Question

Give reasons for the following:
(i) In the contact process, sulphur trioxide formed is dissolved in 98% sulphuric acid and not in water.
(ii) When solution of sulphur dioxide is exposed to air, it gets converted to sulphuric acid.
(iii) When diluting concentrated sulphuric acid, the acid should be added to water and not water to the acid.
(iv) When concentrated sulphuric acid is exposed to air, its volume increases and it becomes slightly dilute.
(v) Sulphuric acid can form two kinds of salts with sodium chloride.
(vi) When barium chloride is added to dilute sulphuric acid, a white precipitate is formed.
(vii) When carbon is heated with concentrated sulphuric acid, carbon dioxide is formed.
(viii) Ammonia gas cannot be dried by passing through concentrated sulphuric acid.
(ix) When concentrated sulphuric acid is added to sugar/glucose, a black mass is left behind.
(x) Concentrated sulphuric acid should not be added to oxalic acid or formic acid in the open laboratory.
(xi) When concentrated sulphuric acid is added to blue crystalline copper sulphate, it turns powdery white.
(xii) Concentrated sulphuric acid must be stored in airtight bottles.
(xiii) Cotton clothes get burnt with concentrated sulphuric acid.
(xiv) H₂SO₄ cannot be prepared by heating sodium sulphate with conc. HCl or HNO₃.

Answer 1

(i) In the contact process, the sulphur trioxide so formed is dissolved in 98% sulphuric acid rather than in water because H₂SO₄ releases a large amount of heat when mixed with water. This leads to the formation of a dense fog of H₂SO₄ , which is not easy to condense.

(ii) When a solution of sulphur dioxide is exposed to air, it slowly forms sulphur trioxide, which reacts with water to give sulphuric acid, as shown below:

SO₂ + H₂O + O₂ → H₂SO₄

(iii) When concentrated sulphuric acid is diluted, the acid should be added to water because during dilution of sulphuric acid, a large amount of heat is released and the excess amount of water will absorb the evolved heat. On the other hand, if water is added to acid, then the evolved heat is not absorbed and this causes the contents of the vessel to spill out and cause burns.

(iv) When concentrated sulphuric acid is exposed to air, its volume increases and it becomes slightly dilute because of its hygroscopic nature, as concentrated sulphuric acid has a tendency to absorb moisture from the air.

(v) Sulphuric acid can form two kinds of salts with sodium chloride viz. sodium bisulphate (NaHSO₄) and sodium sulphate (Na₂SO₄) because it is a diprotic acid. The reactions for the formation of both the salts are given below:

$$\begin{gathered} \text{NaCl}+{\text{H}}_{2}S{O}_4\stackrel{{\text{<200}}^{0}C}{\to }{\text{NaHSO}}_4+\text{HCl} \\ 2\text{NaCl}+{\text{H}}_{2}S{O}_4\stackrel{{\text{>200}}^{0}C}{\to }{\text{Na}}_{\text{2}}{\text{SO}}_4+\text{2HCl} \end{gathered}$$

(vi) When barium chloride is added to dilute sulphuric acid, barium sulphate is formed. Barium sulphate is a white precipitate formed due to the displacement of chloride ion by sulphate ion as shown in the given reaction:

BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl

(vii) When carbon is heated with concentrated sulphuric acid, it is oxidised to carbon dioxide because concentrated sulphuric acid is a strong oxidising agent.

C + 2H₂SO₄ → 2H₂O + 2SO₂ + CO₂

(viii) Ammonia gas when passed through concentrated sulphuric acid reacts with it to form ammonium sulphate as shown in the given reaction:

2NH₃ + H₂SO₄ → (NH₄)₂SO₄

Hence, concentrated sulphuric acid cannot be used to dry ammonia gas.

(ix) When concentrated sulphuric acid is added to sugar/glucose, it acts as a dehydrating agent and removes water from sugar/glucose crystals. As a result, only carbon is left behind as black mass as given in the reaction below:

C₆H₁₂O₆ $\stackrel{\mathrm{conc}.{\mathrm{H}}_2{\mathrm{SO}}_4}{\to }$ 6C + 6H₂O

(x) Concentrated sulphuric acid should not be added to oxalic acid because it oxidises oxalic acid to carbon monoxide and carbon dioxide. Similarly, it oxidises formic acid to carbon monoxide as shown below:

C₂H₂O₄ $\stackrel{\mathrm{conc}.{\mathrm{H}}_2{\mathrm{SO}}_4}{\to }$ H₂O + CO + CO₂
HCOOH $\stackrel{\mathrm{conc}.{\mathrm{H}}_2{\mathrm{SO}}_4}{\to }$ H₂O + CO

(xi) When concentrated sulphuric acid is added to blue crystalline copper sulphate, it loses its water of crystallisation and become anhydrous copper sulphate which is white in colour.

CuSO₄.5H₂O $\stackrel{\mathrm{conc}.{\mathrm{H}}_2{\mathrm{SO}}_4}{\to }$ CuSO₄ + 5H₂O

(xii) Concentrated sulphuric acid must be stored in airtight bottles because of its hygroscopic nature. It absorbs moisture from air as a result its level rises and it gets diluted.

(xiii) Cotton clothes get burnt with concentrated sulphuric acid as it dehydrates the cellulose present in fibres of cotton.

(C₆H₁₀O₅)_n $\stackrel{\mathrm{Conc}.{\mathrm{H}}_2{\mathrm{SO}}_4}{\to }$ 6(C)_n+ 5(H₂O)_n

(xiv) H₂SO₄ cannot be prepared by heating sodium sulphate with conc. HCl or HNO₃ because HCl and HNO₃ are more volatile than H₂SO₄ and will not be able to displace H₂SO₄ from sodium sulphate.