Problem 80 - A Complete Resource Book in Chemistry for JEE Main

Question

The blue colour obtained in the Lassaigne's test is due to formation of the compound (a) \(\mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3}\) (b) \(\mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\) (c) \(\mathrm{Na}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\) (d) \(\mathrm{Fe}_{3}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{4}\)

Step-by-Step Solution

Answer

The blue colour is due to the formation of Prussian Blue: (a) \(\mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3}\).

Step 1: Understanding Lassaigne's Test

Lassaigne's test is a qualitative analysis test used to detect the presence of nitrogen, sulfur, and halogens in organic compounds. The organic sample is fused with sodium metal. Sodium cyanide is formed if nitrogen is present. This sodium cyanide reacts further to form compounds that are detected by colorimetric changes.

Step 2: Identifying the Blue Colour

In Lassaigne's test, the formation of a blue color indicates the presence of nitrogen in the original compound. This blue color is known as Prussian Blue, a well-known complex in chemistry.

Step 3: Understanding Prussian Blue Chemistry

Prussian Blue is an iron-cyanide complex. The chemical composition of Prussian Blue is \textbf{Iron(III) ferricyanide}, which is represented by the chemical formula \(Fe_4[Fe(CN)_6]_3\). This is the compound that results in the characteristic blue color in the test.

Step 4: Matching with Options

Refer to the options given in the problem statement: (a) \(Fe_4[Fe(CN)_6]_3\), (b) \(Fe_4[Fe(CN)_6]\), (c) \(Na_4[Fe(CN)_6]\), (d) \(Fe_3[Fe(CN)_6]_4\). The Prussian Blue matches with option (a), \(Fe_4[Fe(CN)_6]_3\).

Key Concepts

Prussian Blue: The Tell-Tale Sign of Nitrogen

Prussian Blue is a fascinating compound often encountered in chemistry labs, particularly in Lassaigne's test for detecting nitrogen. This vivid blue color alerts chemists to the presence of nitrogen in organic samples. The creation of Prussian Blue involves a reaction between iron salts and ferrocyanides, resulting in an intense blue pigment. What makes Prussian Blue so interesting is its historical usage, not just in labs, but also in art, where it has been used as a pigment. In the context of Lassaigne's test, however, it serves a more scientific purpose by forming when sodium cyanide, generated from the reaction of nitrogen and sodium, interacts with ferrous salts to produce the iron-cyanide complex.

Qualitative Analysis: Identifying Substances by Their Reactions

Qualitative analysis is an essential part of chemistry that involves understanding the composition of a substance based on its reactions and physical properties, rather than its quantity. Lassaigne's test is an example of qualitative analysis used in organic chemistry to identify elements like nitrogen, sulfur, and halogens. These elements react in very specific ways when subjected to certain conditions, such as the formation of characteristic colors or precipitates. In Lassaigne's test, the sample undergoes a sodium fusion, and the resultant compounds are examined for these distinct reactions. The blue color indicating nitrogen is just one part of this greater method of analysis, which can also disclose other elements based on different reaction outcomes.

Iron-Cyanide Complex: The Chemistry Behind the Blue

The term "iron-cyanide complex" may sound complex, but it actually refers to a well-organized structure that results from the combination of iron ions with cyanide ions. In the case of Prussian Blue, the iron-cyanide complex forms when ferrocyanide ions bind to iron(III) ions to create a tight, crystalline structure that has the capability to absorb specific wavelengths of light, hence exhibiting the distinct blue hue. This complex formation is critical in Lassaigne's test, as it is the presence of this configuration that leads to the color change. Unlike simple ionic compounds, these complexes have geometrical arrangements that influence the color we see, making them vital for colorimetric tests in chemistry.

Nitrogen Detection: A Clue in Organic Chemistry

Detecting nitrogen is crucial for chemists, especially when dealing with organic compounds. The presence of nitrogen can significantly affect the behavior and properties of substances. In Lassaigne's test, nitrogen undergoes conversion to sodium cyanide when the organic sample is treated with sodium. This is the preliminary step, where the cyanide ions act as intermediates that later complex with iron salts. This detection method is a tool that allows scientists to infer the molecular composition of organic materials. Identifying nitrogen is not just about noting its presence; it can unlock deeper insights into the nature and potential reactions of an organic compound.