Question

Describe Ostwald’s process for the manufacture of nitric acid. How will you detect NO$_3^-$ radical in this acid? Give equations for reactions involved.

Hint:

Remember the sequence: $NH_3 \to NO \to NO_2 \to HNO_3$ in Ostwald’s process.

Answer 1

Step 1: Ostwald’s process.
Ostwald’s process is used for large-scale manufacture of nitric acid from ammonia. Step 2: Reactions involved.
\[ 4NH_3 + 5O_2 \;\xrightarrow{Pt/Rh,\, 500^\circ C,\, 9\, atm}\; 4NO + 6H_2O \] \[ 2NO + O_2 \;\longrightarrow\; 2NO_2 \] \[ 4NO_2 + 2H_2O + O_2 \;\longrightarrow\; 4HNO_3 \] Step 3: Detection of nitrate ion (NO$_3^-$).
Brown ring test is used. The solution containing nitrate is treated with freshly prepared FeSO$_4$ solution and concentrated H$_2$SO$_4$ is added along the sides of the test tube. A brown ring appears at the junction, indicating the presence of NO$_3^-$. Reaction: \[ NO_3^- + 3Fe^{2+} + 4H^+ \;\longrightarrow\; NO + 3Fe^{3+} + 2H_2O \] \[ Fe^{2+} + NO \;\longrightarrow\; [Fe(NO)]^{2+} \quad \text{(brown ring complex)} \] Conclusion:
Ostwald’s process produces nitric acid from ammonia using catalytic oxidation. The presence of nitrate ions can be confirmed by the brown ring test.