Contact processStep 1:Preparation of sulphur dioxide:

SO2is prepared by burningsulphurin the presence of excess air so that the product combines with oxygen which is helpful for the next stage.

S(s) + O2(g) → SO2(g)

Step 2:Preparation of sulphur trioxide:

Sulphur trioxide is formed when sulphur dioxide reacts with oxygen in a ratio of 1:1 at a temperature of 400°C – 450°C and a pressure of 1-2 atm in the presence of N2O5as a catalyst. This reaction is reversible in nature.

2SO2(g) + O2(g) ⇌ 2SO3(g)

Step 3:Preparation of concentrated sulphuric acid:

The sulphur trioxide formed is first made to react with concentrated sulphuric acid. Sulphur trioxide cannot be dissolved in water directly as it leads to the formation of fog. The product obtained after this reaction is known as oleum. The oleum obtained is then dissolved in water to obtain concentrated sulphuric acid.

H2SO4+ SO3(g) → H2S2O7(l)

H2S2O7(l) + H2O(l) → 2H2SO4

Contact processStep 1:Preparation of sulphur dioxide:

SO2is prepared by burningsulphurin the presence of excess air so that the product combines with oxygen which is helpful for the next stage.

S(s) + O2(g) → SO2(g)

Step 2:Preparation of sulphur trioxide:

Sulphur trioxide is formed when sulphur dioxide reacts with oxygen in a ratio of 1:1 at a temperature of 400°C – 450°C and a pressure of 1-2 atm in the presence of N2O5as a catalyst. This reaction is reversible in nature.

2SO2(g) + O2(g) ⇌ 2SO3(g)

Step 3:Preparation of concentrated sulphuric acid:

The sulphur trioxide formed is first made to react with concentrated sulphuric acid. Sulphur trioxide cannot be dissolved in water directly as it leads to the formation of fog. The product obtained after this reaction is known as oleum. The oleum obtained is then dissolved in water to obtain concentrated sulphuric acid.

H2SO4+ SO3(g) → H2S2O7(l)

H2S2O7(l) + H2O(l) → 2H2SO4

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