Problem 52 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

An aqueous blue coloured solution of a transition metal sulphate reacts with \(\mathrm{H}_{2} \mathrm{~S}\) in acidic medium to give a black precipitate \(\mathbf{A}\), which is insoluble in warm aqueous solution of \(\mathrm{KOH}\). The blue solution on treatment with KI in weakly acidic medium, turns yellow and produces a white precipitate \(\mathbf{B}\). Identify the transition metal ion. Write the chemical reactions involved in the formation of \(\mathbf{A}\) and \(\mathbf{B}\).

Step-by-Step Solution

Answer

The transition metal ion is \(\mathrm{Cu^{2+}}\). Chemical reactions:\[\mathrm{Cu^{2+}_{(aq)}} + \mathrm{H_2S_{(aq)}} \rightarrow \mathrm{CuS_{(s)}} + 2\mathrm{H^+_{(aq)}}\] \[2\mathrm{Cu^{2+}_{(aq)}} + 4\mathrm{I^-_{(aq)}} \rightarrow 2\mathrm{CuI}_{(s)} + \mathrm{I_2}_{(aq)}\]

Step 1: Analyze the Reaction with Hydrogen Sulfide

The given aqueous blue colored solution of a transition metal sulfate (let's denote the metal as M) reacts with hydrogen sulfide, \(\mathrm{H}_2\mathrm{S}\), in an acidic medium to form a black precipitate \(\mathbf{A}\). Black precipitates of metal sulfides are typically indicative of transition metals like copper or lead. We're told this precipitate \(\mathbf{A}\) is insoluble in \(\mathrm{KOH}\), which suggests it is likely \(\mathrm{CuS}\). The reaction can be represented as:\[\mathrm{Cu^{2+}_{(aq)}} + \mathrm{H_2S_{(aq)}} \rightarrow \mathrm{CuS_{(s)}} + 2\mathrm{H^+_{(aq)}}\]

Step 2: Analyze the Reaction with Potassium Iodide

The blue solution on treatment with \(\mathrm{KI}\) in a weakly acidic medium turns yellow and gives a white precipitate \(\mathbf{B}\). This suggests that the transition metal ion present forms a complex or compound that leads to a visible color change and precipitate formation. The iodine liberated from KI can oxidize the \(\mathrm{Cu^{2+}}\) ions to form \(\mathrm{CuI}\) as a white precipitate. The reaction is:\[2\mathrm{Cu^{2+}_{(aq)}} + 4\mathrm{I^-_{(aq)}} \rightarrow 2\mathrm{CuI}_{(s)} + \mathrm{I_2}_{(aq)}\]The formation of \(\mathrm{CuI}\) and the iodine turning the solution yellow confirms the involvement of \(\mathrm{Cu^{2+}}\) ions.

Step 3: Identification of the Transition Metal Ion

Based on the reactions above, the transition metal ion that is both involved in the formation of \(\mathbf{A}\) as \(\mathrm{CuS}\) and \(\mathbf{B}\) as \(\mathrm{CuI}\) is the copper ion, \(\mathrm{Cu^{2+}}\). The reactions with hydrogen sulfide forming black copper sulfide, and with potassium iodide forming white copper(I) iodide confirm this identification.

Key Concepts

Copper Sulfide Formation

Copper sulfide formation involves a reaction between copper ions and sulfide ions in an acidic medium. When copper ions (\(\mathrm{Cu^{2+}} \)) from a copper sulfate solution meet hydrogen sulfide (\(\mathrm{H_2S} \)), a black precipitate forms. This black precipitate is copper(II) sulfide, \(\mathrm{CuS} \), a characteristic of copper in such reactions. This transformation is a classic example of a transition metal reaction where visible color changes and precipitates occur:

• Copper ions (\(\mathrm{Cu^{2+}} \)) react with \(\mathrm{H_2S} \) to form \(\mathrm{CuS} \).
• The reaction involves release of hydrogen ions (\(\mathrm{H^+} \)), maintaining the acidic environment required for the reaction.

The equation representing this reaction is: \[\mathrm{Cu^{2+}_{(aq)}} + \mathrm{H_2S_{(aq)}} \rightarrow \mathrm{CuS_{(s)}} + 2\mathrm{H^+_{(aq)}}.\] This reaction underscores the chemical property of copper forming stable sulfide precipitates, helping to identify transition metal ions.

Precipitation Reactions

Precipitation reactions occur when solutes in a solution react to form an insoluble solid, or precipitate. In the case of copper sulfide, the black precipitate helps identify it as an insoluble compound. These reactions are visually effective, providing clear evidence of a chemical change. Key points to remember about precipitation reactions include:

• Formation of an insoluble solid from two soluble salts.
• Color changes signify the formation of new compounds.
• Can be easily reversed by dissolving the precipitate in a suitable solvent, if possible.

For copper sulfide formation: copper ions and sulfide ions bond to create solid copper(II) sulfide, which can be identified by its distinct black color. Precipitation reactions are vital methods of chemical analysis used to identify components in a mixture.

Oxidation-Reduction Reactions

Oxidation-reduction reactions, or redox reactions, are processes where one element is oxidized and another is reduced. In our problem, redox reactions occur when copper(II) ions interact with iodide ions from potassium iodide, resulting in the formation of copper(I) iodide. Here's the breakdown of what happens:

• Copper(II) ions (\(\mathrm{Cu^{2+}} \)) are reduced to copper(I) ions (\(\mathrm{Cu^+} \)).
• Iodide ions (\(\mathrm{I^-} \)) are oxidized to iodine (\(\mathrm{I_2} \)).

This redox reaction can be seen as:\[2\mathrm{Cu^{2+}_{(aq)}} + 4\mathrm{I^-_{(aq)}} \rightarrow 2\mathrm{CuI}_{(s)} + \mathrm{I_2}_{(aq)}.\] The resulting yellow color and formation of a white copper(I) iodide precipitate (\(\mathrm{CuI} \)) confirm the redox activity and presence of copper ions. Redox reactions are fundamental in chemistry, illustrating the exchange of electrons and connections between specific chemical properties.

Chemical Reaction Identification

Chemical reaction identification is crucial in determining the compounds or ions in a mixture. In our exercise, visual indicators, such as color change and precipitate formation, help recognize chemical reactions and the involved transition metal ion. Here's how one can identify reactions in such scenarios:

• Observe color changes, a direct sign of a chemical transformation.
• Note the formation of any precipitates, which signals the creation of a new insoluble product.
• Consider the solubility of precipitates in various solvents to confirm identities.

In this context, the black \(\mathrm{CuS} \) and white \(\mathrm{CuI} \) precipitates act as solid identifiers of copper ion reactions. By understanding these visible indicators, students can effectively deduce the nature of chemical transformations and identify the metal ions responsible.