# 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

$S+{O}_{2}\to S{O}_{2}\left(g\right)$

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

$4Fe{S}_{2}+11{O}_{2}\to 2F{e}_{2}{O}_{3}+8S{O}_{2}$

In either case, an excess of air is used so that the $S{O}_{2}$ produced is already mixed with oxygen for the next stage. This is reversible reaction and the formation of $S{O}_{3}$ is exothermic in the presence of catalyst ${V}_{2}{O}_{5}$ at 720K.

This cannot be done by simply adding water to the $S{O}_{3}.$ The reaction is so uncontrollable that it creates a fog of ${H}_{2}S{O}_{4}.$ Instead, the $S{O}_{3}$ is first dissolved in conc. ${H}_{2}S{O}_{4}.$

${H}_{2}S{O}_{4}+S{O}_{3}\to {H}_{2}{S}_{2}{O}_{7}$

The product is known as fuming sulphuric acid or oleum to which water is added to get ${H}_{2}S{O}_{4}$

${H}_{2}{S}_{2}{O}_{7}+{H}_{2}O\to 2{H}_{2}S{O}_{4}$

(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).

$B{i}^{5+}+2{e}^{-}\to B{i}^{3+}$

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

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