitric oxide) combine very rapidly with ozone Nitrogen oxides emitted from the exhaust systems of supersonic jet aeroplanes might be slowly depleting the concentration of the ozone layer in the upper atmosphere. NO(g) + O 3 (g) → NO 2 (g)+ O 2 (g) Another threat to this ozone layer is probably posed by the use of freons which are used in aerosol sprays and as refrigerants.

17. Freons Freon is a registered trademark of The Chemours Company, which it uses for a number of halocarbon products. They are stable, nonflammable, moderately toxic gases or liquids which have typically been used as refrigerants and as aerosol propellants. These include CFCs that cause ozone depletion  ( chlorodifluoromethane ) , but also include newer refrigerants which typically include fluorine instead of chlorine and do not deplete the ozone layer. Not all refrigerant is labelled as "Freon" since Freon is a brand name for the refrigerants R-12, R-13B1, R-22, R-502, and R-503 manufactured by The Chemours Company.

18. USES Used as a germicide, disinfectant & for sterilizing water. Used for bleaching oils, ivory, flour, starch, etc. Acts as an oxidizing agent in the manufacture of potassium permanganate.

19. Sulphur Group-16 Elements

20. Allotropic Forms Sulphur

21. Allotropic Forms Sulphur forms numerous allotropes of which the yellow rhombic ( α- sulphur ) and monoclinic ( β - sulphur ) forms are the most important. The stable form at room temperature is rhombic sulphur , transforms to monoclinic sulphur when heated above 369 K.

22. Rhombic sulphur ( a - sulphur ) This allotrope is 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 CS 2 . It is insoluble in water but dissolves to some extent in benzene, alcohol and ether. It is readily soluble in CS 2 .

23. Monoclinic sulphur ( β - Sulphur ) Its m.p . is 393 K and specific gravity 1.98. It is soluble in CS2. This form of sulphur is prepared by melting rhombic sulphur in a dish and cooling, till crust is formed. Two holes are made in the crust and the remaining liquid poured out. On removing the crust, colourless needle shaped crystals of β- sulphur are formed. It is stable above 369 K and transforms into α- sulphur below it.

24. Allotropes 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.

25. Allotropes Several other modifications of sulphur containing 6-20 sulphur atoms per ring have been synthesised in the last two decades. In cyclo-S 6 , the ring adopts the chair form. At elevated temperatures (~1000 K), S 2 is the dominant species and is paramagnetic like O 2 .

26. Sulphur dioxide Sulphur

27. Preparation Sulphur dioxide is formed together with a little (6-8%) sulphur trioxide when sulphur is burnt in air or oxygen: S( s) + O 2 (g) → SO 2 (g) In the laboratory it is readily generated by treating a sulphite with dil. sulphuric acid. SO 3 2- ( aq) + 2H + (aq) → H 2 O(l) + SO 2 (g)

28. Preparation Industrially, it is produced as a by-product of the roasting of sulphide ores. Fe 2 S( s) +11O 2 (g) →2Fe 2 O 3 (s) + 8SO 2 (g) The gas after drying is liquefied under pressure and stored in steel cylinders.

29. Physical properties Sulphur dioxide is a colourless gas with pungent smell and is highly soluble in water. It liquefies at room temperature under a pressure of two atmospheres and boils at 263 K.

30. Chemical properties Sulphur dioxide, when passed through water, forms a solution of sulphurous acid. SO 2 (g) + H 2 O(l) → H 2 SO 3 ( aq) It reacts readily with NaOH solution, forming sodium sulphite , which then reacts with more So 2 to form sodium hydrogen sulphite . 2NaOH + SO 2 → Na 2 SO 3 + H 2 O Na 2 SO 3 + H 2 O + SO 2 → 2NaHSO 3

31. Chemical properties In its reaction with water and alkalies , SO 2 is very similar to that of CO 2 SO 2 reacts with Cl 2 in the presence of charcoal (catalyst) to give sulphuryl chloride. SO 2 (g) + Cl 2 (g) → SO 2 Cl 2 ( l) It is oxidised to sulphur trioxide by oxygen in the presence of vanadium oxide catalyst. 2SO2(g) + O2(g) → 2SO3(g) V2O5

32. When moist, sulphur dioxide behaves as a reducing agent. For example, it converts iron(III) ions to iron(II) ions and decolourises acidified potassium permanganate(VII) solution; the latter reaction is a convenient test for the gas. 2Fe 3+ + SO 2 + 2H 2 O → 2Fe 2+ + SO 4 2- + 4H + + 2Mn 2+ 5SO 2 + 2MnO 4 - + 2H 2 O → 5SO 4 2- + 4H + + 2Mn 2+ Chemical properties

33. Uses ( i ) in refining petroleum and sugar (ii) in bleaching wool and silk (iii) as an anti- chlor , disinfectant and preservative . An  antichlor  is a substance used to decompose residual hypochlorite or chlorine after chlorine-based bleaching, in order to prevent ongoing reactions with, and therefore damage to, the material that has been bleached. 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.

