Problem 111 - A Complete Resource Book in Chemistry for JEE Main

Question

A green crystalline compound shows the following tests: (i) Its aqueous solution gives a dense white precipitate with Barium chloride solution (ii) Its aqueous solution gives a rose-red precipitate with dimethyl glyoxime and \(\mathrm{NH}_{4} \mathrm{OH}\). The compound is (a) \(\mathrm{NiSO}_{4}\) (b) \(\mathrm{FeSO}_{4}\) (c) \(\mathrm{Cr}_{2}\left(\mathrm{SO}_{4}\right)_{3}\) (d) \(\mathrm{FeCl}_{3}\)

Step-by-Step Solution

Answer

The compound is \( \mathrm{NiSO}_4 \).

Step 1: Analyzing the reaction with Barium Chloride

The compound's aqueous solution gives a dense white precipitate with Barium chloride. This indicates the presence of \( \text{SO}_4^{2-} \) ions because Barium ions \((\text{Ba}^{2+})\) react with sulfate ions to form Barium sulfate \((\text{BaSO}_4)\), which is insoluble in water and forms a white precipitate.

Step 2: Understanding the reaction with Dimethyl Glyoxime

The compound's aqueous solution gives a rose-red precipitate with dimethyl glyoxime and \( \text{NH}_4\text{OH} \). This reaction is characteristic of nickel ions \((\text{Ni}^{2+})\), as they form a rose-red complex with dimethyl glyoxime, confirming the presence of nickel in the compound.

Step 3: Identifying the compound

From the reactions, we have determined that the compound contains \( \text{SO}_4^{2-} \) ions and \( \text{Ni}^{2+} \) ions. The only option that meets both criteria is \( \text{NiSO}_4 \), which contains nickel and sulfate ions.

Key Concepts

Qualitative Analysis

Qualitative analysis is a method used in chemistry to identify the constituents or components within a chemical compound or mixture. Unlike quantitative analysis, which determines the amount or concentration of a substance, qualitative analysis focuses on identifying what substances are present.
For instance, the green crystalline compound in the exercise undergoes a series of tests to detect specific ions. By applying reagents that yield observable outcomes, such as color changes or precipitate formation, chemists can effectively deduce the identity of unknown substances.
This method, which involves systematic testing, seeks to provide insight into the composition based on characteristic chemical reactions. The process can involve both traditional chemical tests, typical in inorganic chemistry, and more advanced instrumental techniques.

Chemical Reactions

Chemical reactions are processes where reactants convert into products via chemical changes. These reactions are foundational to both theoretical and practical chemistry, making it essential for students to understand how they operate.
In our exercise, the characterization reactions provide insight into the compound’s composition. To accurately interpret these reactions, understanding the nature of reactants and the resulting products is critical.
During a chemical reaction, particles collide with sufficient energy to break or form chemical bonds, leading to new substances. Monitoring these changes can reveal a lot about a compound's structure and elemental makeup, just as the formation of precipitates or color change indicates specific ions in the solution.

Precipitation Reactions

Precipitation reactions are crucial for identifying ions in solution as they involve the formation of a solid, known as a precipitate, from two aqueous solutions. This method is common in qualitative analysis for determining the presence of certain ions.

• When the green compound reacts with Barium chloride, a dense white precipitate forms. This signals the presence of sulfate ions \(\text{SO}_4^{2-}\) because Barium ions \(\text{Ba}^{2+}\) react with sulfate to yield insoluble Barium sulfate \(\text{BaSO}_4\), which precipitates out of the solution.
• Precipitation is not only a verification method but also plays a vital role in techniques aimed at removing unwanted ions from mixtures.

These reactions offer visible evidence of specific ions and form the basis for many analytical procedures in chemistry.

Nickel Complexes

Nickel complexes are compounds containing nickel ions bonded to ligands, which are molecules or ions that donate electron pairs. These complexes often exhibit distinct colors that can be used to identify the presence of nickel in a solution.

• For example, in the provided exercise, nickel ions \(\text{Ni}^{2+}\) react with dimethyl glyoxime in the presence of ammonia to form a rose-red precipitate. This reaction is a hallmark of nickel presence and is frequently applied in qualitative analysis.
• The reaction with dimethyl glyoxime results in a complex where the nickel ion is chelated, or securely bound by two bidentate ligands, contributing to the rose-red color.

Nickel complexes demonstrate the powerful role of coordination chemistry in identifying and studying metal ions in inorganic chemistry.