Question

The following changes represented by letters A, B, C, D can bring about the conversion of iron pyrites (FeS₂) to sulphuric acid (H₂SO₄) in the contact process.
${\mathrm{FeS}}_2\stackrel{A}{\to }{\mathrm{SO}}_2\stackrel{B}{\to }{\mathrm{SO}}_3\stackrel{C}{\to }\mathrm{oleum}\stackrel{D}{\to }{\mathrm{H}}_2{\mathrm{SO}}_4$
(i) Write the balanced equation to show change 'A'.
(ii) Name the above reaction bringing about change 'A'.
(iii) Write the formula of oleum.
(iv) Name the substance C.
(v) What is 'D'? Why is it required?
(vi) State giving reasons, whether 'D' can be used in place of 'C'.

Answer 1

(i) The balanced chemical reaction for conversion of FeS₂ to SO₂ is:

4FeS₂ + 11O₂ → 2Fe₂O₃ + 8SO₂

(ii) The above reaction is an example of redox reaction.
(iii) Formula of oleum is H₂S₂O₇.
(iv) The complete sequence of the reaction is as follows:

${\mathrm{FeS}}_2\stackrel{{\mathrm{O}}_2}{\to }{\mathrm{SO}}_2\stackrel{{\mathrm{O}}_2}{\to }{\mathrm{SO}}_3\stackrel{{\mathrm{H}}_2{\mathrm{SO}}_4}{\to }\mathrm{Oleum}\stackrel{{\mathrm{H}}_2\mathrm{O}}{\to }{\mathrm{H}}_2{\mathrm{SO}}_4$

Thus, C is sulphuric acid.

(v) D is soft water. Soft water is required to dilute oleum for producing the sulphuric acid of required concentration.
(vi) D cannot be used in place of C. Sulphur trioxide (B) dissolves in water (D) forms sulphuric acid with the liberation of large amount of heat. Large amount of heat evolved causes the formation of dense fog of sulphuric acid which does not condense easily. Therefore, water (D) cannot be used in place of sulphuric acid (C).