In the Contact process

(i) State how you would convert (a) sulphur (b) iron pyrites to sulphur dioxide in the first step of the Contact process.

(ii) State the conditions i.e. catalyst, promoter, temperature and pressure in the catalytic oxidation of sulphur dioxide to sulphur trioxide in the Contact tower. Give a balanced equation for the same.

(iii) State why the above catalytic oxidation reaction supplies energy.

(iv) Give a reason why – vanadium pentoxide is preferred to platinum during the catalytic oxidation of sulphur dioxide.

(v) Give a reason why the catalyst mass is heated electrically – only initially.

(vi) State why sulphur trioxide vapours are absorbed in concentrated sulphuric acid and not in water to obtain sulphuric acid.

(i) Sulphur dioxide is produced by burning sulphur [S] or iron pyrites [FeS₂]

S + O₂ ⟶ SO₂

4FeS₂ + 11O₂ ⟶ 2Fe₂O₃ + 8SO₂

(ii) Catalyst : Vanadium pentoxide [V₂O₅] or platinum [Pt.]

Temperature : 450 - 500°C , Pressure : 1 to 2 atmosphere

$2\text{SO}$2 + \text{O}2 \xrightleftharpoons[450-500\text{\degree C} /1 - 2 \text{ atmos}]{\text{V}2\text{O}5} 2\text{SO}_3 + \Delta

Conversion ratio: 98% of sulphur dioxide converted to sulphur trioxide.

(iii) The catalytic oxidation of SO₂ to SO₃ is an exothermic reaction, hence, supplies energy.

(iv) Vanadium pentoxide is preferred to platinized asbestos as a catalyst since it is comparatively cheaper and less easily poisoned or susceptible to impurities.

(v) The catalyst mass is only initially heated electrically, since the catalytic oxidation of sulphur dioxide is an exothermic reaction and the heat produced maintains the temperature at 450 – 500°C.

(vi) Even though sulphur trioxide is an acid anhydride of sulphuric acid it is not directly absorbed in water to give sulphuric acid.

The reaction is highly exothermic resulting in production of a dense fog of sulphuric acid particles which do not condense easily.

Hence sulphur trioxide vapours are dissolved in conc. sulphuric acid to give oleum which on dilution with the requisite amount of soft water in the dilution tank gives sulphuric acid of the desired concentration [about 98%].