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ICSE Class X Sample / Model Paper 2025 : Physics : Paper

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Sulphuric acid is rightly called the 'King of Chemicals because there is no other manufactured compound which is used by such a large number of key industries. It has been known for a long time. In the later Middle Ages, it was obtained as an oily viscous liquid by heating crystals of green vitriol, and was, therefore, known by the name of oil of vitriol. In the free state, sulphuric acid is found in certain mineral springs. In the combined state, as Barytes [BaSO4] Gypsum [CaSO4. 2H2O] and Kieserite [MgSO4.H2O] etc. PREPARATION OF SULPHURIC ACID (1) By oxidation of an aqueous solution of sulphur dioxide with oxygen, chlorine or bromine. SO2 + 2H2O + O2 2H2SO4 SO2 + 2H2O + Cl2 H2SO4 +2HCl SO2 + 2H2O + Br2 H2SO4 + 2HBr The colour of Cl2 and Br2 fades. (2) Sulphur with conc. nitric acid S + 6HNO3 H2SO4 + 6NO2 + 2H2O MANUFACTURE OF SULPHURIC ACID (CONTACT PROCESS) The various steps in the Contact process are as follows: (1) Production of sulphur dioxide: Sulphur dioxide is produced by roasting metallic sulphides in air 4FeS2 (Iron pyrites) + 1102 2Fe2O3 + 8SO2. OR by burning Louisiana sulphur (99.5% pure) in purified air. S+O SO2 (2) Purification of gases: The mixture of sulphur dioxide and air contains various impurities which must be removed, otherwise the catalyst loses its efficiency. The mixture is passed through a purifier called electric precipitator, consisting of a chamber with wires at high electric potential. The electric charge attracts solid particles which are removed. The mixture is then led to a water scrubber where it is completely freed from dust particles. It is then dried by a spray of concentrated sulphuric acid in another chamber. It then goes into the arsenic purifier where every trace of arsenic oxide is removed. (3) Catalytic oxidation of sulphur dioxide. The clean dried gaseous mixture of sulphur dioxide and air is passed through tower loosely packed with vanadium pentoxide or platinum on perforated shelves. The catalyst is placed in vertical iron pipes inside a cylindrical iron tower called the converter. Here, the preheated mixture of sulphur dioxide and air (oxygen) on passing through catalyst pipes forms sulphur trioxide. 2SO2 + O2 2SO3 + Since the reaction is highly exothermic, the catalyst is heated only in the beginning to about 450 C.This temperature is maintained by the heat evolved during the reaction. (4) Absorption of sulphur trioxide in sulphuric acid. The gas is cooled in a heat exchanger and is then absorbed in absorption tower in concentrated sulphuric acid (98%). SO3 + H2SO4 H2S2O7 (oleum or pyrosulphuric acid) (5) Dilution of oleum to obtain sulphuric acid. It is diluted in dilution tank by adding calculated amount of water to obtain sulphuric acid of the desired strength. H2S2O7+ H2O 2H2SO4 Favourable conditions for the conversion of SO2 process to SO3 (contact process) (i) Exothermic reactions are favoured at low temperature: The temperature should be as low as possible. The yield has been found to be maximum at about 410-450 C. (ii) High pressure: High pressure favours the reaction because the product formed has less volume than reactant. Pressure of 1 - 2 atmosphere is used. (iii) Excess of oxygen: This increases the production of sulphur trioxide. (iv) A suitable catalyst: Platinum is more efficient as a catalyst than vanadium pentoxide but platinum is more expensive and also it easily In gets poisoned by impurities like arsenic (III) oxide. So Vanadium pentoxide is used as a catalyst. Summary of all reactions in Contact Process S + O2 SO2 (Formation of Sulphur dioxide) 2SO2 + O2 2SO3 (Conversion of SO2 to SO3) SO3 + H2SO4 H2S2O7 (Conversion of SO3 to Oleum) H2S2O7 + H2O H2SO4 (Dilution of Oleum to H2SO4) PROPERTIES OF SULPHURIC ACID Physical Properties: 1) Colourless, Odourless, slightly sour in taste. 2) It is hygroscopic liquid so it absorbs moisture. Hence, it is always stoppered. 3) Pure Acid is non-conductor but Dil. Acid is good conductor. 