Question

Treatment of a gas \(X\) with a freshly prepared ferrous sulphate solution gives a compound \(Y\) as a brown ring. The compounds \(X\) and \(Y\) are :

• A. \(NO\) and \([Fe(NO)]SO_4\)
• B. \(NO_2\) and \([Fe(NO_2)]SO_4\)
• C. \(N_2O\) and \([Fe(N_2O)]SO_4\)
• D. \(N_2O_4\) and \([Fe(N_2O_4)]SO_4\)

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
This is the classic Brown Ring Test used for the detection of nitrate (\(NO_{3}^{-}\)) or nitrite (\(NO_{2}^{-}\)) ions.
Step 2: Detailed Explanation:
In the test, the nitrate/nitrite ion is reduced to Nitric Oxide (NO) gas by \(Fe^{2+}\) in an acidic medium.
\[ NO_{3}^{-} + 3Fe^{2+} + 4H^{+} \rightarrow NO + 3Fe^{3+} + 2H_{2}O \]
This NO gas then reacts with the excess aqueous ferrous sulphate (\([Fe(H_{2}O)_{6}]^{2+}\)) present in the solution to form a coordination complex:
\[ [Fe(H_{2}O)_{6}]^{2+} + NO \rightarrow [Fe(H_{2}O)_{5}(NO)]^{2+} + H_{2}O \]
The brown colored complex is \([Fe(H_{2}O)_{5}(NO)]^{2+}\). Its corresponding sulphate salt is \([Fe(NO)]SO_{4}\) (in simplified form).
In this complex, iron is in the +1 oxidation state and NO is present as the nitrosonium ion (\(NO^{+}\)).
Step 3: Final Answer:
Gas 'X' is \(NO\) and the brown complex 'Y' is \([Fe(NO)]SO_{4}\).