Problem 138 - A Complete Resource Book in Chemistry for JEE Main

Question

A is a coloured crystalline solid which is easily soluble in water. Addition of aqueous KCN gives a precipitate which dissolves in excess of reagent whereas addition of KI aqueous solution gives a white grey precipitate with the liberation of iodine. On gently heating, A loses \(80 \%\) of its water of crystallization. Identify A here? (a) \(\mathrm{ZnSO}_{4} \cdot 7 \mathrm{H}_{2} \mathrm{O}\) (b) \(\mathrm{FeSO}_{4} \cdot 7 \mathrm{H}_{2} \mathrm{O}\) (c) \(\mathrm{CuSO}_{4} \cdot 5 \mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{Hg}_{2} \mathrm{Cl}_{2}\)

Step-by-Step Solution

Answer

Compound A is CuSO₄·5H₂O.

Step 1: Understanding the Problem

We need to identify the compound A based on its reactions and properties. We know it is a coloured crystalline solid, easily soluble in water, and reacts with additional reagents, specifically aqueous KCN and KI, resulting in specific changes.

Step 2: Analyzing Solubility and Reactivity

Examine the options. Options (a) and (b), ZnSO₄·7H₂O and FeSO₄·7H₂O, are typically not colored crystalline solids consistently with the description, while CuSO₄·5H₂O, option (c), is a known colored compound. Option (d), Hg₂Cl₂, is a colored solid but doesn't react with aqueous solutions as described.

Step 3: Reaction with KCN

With KCN, option (c) CuSO₄ can form a precipitate that dissolves in excess reagent due to the complex ion formation, Cu(CN)₄²⁻, which is consistent with the given reaction observation.

Step 4: Reaction with KI

Considering the KI reaction, addition of KI to CuSO₄ can result in a greyish precipitate due to the formation of CuI and the iodine liberation reaction, forming a consistent procedural result.

Step 5: Checking Water of Crystallization

CuSO₄·5H₂O, upon gentle heating, loses some of its water of crystallization, matching the description of losing 80% of its water. This makes option (c) the most plausible choice.

Key Concepts

Transition Metal Complexes

Transition metals are known for their ability to form complexes, thanks to their partially filled d orbitals. This unique property allows them to form coordinate bonds with other molecules, typically known as ligands. Key Characteristics of Transition Metal Complexes:

• Coordination Chemistry: Transition metals can coordinate with various ligands, forming complex structures. For instance, copper(II) sulfate forms complexes like \[\text{Cu(CN)}_4^{2-}\] with cyanide ions.
• Variable Oxidation States: Transition metals can exist in multiple oxidation states, making them versatile in chemical reactions.
• Colored Compounds: Many complexes are intensely colored, such as the blue color of CuSO₄ · 5H₂O. This is due to the d-d electronic transitions.

Understanding these complexes helps us explain various reactions and properties of transition metals, like the solubility and color changes observed when reacting with different compounds.

Hydration of Compounds

Hydration refers to the incorporation of water molecules into a compound, forming a hydrated structure. This happens in various crystalline solids, where water molecules are integrated into the crystal lattice. Important Notes on the Hydration:

• Water of Crystallization: It represents water molecules that become an integral part of the crystalline structure. An example is CuSO₄·5H₂O, which contains five water molecules per formula unit.
• Role of Hydration in Physical Properties: Hydration can significantly affect the appearance, stability, and solubility of the compounds.
• Dehydration Process: When heated, hydrated compounds can lose their water of crystallization, often resulting in a color change. For example, blue CuSO₄·5H₂O turns white when it loses its water and becomes anhydrous CuSO₄.

Hydration is crucial for determining the conditions under which a certain compound will lose its water of crystallization, impacting the compound's overall reactivity and chemical properties.

Reactivity with Cyanide

Cyanide, often regarded for its potent reactivity, interacts with metal ions to form complex ions. The reaction of transition metal ions with cyanide is important for various analytical and chemical processes. Cyanide and Metal Reactions:

• Complex Formation: Transition metals such as copper with cyanide form complex anions, such as \[\text{Cu(CN)}_4^{2-}\], which can dissolve back in the solution, explaining why excess cyanide can redissolve a precipitate.
• Stability of Complexes: These complexes are often stable due to the strong binding affinity of cyanide ions to metal centers, impacting the solubility and reactivity of the compounds.
• Precipitation and Resolution: Initially, complexes might precipitate out of solution, but in presence of excess reagent, they can redissolve, as seen with the reaction of KCN with CuSO₄.

Understanding these reactions is essential for interpreting the behavior of metal salts in various industrial and laboratory conditions.

Precipitation Reactions

Precipitation reactions are fundamental in chemistry, occurring when two soluble solutions react to form an insoluble solid, known as a precipitate. Transition metal salts often participate in these reactions due to their varying solubility characteristics. Key Points on Precipitation Reactions:

• Formation of Precipitates: These reactions involve the formation of a solid from a solution, such as the reaction between CuSO₄ and KI forming a CuI precipitate.
• Solubility Product: The concept of the solubility product constant (\(K_{sp}\)) helps predict the formation of a precipitate based on the ion concentrations in a solution.
• Color Change and Indicators: Precipitation often goes with a color change, like the greyish color due to the formation of CuI.
• Applications: Precipitation is useful in purification processes, qualitative analysis, and removing ions from solutions.

Being familiar with the principles of precipitation reactions is crucial for predicting and controlling outcomes in chemical syntheses and analyses.