4) At 338 C it forms constant boiling mixture having 98.5% of acid. 5) Conc. H2SO4 is corrosive so it chars the skin black. Chemical Properties of Dilute H2SO4: Pure (100% H2SO4) hydrogen sulphate is not an acid as it does not ionise.. It is a dibasic acid so it ionizes in 2 steps and form 2 salts. Dilution of H2SO4 : Water is never poured on acid to dilute it as large amount of heat is evolved which changes poured water to steam. The steam so formed causes spurting of acid which can cause burn injuries, so dilution is done by pouring acid on a given amount of water in a controlled manner by continuous stirring, else acid being heavier will settle down. The evolved heat is dissipated in the water itself. (a) Properties of dilute sulphuric acid Acidic property: (1) Dilute sulphuric acid reacts with metals, which are above hydrogen in the activity series to form metallic sulphate and hydrogen. (2) It neutralises bases (metal oxides and metal hydroxides) to form salts and water. (3) It liberates carbon dioxide from metallic carbonates and bicarbonates. (4) It evolves hydrogen sulphide from metal sulphides. (5) It evolves sulphur dioxide from sulphites and hydrogen sulphites (bisulphites). (b) Properties of concentrated sulphuric acid (1) Non-volatile nature Concentrated sulphuric acid has a high boiling point (338 C) and so, it is considered to be a non-volatile acid. It is, therefore, used for preparing volatile acids like hydrochloric acid, nitric acid and acetic acid . (2) As an oxidising agent The oxidising property of concentrated sulphuric acid is due to the fact that on thermal decomposition, it yields nascent oxygen [O]. H2SO4 H2O + SO2 + [O] Nascent oxygen oxidises non-metals, metals and inorganic compounds. C + 2H2SO4 CO2 + 2H2O + 2SO2 . Cu + 2H2SO4 CuSO4 + 2H2O + 2SO2 (3) As a dehydrating agent H2SO4 has a great affinity for water. It readily removes elements of water from other compounds i.e., it acts as a dehydrating agent. (a) Conc. sulphuric acid removes water of crystallisation from salts. (b) All carbohydrates such as glucose, sugar, and cellulose (paper, cotton, wood, etc.) react immediately to give a black spongy mass of carbon which rises up. (c) Organic acids and organic compounds are dehydrated by conc. H2SO4 C12H22O11 (Cane Sugar) 12 C (Sugar Charcoal) + 11H2O CuSO4 (Blue) .5H2O CuSO4 (White) + 5H2O (4) Preparation of insoluble sulphates Sulphuric acid precipitates the insoluble sulphates of lead, barium and calcium from the solutions of their salts. BaCl2 + H2SO4 -----BaSO4+ 2HCI (White prep is insoluble in any acid.) Uses of Sulphuric Acid In the preparation of halogens, CO, CO and SO . Extraction of metals: Leaching of metallic compounds produces sulphates which give the metal in pure form on electrolysis. Pickling of metals: Removes metallic impurities from the surface of metals before galvanising. Industrial uses (i) In the manufacture of fertilisers such as ammonium sulphate [(NH4)2SO4 ] and superphosphate of lime [Ca(H2PO4)2 + CaSO4]. (ii) In the manufacture of explosives such as trinitrotoluene and picric acid. TESTS FOR SULPHURIC ACID AND SULPHATES (1) When barium chloride solution is added to psulphuric acid or any soluble metal sulphate, white precipitate of barium sulphate is formed. BaCl2 + H2SO4 -----BaSO4+ 2HCI The white precipitate of barium sulphate is insoluble in any mineral acid like hydrochloric acid or nitric acid. (2) Conc. sulphuric acid on heating with copper evolves sulphur dioxide. Cu + 2H2SO4 CuSO4 + 2H2O + SO (Conc.) Sulphur dioxide turns acidified potassium dichromate solution green (Reducing property of SO2). (3) Conc. sulphuric acid on heating with NaCl evolves pungent fumes of HCl gas. NaCl + H2SO4 (Conc.) NaHSO4 + HCl (White Fumes) HCI gas gives dense white fumes of ammonium chloride, with a glass rod dipped in ammonium hydroxide. HCI + NH4OH NH4CI+ H2O (dense white fumes) Difference Between Dilute H2SO4 & Concentrated H2SO4 Dilute H2SO4 i) It ionises and behave as strong acid. Concentrated H2SO4 i) It is a weak acid. ii) Strong electrolyte. ii) Weak electrolyte. iii) Don't act as an oxidising agent. iii) Act as an oxidising agent. iv) Don't act as drying or dehydrating Agent. iv) Act as drying or dehydrating Agent. Test for Sulphuric Acid: Cu + H2SO4 (Conc.) CuSO4 + H2O + SO2 NaCl + H2SO4 NaHSO4 + HCl 3) Preparation of Insoluble Sulphites - H2SO4 precipitates the insoluble sulphate of lead, barium and calcium. BaCl2 + H2SO4 BaSO4 (white) Properties of Conc. Sulphuric Acid: Non-Volatile Nature: NaCl + H2SO4 NaHSO4 + HCl KNO3 + H2SO4 NaHSO4 + HCl

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