Problem 61 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

A light bluish green crystalline compound responds to the following tests : (i) Its aqueous solution gives a brown precipitate or colour with alkaline \(\mathrm{K}_{2}\left[\mathrm{HgI}_{4}\right]\) solution. (ii) Its aqueous solution gives a blue colour with \(\mathrm{K}_{3}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\) solution. (iii) Its solution in hydrochloric acid gives a white precipitate with \(\mathrm{BaCl}_{2}\) solution. Identify the ions present and suggest the formula of the compound.

Step-by-Step Solution

Answer

The ions present are \(\mathrm{Cu}^{2+}\), \(\mathrm{Fe}^{2+}\), and \(\mathrm{SO}_{4}^{2-}\). The compound could be \(\mathrm{CuSO}_{4} \cdot \mathrm{FeSO}_{4}\).

Step 1: Analyzing the First Test

The first test indicates that the compound's solution gives a brown precipitate or color with alkaline \(\mathrm{K}_{2}[\mathrm{HgI}_{4}]\). This suggests the presence of \(\mathrm{Cu}^{2+}\) ions in the compound, as copper ions react with \(\mathrm{K}_{2}[\mathrm{HgI}_{4}]\) to produce a brown color.

Step 2: Analyzing the Second Test

The second test indicates the formation of a blue color with \(\mathrm{K}_{3}[\mathrm{Fe(CN)}_{6}]\). This change suggests the presence of \(\mathrm{Fe}^{2+}\) ions, because ferrocyanide reacts with iron ions to produce a blue complex known as Turnbull's blue.

Step 3: Analyzing the Third Test

The compound's solution, when mixed with \(\mathrm{BaCl}_{2}\), leads to a white precipitate. This observation points to the presence of the \(\mathrm{SO}_{4}^{2-}\) ions, as barium sulfate (\(\mathrm{BaSO}_{4}\)) is an insoluble white compound formed under these conditions.

Step 4: Combining the Findings

Combining the results from the tests, we gather that the compound contains \(\mathrm{Cu}^{2+}\), \(\mathrm{Fe}^{2+}\), and \(\mathrm{SO}_{4}^{2-}\) ions. Hence, all three ions should be parts of the compound's molecular structure.

Step 5: Suggesting the Formula

Given that each \(\mathrm{Cu}^{2+}\) and \(\mathrm{Fe}^{2+}\) ion would combine with one mole of \(\mathrm{SO}_{4}^{2-}\), the most plausible formula for this compound could be a mixed salt such as \(\mathrm{CuSO}_{4} \cdot \mathrm{FeSO}_{4}\). The color and properties align with the tests performed.

Key Concepts

Qualitative Analysis

Qualitative Analysis is a branch of chemistry focusing on determining the components of a chemical sample. It doesn't measure how much of a substance is present but rather identifies which substances are in a mixture. In this exercise, qualitative analysis is used to identify ions in a bluish green crystalline compound.

Tests are performed using solutions that react with suspected ions to produce distinctive colors or precipitates. For example:

• The test with alkaline \(\mathrm{K}_{2}[\mathrm{HgI}_{4}]\) produces a brown color, indicating the presence of \(\mathrm{Cu}^{2+}\) ions.
• The test with \(\mathrm{K}_{3}\left[\mathrm{Fe(CN)}_{6}\right]\) yields a blue color, pointing to \(\mathrm{Fe}^{2+}\) ions.
• The reaction with \(\mathrm{BaCl}_{2}\) resulting in a white precipitate indicates \(\mathrm{SO}_{4}^{2-}\) ions.

Each test provides specific clues about the ions in the sample.

Complex Ions

Complex ions are ions made from a central metal ion bonded to molecules or ions called ligands. These ligands have pairs of electrons available to form coordinate bonds with the metal ion.

In the given exercise, the interaction of the compound with \(\mathrm{K}_{3}\left[\mathrm{Fe(CN)}_{6}\right]\) leads to the formation of a blue complex known as Turnbull's blue.

This arises because:\(\text{Fe}^{2+}\) ions from the sample form a coordination compound with the cyanide ions \(\left(\mathrm{CN}^{-}\right)\). Turnbull's blue is a result of this complex formation:

• The complex ion formed is \(\left[\mathrm{Fe}^{3+}\left(\mathrm{Fe(CN)}_{6}\right]^{4-}\).
• This complex is attributed to its distinct blue color, a significant indicator in qualitative analysis.

Understanding complex ions help in predicting the compounds' behaviors and interactions in various reactions.

Precipitation Reactions

Precipitation reactions occur when two liquids react and form an insoluble solid, called a precipitate. These reactions are important in qualitative analysis to help identify ions in a solution.

In the context of the exercise:

• When the aqueous solution of the compound reacts with \(\mathrm{BaCl}_{2}\), a white precipitate, \(\mathrm{BaSO}_{4}\), forms which is insoluble in water.
• The appearance of this precipitate is characteristic of \(\mathrm{SO}_{4}^{2-}\) ions in the solution.

Such reactions provide direct evidence of ion presence. They are pivotal in determining the presence of specific anions and cations based on the color and solubility of the precipitates formed.