Brown ring in nitrates test is formed when:

Answer the question on basis of passage given below:

Nitrogen differs from the rest of the members of its group due to its small size, high electronegativity, high ionisation enthalpy and non-availability of d-orbitals. Nitrogen has unique ability to form $pπ-pπ$ multiple bonds with itself and with other elements having small size and high electronegativity (eg, C, O) Heavier elements of this group do not form $pπ-pπ$ bonds as their atomic orbitals are so large and diffused that they cannot have effective overlapping.

Answer & explanation

Correct answer: option 1

The correct answer is Option (1) → $Fe^{2+}$ oxidises.

The brown ring test is a classic qualitative analysis method used to detect the presence of nitrate ions ($ \text{NO}_3^- $) in a solution. Here’s a detailed explanation of the process:

Procedure

Preparation: To perform the test, a small amount of the solution suspected of containing nitrates is taken in a test tube.

Addition of Sulfuric Acid: Concentrated sulfuric acid ($ \text{H}_2\text{SO}_4 $) is carefully added to the test tube. The sulfuric acid is important as it provides the acidic medium necessary for the reaction.

Addition of Ferrous Sulfate: Next, a few drops of ferrous sulfate ($ \text{FeSO}_4 $) solution are added.

Chemical Reactions

In this acidic medium, $ \text{Fe}^{2+} $ ions can react with the nitrate ions. The reaction involves the oxidation of $ \text{Fe}^{2+} $ ions to $ \text{Fe}^{3+} $ and the reduction of nitrate ions.

The simplified overall reaction can be represented as follows:

$\text{3Fe}^{2+} + \text{NO}_3^- + \text{4H}^+ \rightarrow \text{3Fe}^{3+} + \text{NO} + \text{2H}_2\text{O}$

Here, the $ \text{NO}_3^- $ ion is reduced to nitric oxide ($ \text{NO} $), while the $ \text{Fe}^{2+} $ ion is oxidized to $ \text{Fe}^{3+} $.

Formation of the Brown Ring

The $ \text{Fe}^{3+} $ ions then form a complex with nitric oxide in the presence of water:

$\text{Fe}^{3+} + \text{NO} + \text{5H}_2\text{O} \rightarrow \text{[Fe(H}_2\text{O)}_5\text{NO]}^{2+}$

This complex is responsible for the characteristic brown ring observed at the interface between the sulfuric acid and the aqueous layer in the test tube.

Summary

Oxidation: The ferrous ions $ \text{Fe}^{2+} $ are oxidized to ferric ions $ \text{Fe}^{3+} $.

Reduction: The nitrate ions $ \text{NO}_3^- $ are reduced to nitric oxide $ \text{NO} $.

Complex Formation: The resulting $ \text{Fe}^{3+} $ ions and $ \text{NO} $ form the brown-colored complex $ \text{[Fe(H}_2\text{O)}_5\text{NO]}^{2+} $.

This test is highly specific for nitrates, making it a reliable method for their detection in various solution

Answer the question on basis of passage given below:

Brown ring in nitrates test is formed when:

Answer & explanation

Correct answer: option 1

The correct answer is Option (1) → $Fe^{2+}$ oxidises.

The brown ring test is a classic qualitative analysis method used to detect the presence of nitrate ions ($ \text{NO}_3^- $) in a solution. Here’s a detailed explanation of the process:

Procedure

Preparation: To perform the test, a small amount of the solution suspected of containing nitrates is taken in a test tube.

Addition of Ferrous Sulfate: Next, a few drops of ferrous sulfate ($ \text{FeSO}_4 $) solution are added.

Chemical Reactions

The simplified overall reaction can be represented as follows:

$\text{3Fe}^{2+} + \text{NO}_3^- + \text{4H}^+ \rightarrow \text{3Fe}^{3+} + \text{NO} + \text{2H}_2\text{O}$

Here, the $ \text{NO}_3^- $ ion is reduced to nitric oxide ($ \text{NO} $), while the $ \text{Fe}^{2+} $ ion is oxidized to $ \text{Fe}^{3+} $.

Formation of the Brown Ring

The $ \text{Fe}^{3+} $ ions then form a complex with nitric oxide in the presence of water:

$\text{Fe}^{3+} + \text{NO} + \text{5H}_2\text{O} \rightarrow \text{[Fe(H}_2\text{O)}_5\text{NO]}^{2+}$

This complex is responsible for the characteristic brown ring observed at the interface between the sulfuric acid and the aqueous layer in the test tube.

Summary

Oxidation: The ferrous ions $ \text{Fe}^{2+} $ are oxidized to ferric ions $ \text{Fe}^{3+} $.

Reduction: The nitrate ions $ \text{NO}_3^- $ are reduced to nitric oxide $ \text{NO} $.

Complex Formation: The resulting $ \text{Fe}^{3+} $ ions and $ \text{NO} $ form the brown-colored complex $ \text{[Fe(H}_2\text{O)}_5\text{NO]}^{2+} $.

This test is highly specific for nitrates, making it a reliable method for their detection in various solution