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sulphuric acid

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Sulphur Dioxide and Sulphuric Acid 1. Sulphur dioxide gas can be prepared by the following chemical methods : (i) By burning sulphur in air. burning S + O2 SO2 (ii) By roasting metallic sulphides in air burning 2PbS + 3O2 2PbO + 2SO2 (iii) By treating metallic sulphites with dil. H2SO4 Na2SO3 + H2SO4 (dil.) Na2SO4 + H2O + SO2 2. In the laboratory, sulphur dioxide is prepared (i) by heating conc. sulphuric acid with copper turnings (ii) by treating dilute sulphuric acid with sodium sulphite. The gas is dried by passing it through conc. sulphuric acid. It is collected by the upward displacement of air. Cu + 2H2SO4 (conc.) CuSO4 + 2H2O + SO2 (g) Na2SO3 + H2SO4 (dil.) Na2SO4 + H2O + SO2 (g) 3. Physical properties of sulphur dioxide gas : 1. It is a colourless gas, having a pungent and suffocating odour, which always causes coughing. 2. It has a sour taste and is fairly soluble in water. 3. It is 2.2 times heavier than air. 4. It is highly poisonous in nature. 4. Chemical properties of sulphur dioxide gas : 1. It is neither combustible nor does it support combustion. 2. It turns moist blue litmus paper red and then bleaches it. Hence, it is acidic as well as a bleaching agent in nature. 3. It dissolves in water to form sulphurous acid. SO2 + H2O H2SO3 1 Key Points 4. It reacts with alkalis to form two series of salts, i.e., sulphites and bisulphites. NaOH + SO2 NaHSO3 NaOH + NaHSO3 Na2SO3 + H2O 5. It reacts with solutions of soluble carbonates and liberates carbon dioxide gas. Na2CO3 + 2SO2 + H2O 2NaHSO3 + CO2 (g) 6. Bleaching properties of sulphur dioxide are based on the fact that, it easily liberates nascent hydrogen when treated with water. The nascent hydrogen reacts with oxygen atoms present in vegetable colouring matter and hence bleaches it. SO2 + 2H2O H2SO4 + 2 [H] Veg. colouring matter + 2 [H] Colourless product + Water 7. The bleaching done by sulphur dioxide is temporary in nature. The bleached article slowly regains its orginal colour by absorbing atoms of oxygen from air. However, bleaching done by sulphur dioxide does not damage the article. 8. Two volumes of dry SO2 and one volume of dry oxygen react in the presence of platinised asbestos to form sulphur trioxide. 2SO2 + O2 2SO3 9. Dry sulphur dioxide reacts with dry chlorine to form sulphuryl chloride. Sunlight SO2 + Cl2 SO2Cl2 10. Moist sulphur dioxide reacts with chlorine to form a mixture of sulphuric acid and hydrochloric acid. SO2 + Cl2 + 2H2O H2SO4 + 2HCl 11. Burning, highly active metals react with it to form their oxides or sulphites. 4Na + 3SO2 Na2SO3 + Na2S2O3 2Mg + SO2 2MgO + S 12. It reduces bromine water to hydrobromic acid. SO2 + Br2 + 2H2O H2SO4 + 2HBr 2 Key Points 5. 6. 7. 8. 13. It reduces iodine solution to hydroiodic acid. SO2 + I2 + 2H2O H2SO4 + 2HI 14. It reduces ferric salts to ferrous salts. Fe2(SO4)3 + SO2 + 2H2O 2FeSO4 + 2H2SO4 15. It reduces orange-coloured potassium dichromate solution to green-coloured solution. K2Cr2O7 + H2SO4 + 3SO2 K2SO4 + Cr2(SO4)3 + H2O 16. It reduces potassium permanganate sol. (pink) to colourless solution. 2KMnO4 + 5SO2 + 2H2O K2SO4 + 2MnSO4 + 2H2SO4 17. It oxidises moist hydrogen sulphide to sulphur. 2H2S + SO2 2H2O + 3S Uses of sulphur dioxide : 1. It is used for bleaching delicate objects such as wool, silk, feathers, etc. 2. It is used in bleaching cane juice in sugar mills. 