34. Oxoacids Sulphur

35. Oxoacids Sulphur forms a number of oxoacids such as H 2 SO 3 , H 2 S 2 O 3 , H 2 S 2 O 4 , H 2 S 2 O 5 , H 2 S x O 6 (x = 2 to 5), H 2 SO 4 , H 2 S 2 O 7 , H 2 SO 5 , H 2 S 2 O 8 . Some of these acids are unstable and cannot be isolated. They are known in aqueous solution or in the form of their salts.

36. Oxoacids

37. Sulphuric Acid Sulphur

38. Manufacturing Sulphuric acid is one of the most important industrial chemicals worldwide . Manufactured by the Contact Process involving three steps: ( i ) burning of sulphur or sulphide ores in air to generate SO 2 . (ii) conversion of SO 2 to SO 3 by the reaction with oxygen in the presence of a catalyst (V 2 O 5 ) (iii) absorption of SO 3 in H 2 SO 4 to give Oleum (H 2 S 2 O 7 ).

39. The SO 2 produced is purified by removing dust and other impurities such as arsenic compounds. The key step in the manufacture of H 2 SO 4 is the catalytic oxidation with V 2 O 5 (catalyst). 2SO 2 (g) + O2(g) → 2SO 3 (g) The reaction is exothermic, reversible and the forward reaction leads to a decrease in volume. Therefore, low temperature and high pressure are the favourable conditions for maximum yield. But the temperature should not be very low otherwise rate of reaction will become slow. Manufacturing

40. In practice, the plant is operated at a pressure of 2 bar and a temperature of 720 K. The SO 3 gas from the catalytic converter is absorbed in concentrated H 2 SO 4 to produce oleum . Dilution of oleum with water gives H 2 SO 4 of the desired concentration. In the industry two steps are carried out simultaneously to make the process a continuous one and also to reduce the cost. SO 3 + H 2 SO 4 → H 2 S 2 O 7 The sulphuric acid obtained by Contact process is 96-98% pure. Manufacturing

41. Properties Sulphuric Acid

42. Properties colourless - dense - oily liquid - specific gravity of 1.84 at 298 K - freezes at 283 K & boils at 611 K . It dissolves in water with the evolution of a large quantity of heat. Hence, care must be taken while preparing sulphuric acid solution from concentrated sulphuric acid. The concentrated acid must be added slowly into water with constant stirring.

43. CheMical Properties The 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 (d) ability to act as an oxidising agent.

44. In aqueous solution, sulphuric acid ionises in two steps. H 2 SO 4 (aq) + H 2 O(l) → H 3 O+(aq) + HSO 4 - ( aq ); K a 1 = very large (K a 1 >10) HSO 4 – (aq) + H 2 O(l) → H 3 O + (aq) + SO 4 2- (aq) ; K a 2 = 1.2 × 10 –2 The larger value of K a1 (K a1 >10) means that H2SO4 is largely dissociated into H + and HSO4 – .Greater the value of dissociation constant ( K a ), the stronger is the acid. forms two series of salts: normal sulphates (such as sodium sulphate and copper sulphate ) and acid sulphates (e.g., sodium hydrogen sulphate ). CheMical Properties

45. because of its low volatility can be used to manufacture more volatile acids from their corresponding salts. 2 MX + H 2 SO 4 → 2 HX + M 2 SO 4 (X = F, Cl , NO3) (M = Metal) Conc. sulphuric acid is a strong dehydrating agent. Many wet gases can be dried by passing them through sulphuric acid, provided the gases do not react with the acid. Sulphuric acid removes water from organic compounds; it is evident by its charring action on carbohydrates. C 12 H 22 O 11 + H 2 SO 4 →12C + 11H 2 O CheMical Properties

46. Hot concentrated sulphuric acid is a moderately strong oxidising agent. In this respect, it is intermediate between phosphoric and nitric acids. Both metals and non-metals are oxidised by concentrated sulphuric acid, which is reduced to SO2. Cu + 2H 2 SO 4 (conc.) → CuSO 4 + SO 2 + 2H 2 O S + 2H 2 SO 4 (conc.) → 3SO 2 + 2H 2 O C + 2H 2 SO 4 (conc.) → CO 2 + 2 SO 2 + 2 H 2 O CheMical Properties

47. Uses Very important industrial chemical. A nation’s industrial strength can be judged by the quantity of sulphuric acid it. Produces and consumes. Needed for manufacture of hundreds of other compounds and also in many industrial processes. The bulk of sulphuric acid produced is used in the manufacture of fertilisers ( e.G. , Ammonium sulphate , superphosphate).

48. (a) petroleum refining (b) manufacture of pigments, paints and dyestuff intermediates (c) detergent industry (d) metallurgical applications (e.g., cleansing metals before enameling, electroplating and galvanising ) (e) storage batteries (f) in the manufacture of nitrocellulose products (g) as a laboratory reagent. Other Uses