NEET Chemistry The p-Block Elements (XII) Questions Solved

BOARD

(a) Describe the conditions and the steps involved in the manufacture of sulphuric acid by contact process. Write the necessary reactions. (No diagram is required)

(b) Give reasons :

(i) Bond dissociation energy of F2 is less than that of Cl2.

(ii) Nitric oxide becomes brown when released in air.

(a) Contact process : Burning sulphur in an excess of air 

        S+O2SO2g

or, By heating sulphide ores like pyrites in an excess of air :

       4FeS2+11O22Fe2O3+8SO2

In either case, an excess of air is used so that the SO2 produced is already mixed with oxygen for the next stage. This is reversible reaction and the formation of SO3 is exothermic in the presence of catalyst V2O5 at 720K.

2SO2+O22SO3 H = -196KJ/mol

This cannot be done by simply adding water to the SO3. The reaction is so uncontrollable that it creates a fog of H2SO4. Instead, the SO3 is first dissolved in conc. H2SO4.

               H2SO4+SO3H2S2O7

The product is known as fuming sulphuric acid or oleum to which water is added to get H2SO4

      H2S2O7+H2O2H2SO4

(b) (i) Bond dissociation energy of F2 is less than that of Cl2 because of large electron-electron repulsion among the lone pair in F2 molecule.

(ii) Nitric oxide becomes brown when released in air because of the formation of NO2 gas.

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