Sulphuric acid (H2SO4) : Manufacture by contact process, properties, uses, test and structure.

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Sulphuric acid is widely used chemical in industries. It is used in the manufacture of fertilizers, drugs, dyes, polymers, etc. thousands of tons of sulphuric acid is manufactured every year worldwide. So it is called king of chemicals.

Manufacture of sulphuric acid(H2SO4) by contact process

  • In early days, sulphuric acid used to be maniufactured by lead chamber process.
  • Contact process is modern method. The acid obtained is pure( free from impurities) and is quite concentrated (96-99%). The contact process, it’s name is mainly from the fact that the conversion of sulphur dioxide into sulphur trioxide is carried out in contact with surface of catalyst.

Principle :

 1. Production of sulphur dioxide : Sulphur dioxide gas can be prepared either by burning sulphur or roasting of iron pyrites.

   

 2. Catalytic oxidation of sulphur dioxide : Sulphur dioxide is oxidized to sulphur trioxide in presence of catalyst vanadium pentoxide at about 4500C temperature and 2 atm pressure.

   

 3. Conversion of SO3 into H2SO4 : sulphur trioxide obtained is absorbed in 98% H2SO4 to produce oleum or fuming sulphuric acid. The oleum is diluted with calculated amount of water to get desired concentration of H2SO4.

   

Conditions for optimum yield of H2SO4 :

Formation of SO2 to SO3 is one of the must important steps in the manufacture of sulphuric acid. The production of H2SO4 entirely depends on the amount of SO3 formed. As reaction is reversible, exothermic and proceeds with decrease in volume, Le- Chatelier’s principle can be applied for the maximum yield of sulphur trioxide.

  • Low temperature : This reaction is exothermic . So, low temperature is required for maximum yield. But too low temperature is too slow to attain equilibrium. So an optimum temperature of about 4500C is supplied.
  • High concentration of reactants : High concentration of SO2 and O2 is used for more production of SO3 .
  • High pressure : High pressure favours the reaction because the product formed has less volume than reactant . But the acid resistant tower which are able to withstand high pressure are difficult to build. Hence, an optimum pressure of about 2 atm is applied.
  • Use of catalyst : Rate of reaction is increased by the use of positive catalyst . So, vanadium pentoxide is used as catalyst for higher yield of H2SO4.

manufacture of sulphuric acid(H2SO4) by contact process

Details of the plant or process :

 [I] Sulphur or pyrite burner : SO2 gas is obtained by burning sulphur or iron pyrite with air in sulphur or pyrite burner.

   

 [II] Purification unit : The impure SO2 gas obtained is purified by the purification unit.

  1. Electrical dust precipitator : Dust particles present in sulphur dioxide gas is precipitated in electrical dust precipitator by the influence of high potential difference applied between the metallic conductors fitted in the chamber.
  2. Steam chamber : The lighter dust particles are settled down by using steam in steam chamber.
  3. Cooler : The gases coming out from the steam chamber are passed through cooler. The gases get cooled down to about 1000C.
  4. Washing tower(scrubber) : The cooled SO2 gas is then passed into a tower called scrubber which is packed with quartz and water is sprayed from the top. The water soluble impurities are washed away.
  5. Drying tower : The moisture present in sulphur dioxide is absorbed by the spray of conc.H2SO4 in drying tower.
  6. Arsenic purifier : arsenous oxide ( As2O3) present in gas is absorbed by ferric hydroxide( because it may causes catalytic poisoning).

 [III] Testing box : The purity of sulphur dioxide is checked in testing box by passing through a darkened box.

 [IV] Preheater : The purified mixture of SO2 and O2 is heated upto 4500C.

 [V] Contact chamber : The SO2 gas is catalytically oxidized to sulphur trioxide (SO3) in the presence of V2O5 as catalyst at 4500C temperature and 2 atm pressure.

 [VI] Absorption tower : This tower is packed with quartz (or acid proof stone) in which 98% H2SO4 is sprayed from the top of the tower. Concentrated H2SO4 absorbs sulphur trioxide to form oleum( or pyrosulphuric acid). Oleum is then treated with calculated amount of water to form sulphuric acid of desired concentration.

