Question

Explain the principle of manufacturing sulphuric acid by contact process.

Hint:

Remember: Catalyst = V$_2$O$_5$, Temperature ≈ 450°C, Pressure ≈ 2 atm. Always absorb SO$_3$ in H$_2$SO$_4$ (not directly in water).

Answer 1

Step 1: Principle.
The manufacture of sulphuric acid by the Contact Process is based on the catalytic oxidation of sulphur dioxide (SO$_2$) to sulphur trioxide (SO$_3$) using vanadium(V) oxide (V$_2$O$_5$) as a catalyst.
Step 2: Chemical reactions involved.
\[ \text{(a) Formation of SO}_2: \quad \text{S} + \text{O}_2 \rightarrow \text{SO}_2 \] \[ \text{(b) Oxidation of SO}_2 \text{ to SO}_3: \quad 2\text{SO}_2 + \text{O}_2 \xrightleftharpoons[\text{V}_2\text{O}_5]{450^\circ\text{C}, 2 \text{ atm}} 2\text{SO}_3 + \text{Heat} \] Step 3: Conditions for maximum yield.
- Catalyst: V$_2$O$_5$ (Vanadium(V) oxide)
- Temperature: around 450°C
- Pressure: about 1–2 atm
- The reaction is exothermic and reversible, hence low temperature and moderate pressure favor the forward reaction.
Step 4: Formation of sulphuric acid.
\[ \text{SO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{H}_2\text{S}_2\text{O}_7 \quad (\text{oleum}) \] \[ \text{H}_2\text{S}_2\text{O}_7 + \text{H}_2\text{O} \rightarrow 2\text{H}_2\text{SO}_4 \] Step 5: Summary of process.
\[ \text{S or FeS}_2 \rightarrow \text{SO}_2 \xrightarrow{\text{O}_2, \text{V}_2\text{O}_5} \text{SO}_3 \xrightarrow{\text{H}_2\text{SO}_4} \text{H}_2\text{S}_2\text{O}_7 \xrightarrow{\text{H}_2\text{O}} \text{H}_2\text{SO}_4 \] Step 6: Conclusion.
Thus, in the contact process, sulphuric acid is produced economically and efficiently through catalytic oxidation and absorption steps.