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

BOARD

(a) With the help of chemical equations explain the principle of contact process in brief for the manufacture of sulphuric acid by contact process.

(b) Bismuth is a strong oxidizing agent in the pentavalent state. Explain.

(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=-196 KJ/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) The stability of +5 oxidation state decreases and that of +3 state increases due to inert pair effect down the group therefore Bi (V) accepts two electrons and gets reduced to Bi(III).

       Bi5++2e-Bi3+

So, Bi (V) is more stronger oxidising agent.

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