Problem 83 - A Complete Resource Book in Chemistry for JEE Main

Question

The brown ring test for \(\mathrm{NO}_{2}^{-}\)and \(\mathrm{NO}_{3}^{-}\)is due to the formation of complex ion with formula (a) \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}\) (b) \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5} \mathrm{NO}\right]^{2+}\) (c) \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)(\mathrm{NO})_{5}\right]^{2+}\) (d) \(\left[\mathrm{Fe}(\mathrm{NO})(\mathrm{CN})_{5}\right]^{2-}\)

Step-by-Step Solution

Answer

Option (b) \([\mathrm{Fe}(\mathrm{H}_2\mathrm{O})_5\mathrm{NO}]^{2+}\) is correct.

Step 1: Understanding Brown Ring Test

The brown ring test is a qualitative analysis technique used for the detection of nitrate (\(\mathrm{NO}_3^-\)) ions. This test involves the reaction between nitrate ions and ferrous sulfate (\(\mathrm{FeSO}_4\)) in the presence of concentrated sulfuric acid (\(\mathrm{H}_2\mathrm{SO}_4\)). A brown ring forms at the junction of the two layers, indicating the presence of nitrate ions.

Step 2: Formation of the Complex Ion

During the brown ring test, the formation of a brown ring is due to a particular complex. By adding ferrous sulfate, the nitrate ion reacts to form a nitrosyl complex. Specifically, \([\mathrm{Fe}(\mathrm{H}_2\mathrm{O})_5\mathrm{NO}]^{2+}\) is formed, where nitrogen monoxide (\(\mathrm{NO}\)) is ligated to iron.

Step 3: Analyzing the Options

Examine the four given options for the formula of the complex formed. Option (b) \([\mathrm{Fe}(\mathrm{H}_2\mathrm{O})_5\mathrm{NO}]^{2+}\) matches the description of the brown ring complex, where five water molecules and one NO molecule surround the iron center.

Step 4: Conclusion

The formula \([\mathrm{Fe}(\mathrm{H}_2\mathrm{O})_5\mathrm{NO}]^{2+}\) in option (b) corresponds to the known complex responsible for the brown ring, confirming it as the correct choice.

Key Concepts

Qualitative Analysis

Qualitative analysis is a branch of analytical chemistry focused on identifying the presence or absence of particular chemical species in a sample. Rather than determining how much of a substance is present, qualitative analysis pinpoints specific components. This approach is especially useful in identifying ions or compounds in various sample matrices, from soil to water.

Common qualitative tests often involve changes in color, formation of precipitates, or production of gas. The brown ring test, for instance, uses color change to indicate the presence of nitrate ions.

• Color changes suggest the formation of new complexes.
• Precipitates indicate chemical reactions resulting in insoluble products.
• Gas evolution can show reactions forming gaseous products.

Such analyses are essential in environmental studies, medical diagnostics, and forensic investigations, offering scientists a low-cost and quick way to ascertain what substances are present.

Nitrate Detection

Detecting nitrates is crucial in various fields such as agriculture, water quality monitoring, and medical testing. Nitrates can be harmful in high concentrations, affecting both human health and the environment.

The brown ring test is a classic method for determining the presence of nitrate ions. In this test, when ferrous sulfate is added to a solution containing nitrate ions, under acidic conditions, a distinctive brown ring is formed. This forms because of a complex ion interaction at a liquid interface.

The process involves the reduction of nitrate ions to nitrogen monoxide (NO), which then forms a complex with iron in the ferrous sulfate.

• Concentrated sulfuric acid is carefully added to ensure the proper reaction environment.
• The test is sensitive, meaning a small amount of nitrate can show positive results.
• Proper control of test conditions is necessary to avoid false positives caused by other factors.

This test is qualitative, not quantitative; it tells us about presence, but not concentration.

Complex Ion Formation

The formation of complex ions is central to the brown ring test. A complex ion is a charged species consisting of a central metal ion surrounded by molecule or ion ligands. The complex ion in the brown ring test is formed from the reaction between ferrous sulfate and nitrates.

The key complex, \([ ext{Fe}( ext{H}_2 ext{O})_5 ext{NO}]^{2+}\), forms when the nitrate is reduced to \( ext{NO}\) (nitrogen monoxide) and latches onto the ferrous iron.

• This complex is distinctively brown, appearing as a ring at the interface of the two liquid layers due to its formation process.
• The unique arrangement of oxygen, nitrogen, and water molecules around iron highlights the diverse structures that complex ions can adopt.
• These formations often exhibit striking colors due to their electronic configurations, making them useful in visual detection methods.

Complex ions such as these are pivotal in analytical chemistry, enabling the detection and identification of various chemical species.

Ferrous Sulfate Reaction

Ferrous sulfate plays a crucial role in the brown ring test due to its ability to form complexes with nitrate ions. Ferrous sulfate ( ext{FeSO}_4) contains Fe(II) ions, which are essential for reducing the nitrate ion during the test.

Upon adding concentrated  ext{H}_2 ext{SO}_4, a layer of acid forms underneath the test solution, creating an environment for the reaction to take place. As nitrate ions reduce to nitrogen monoxide, they react with ferrous ions to form the ferrous nitrosyl complex, \([ ext{Fe}( ext{H}_2 ext{O})_5 ext{NO}]^{2+}\).

• This specific reaction is characteristic of ferrous iron's chemistry, showcasing its versatility in forming complexes.
• Complex ions such as these can be manipulated in various environmental conditions to display unique properties.
• The reaction provides an educational insight into transition metal and ligand interactions.

Understanding the role of ferrous sulfate in this process is critical for appreciating the full picture of how the brown ring test operates.