3. It is used in the refining of kerosene oil. 4. It is used in the manufacture of sulphuric acid. 5. It is used in making industrial sulphites such as, sodium hydrogen sulphite. Tests of sulphur dioxide : 1. It has a pungent and choking smell which causes coughing. 2. It turns reddish-brown ferric sulphate solution to green-coloured ferrous sulphate solution. 3. It decolourises bromine water. 4. It turns acidified potassium dichromate solution green. Hydrogen sulphide gas is prepared by treating metallic sulphides with dil. H2SO4 FeS + H2SO4(dil.) FeSO4 + H2S (g) Hydrogen sulphide gas (i) colourless (ii) foul smelling like rotten eggs, (iii) fairly soluble in water, (iv) sour taste and, (v) heavier than air. 3 Key Points 9. Hydrogen sulphide gas is used as an analytical tool as it reacts with soluble metallic salts in acidic or alkaline medium to form their metallic sulphides, which are insoluble in water and have a specific colour. Acidic 2AgNO3 (sol.) + H2S (g) 2HNO3 + Ag2S (black ppt.) medium Acidic H2SO4 + CuS (black ppt.) CuSO4 (sol.) + H2S (g) medium Acidic H2SO4 + HgS (black ppt.) HgSO4 (sol) + H2S (g) medium Acidic 2HNO3 + CdS (yellow ppt.) Cd(NO3)2 (sol) + H2S(g) medium Alkaline MnSO4(sol) + H2S (g) H2SO4 + MnS (flesh colour ppt.) medium Alkaline 2HCl + NiS (black ppt.) NiCl2 (sol) + H2S (g) medium Alkaline 2HCl + CoS (black ppt.) CoCl2 + H2S (g) medium Alkaline 2HNO3 + ZnS (white ppt.) Zn(NO3)2 + H2S (g) medium 10. Sulphuric acid can be prepared in the laboratory by any of the following chemical methods : (i) By heating hydrated ferrous sulphate in the absence of air. 2FeSO4 7H2O Fe2O3 + SO2 + H2SO4 + 13H2O (ii) By the oxidation of sulphurous acid. H2SO3 + [O] H2SO4 (iii) By dissolving sulphur trioxide in water. SO3 + H2O H2SO4 (iv) By dissolving sulphuryl chloride in water. SO2Cl2 + 2H2O H2SO4 + 2HCl (v) By boiling sulphur with nitric acid S + 6HNO3 (conc.) 6NO2 + 2H2O + H2SO4 4 Key Points 11. Contact process for the manufacture of sulphuric acid : (i) Pure sulphur is burnt in excess of oxygen in sulphur burners to form a mixture of sulphur dioxide and oxygen. The mixture is cooled and then washed with water to remove any dust particles as impurity. S + O2 SO2 The moist mixture is dried in conc. sulphuric acid and then passed through arsenic purifier, so as to remove any arsenious oxide present in it. The purified mixture is then passed through testing box. (ii) The purified mixture is finally introduced into catalytic oxidation chamber which contains a series of vertical tubes filled with vanadium pentoxide or platinised asbestos. Sulphur dioxide on coming in contact with catalyst is oxidised to sulphur trioxide. V2O5 2SO2 + O2 2SO3 at 450 C (iii) The sulphur trioxide gas is then absorbed in conc. sulphuric acid to form pyrosulphuric acid or oleum. SO3 + H2SO4 (conc.) H2S2O7 (iv) The pyrosulphuric acid is treated with calculated amount of water to form conc. sulphuric acid. H2S2O7 + H2O 2H2SO4 12. Physical properties of sulphuric acid : 1. Pure sulphuric acid is a colourless, odourless, syrupy liquid. 2. Its density is 1.84 g cm 3 at 15 C. 3. 98.5% sulphuric acid has boiling point 338 C. 4. It is hygroscopic in nature and absorbs a large amount of water vapour from air. 5. It dissolves in water; producing a large amount of heat of hydration. 6. It is highly corrosive in nature and chars the skin black. 7. Pure sulphuric acid is a poor conductor of electricity. However, dilute sulphuric acid is a very good conductor of electricity. 