Problem 7 - A Complete Resource Book in Chemistry for JEE Main

Question

In Lassaigne's test for nitrogen, the blue colour is due to the formation of (a) ferriferrocyanide (b) sodium cyanide (c) potassium fericyanide (d) sodium ferrocyanide

Step-by-Step Solution

Answer

The blue color is due to ferriferrocyanide (option a).

Step 1: Understanding the Lassaigne's Test

Lassaigne's test is a qualitative analysis used to detect the presence of nitrogen, sulfur, and halogens in organic compounds. For nitrogen, the compound is fused with sodium to convert nitrogen into sodium cyanide (\( NaCN \)).

Step 2: Forming Ferrocyanide

In the presence of ferric ions (Fe³⁺), the cyanide ions from sodium cyanide react to form a complex known as ferriferrocyanide (\( Fe_4[Fe(CN)_6]_3 \)), which is responsible for the formation of blue color. This blue-colored compound is also known as Prussian blue.

Step 3: Choosing the Correct Option

The options given are: (a) ferriferrocyanide, (b) sodium cyanide, (c) potassium fericyanide, and (d) sodium ferrocyanide. Based on our understanding, the blue color in Lassaigne's test for nitrogen is due to ferriferrocyanide.

Key Concepts

Nitrogen Detection

Detecting nitrogen in organic compounds is a crucial part of Lassaigne's test. This test helps identify the presence of not only nitrogen but also sulfur and halogens in compounds. Here's how it works for nitrogen: The organic compound is first heated with a small piece of metallic sodium. This process converts any nitrogen in the compound into sodium cyanide ( NaCN ).
The transformation relies on the high reactivity of sodium, which assists in breaking the strong bonds that hold nitrogen within organic molecular structures.
Once converted into sodium cyanide, nitrogen is ready for the next stage of the test, which involves the formation of a complex that gives a distinct color that is easy to detect.

Ferriferrocyanide Formation

Ferriferrocyanide plays a crucial role in Lassaigne's test by providing the visual cue that confirms the presence of nitrogen. Once sodium cyanide is formed, it interacts with iron ions (Fe^{3+}) present in the solution.
Here’s what happens step-by-step:

• The cyanide ions (CN^−) released by sodium cyanide dissolve in water and react with ferric ions (Fe^{3+}).
• This reaction forms soluble ferriferrocyanide, with the chemical formula \( Fe_4[Fe(CN)_6]_3 \).
• It is this compound that causes the solution to appear blue.

This blue compound is better known as Prussian blue and is a classic indication of the presence of nitrogen when performing Lassaigne’s test. The formation of Prussian blue signifies the complex interplay between the cyanide ions and ferric ions in the test.

Qualitative Analysis in Chemistry

Qualitative analysis is a branch of chemistry focused on identifying the chemical composition of substances. Unlike quantitative analysis, which measures the amount of certain substances, qualitative methods aim to discover what substances are present.
Lassaigne's test is a great example of qualitative analysis. It focuses on the presence or absence of specific elements like nitrogen, sulfur, and halogens in organic compounds rather than how much of each is present.
Below are some key features of qualitative analysis in chemistry:

• Characteristic Reactions: Specific reactions, such as color changes, are used to identify elements or compounds.
• Simple and Direct: No complex equipment is necessary; most tests rely on straightforward chemical reactions.
• Wide Application: Valuable for quickly identifying the presence of certain key elements in both research and educational contexts.

Overall, qualitative analysis plays a vital role in chemistry by providing quick and identifiable outcomes for the presence of certain elements, making it an invaluable tool in laboratory settings.