Physical Properties of H2SO4:

  • It is colourless, hygroscopic, syrupy liquid.
  • It is highly soluble in water (due to formation of intermolecular hydrogen bond). It dissolves in water with the liberation of large quantity of heat(19Kcal/mol) which may causes the explosion or spurt the acid out of the container.

      H2SO4 + nH2O → H2SO4.nH2O + Heat

Therefore, sulphuric acid is diluted by adding the acid slowly to water with constant stirring and not by adding water to the acid.

  • It’s melting point is 100C and boiling point is 3380C.
  • Pure H2SO4 is covalent compound and bad conductor of electricity. However aqueous H2SO4 conducts electricity.
  • Litmus test : Sulphuric acid is covalent compound. In pure state it does not change the colour of the blue litmus paper. However, aqueous acid gets ionized and produces hydrogen(or hydronium) ion into solution. Due to which it can change the colour of blue litmus paper to red.

Chemical properties of H2SO4 :

 1. Decomposition of H2SO4 :

On heating H2SO4 decomposes into SO2, H2O and O2.

        decomposition of H2SO4

 2. Acidic nature of H2SO4:

H2SO4 is a strong dibasic (diprotic) acid and ionizes in two steps.

       

It gives two series of salts like bisulphate and sulphate when reacted with base.Eg.

    reaction of sulphuric acid with bases

 3. H2SO4 as an oxidizing agent :

 i. Action with metals : More electropositive metals ( i.e. metals lying above hydrogen in electrochemical series) like Zn, Fe, Mg, Al, etc. react with dil H2SO4 to produce hydrogen gas. Here H2SO4 reduces to H2 and metals oxidize to metal sulphates.

   reaction of dilute H2SO4 with metals

Conc. H2SO4 oxidizes Zn, Cu, Ag, Hg, etc. to respective metal sulphates and H2SO4 gets reduced to SO2.

  reaction of H2SO4 with Cu

 

 ii. Action with non metals :

  • Carbon is oxidized to carbondioxide.

   

  • Sulphur is oxidized to sulphurdioxide.

   

  • Phosphorus is oxidized to phosphoric acid.

   reaction of sulphuric acid with phosphorus

 iii. Action with some other reducing agents :

  • H2SO4 oxidizes H2S to S and itself reduces to SO2.

   

  • HBr and HI are oxidized to Br2 and I2 respectively.

   

{Note : HCl can not be oxidized to Cl2 because of strong bond.}

 4. Sulphuric acid as dehydrating agent :

H2SO4 is a good dehydrating agent. It absorbs water molecules from sugar, cellulose, copper sulphate crystals, etc.

  • With sugar (charring action) : When conc. H2SO4 is treated with sugar, wood, paper, etc. it absorbs water and a black mass of carbon is formed. This process is called charring.

   sulphuric acid as dehydrating agent

  • With copper sulphate crystals : It removes water of crystallization from hydrated salts.

 

  • With oxalic acid crystals and formic acid:

 

Structure : Lewis structure of H2SO4

Lewis structure of H2SO4

Uses of sulphuric acid :

  • It is used for the manufacture of fertilizers.
  • It is used in manufacture of HCl, HNO3, H3PO4, etc.
  • It is used as an oxidizing agent.
  • It is used as a drying and dehydrating agent.

Test of H2SO4 :

Sulphuric acid gives H+ ions and SO4– – ions in aqueous solution. The presence of H+ ion can be detected by litmus paper. The presence of SO4– – can be detected by BaCl2 solution which gives white precipitate.

test reaction of sulphuric acid H2SO4

Sodium thiosulphate(hypo) :

Molecular formula : Na2S2O3. 5H2O

It is commonly called hypo.

Uses of sodium thiosulphate(hypo) :
  • It is used as a fixer in photography. { i.e. for fixing silver bromide in photographic plate}
  • It is used in (iodometric) titration for the estimation of iodine.
  • It is used as an antichlor agent to remove excess of chlorine from bleached textiles/fibers.

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