5 Key Points 13. Chemical properties of sulphuric acid : 1. Sulphuric acid dissolves in water to furnish H3O+ ions, and hence turns (a) blue litmus solution red (b) methyl orange solution pink (c) phenolphthalein solution colourless. 2. It reacts with metallic oxides to form their respective salts and water. CuO + H2SO4 (dil.) CuSO4 + H2O 3. It reacts with alkalis to form two series of salts, i.e., bisulphates and sulphates. NaOH (dil.) + H2SO4 (dil.) NaHSO4 + H2O NaOH (dil.) + NaHSO4 Na2SO4 + H2O 4. It reacts with metallic carbonates to form their respective sulphates, carbon dioxide gas and water. Na2CO3 + H2SO4 (dil.) Na2SO4 + H2O + CO2 5. It reacts with metallic sulphites to form their respective sulphates; sulphur dioxide gas and water. Na2SO3 + H2SO4 (dil.) Na2SO4 + H2O + SO2 6. It reacts with metallic sulphides to form their respective sulphates and hydrogen sulphide. FeS + H2SO4 (dil.) FeSO4 + H2S 7. It reacts with metallic nitrites to form their respective sulphates and oxides of nitrogen. 2NaNO2 + H2SO4 (dil.) Na2SO4 + H2O + NO + NO2 8. It reacts with metallic nitrates on heating to form their respective sulphates and nitric acid. heat 2KNO3 + H2SO4 (conc.) K2SO4 + 2HNO3 9. It reacts with metallic chlorides on heating to form their respective sulphates and HCl gas. heat MgCl2 + H2SO4 (conc.) MgSO4 + 2HCl 10. Conc. sulphuric acid on boiling with carbon oxidises it to carbon dioxide. heat C + 2H2SO4 (conc.) 2H2O + CO2 + 2SO2 6 Key Points 11. Conc. sulphuric acid on boiling with sulphur oxidises it to sulphur dioxide. heat S + 2H2SO4 (conc.) 2H2O + 3SO2 12. Conc. sulphuric acid on boiling with red phosphorus oxidises it to phosphoric acid. heat 2P + 5H2SO4 (conc.) 2H3PO4 + 5SO2 + 2H2O 13. Dilute sulphuric acid reacts with active metals to liberate hydrogen. heat Zn + H2SO4 (dil.) ZnSO4 + H2(g) 14. Conc. sulphuric acid reacts with metals on boiling to form sulphur dioxide gas. heat Cu + 2H2SO4 (conc.) CuSO4 + 2H2O + SO2 15. Conc. sulphuric acid dehydrates blue copper sulphate crystals to white anhydrous CuSO4. CuSO4.5H2O + 5H2SO4 CuSO4 + 5H2SO4 . H2O 16. Conc. sulphuric acid dehydrates (i) glucose to carbon (ii) oxalic acid to a mixture of carbon monoxide and carbon dioxide (iii) formic acid to carbon monoxide (iv) ethyl alcohol to ethylene and (v) cellulose (wood) to carbon. C6H12O6 + 6H2SO4 (conc.) 6C + 6H2SO4. H2O (COOH)2 + H2SO4 (conc.) CO + CO2 + H2SO4.H2O HCOOH + H2SO4 (conc.) CO + H2SO4.H2O C2H5OH + H2SO4 (conc.) C2H4 + H2SO4.H2O (C6H10O5)n + 5(H2SO4)n 6Cn+ (5H2SO4.H2O)n 14. Uses of sulphuric acid : 1. It is used in the manufacture of artificial fertilisers such as ammonium sulphate, ammonium dihydrogen phosphate. 2. It is used in the manufacture of explosives like T.N.T. and T.N.G. 3. It is used in the manufacture of plastics, artificial rubber and cellophane. 4. It is used in the manufacture of nitric acid, hydrochloric acid and phosphoric acid. 7 Key Points 5. It is used as an electrolyte in lead-acid batteries. 6. It is used in refining petroleum. 7. It is used in the manufacture of dyes and drugs. 8. It is used as an important laboratory reagent. 15. Tests for sulphuric acid : 1. When conc. sulphuric acid is boiled with copper turnings, it gives off sulphur dioxide gas, which turns acidified potassium dichromate paper green. 2. Dilute sulphuric acid forms white precipitate with (i) barium chloride solution (ii) lead acetate solution. The precipitates so formed are insoluble in all acids. 8 Key Points

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