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) \(\mathrm{ZnSO}_{4} .7 \mathrm{H}_{2} \mathrm{O}\) (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

A is CuSO₄·5H₂O.

Step 1: Analyze the Properties of the Substance

We begin by analyzing the given properties of the substance A. It is described as a colored crystalline solid that is easily soluble in water. The options provided are: (a) ZnSO₄·7H₂O, (b) FeSO₄·7H₂O, (c) CuSO₄·5H₂O, and (d) Hg₂Cl₂.

Step 2: Reaction with KCN

A gives a precipitate with aqueous KCN that dissolves in an excess amount of the reagent. This behavior is indicative of the presence of metal cations that usually form complexes with CN⁻ ions, such as Copper (II) ions forming a soluble complex with excess KCN.

Step 3: Reaction with KI

The reaction with KI produces a white-grey precipitate along with the liberation of iodine. This suggests an oxidation-reduction reaction, where the anion of the compound is participating in the reaction to liberate iodine, indicating the possible presence of Copper ions.

Step 4: Heating to Lose Water of Crystallization

Upon gently heating, A loses 80% of its water of crystallization. Among the options, CuSO₄·5H₂O is known to lose its water of crystallization on heating, transitioning from blue (hydrated) to white or gray (anhydrous).

Step 5: Conclusion - Identifying the Compound

Considering all the reactions and properties, the compound that matches all the criteria, including being colored, forming precipitates as described, and losing water of crystallization on heating, is (c) CuSO₄·5H₂O.

Key Concepts

Crystalline Solids

Crystalline solids are a type of solid material where the atoms or molecules are organized in a highly ordered structure. This repeating pattern extends in all three spatial dimensions. A classic example of a crystalline solid is table salt, sodium chloride, where the arrangement of sodium and chloride ions forms a regular grid-like pattern.
Crystalline materials are characterized by their well-defined edges and angles and are often known for their pretty external forms. They can also be easily identifiable by their geometric shapes.

• Properties: Crystalline solids possess a definite melting point, implying that they change from solid to liquid state at a specific temperature.
• Physical evidence: They usually appear transparent and reflect light beautifully, creating their unique luster.
• Real-world significance: Many naturally occurring substances like quartz and diamond are crystalline solids, playing important roles in jewelry, electronics, and various scientific applications.

Water of Crystallization

Water of crystallization refers to the water molecules that are part of the crystal structure of some compounds. This water is not just trapped physically but is integral to the crystal lattice that makes up the solid's structure.
For example, in the compound copper(II) sulfate pentahydrate (\( \mathrm{CuSO}_{4} \cdot 5 \mathrm{H}_{2} \mathrm{O} \)), the water molecules are essential for the crystal's formation and stability.

• Importance: Water of crystallization affects the color, weight, and melting point of the crystalline solid. Without it, many compounds may turn into a completely different substance visually and chemically.
• Deprivation of water: Heating can remove this water, a process known as dehydration, transforming the compound into its anhydrous state.
• Effects of dehydration: In the case of copper(II) sulfate, it transitions from a vibrant blue color to a pale grayish white when it's deprived of its water of crystallization.

Chemical Reactions with KCN

Potassium cyanide (KCN) is a highly reactive chemical compound, often known for forming complexes with metal cations. In a typical reaction with KCN, a metal ion first precipitates as an insoluble compound.
However, on the addition of excess KCN, the metal can form a soluble complex due to the binding properties of cyanide ions, illustrating interesting coordination chemistry.

• Complex formation: This involves the metal ions binding with KCN to create a coordination complex, significantly changing the solubility of the metal in solution.
• Example: Such reactions are seen with copper ions, where initially insoluble precipitates dissolve as complexes are formed.
• Reactivity concerns: It's crucial to handle KCN with care due to its high toxicity and potential to release dangerous hydrogen cyanide gas.

Oxidation-Reduction Reactions

Oxidation-reduction reactions, frequently abbreviated as redox reactions, are chemical processes that involve the transfer of electrons between two substances. One substance undergoes oxidation (loses electrons), and another undergoes reduction (gains electrons).
For instance, in the given exercise, when compound A reacts with potassium iodide (KI), it results in the liberation of iodine through an oxidation-reduction mechanism.

• Mechanics of redox: In this process, one substance increases its oxidation state, while another decreases its oxidation state.
• Real-world relevance: Redox reactions are foundational to many essential processes, including metabolism in living organisms, corrosion of metals, and energy production in batteries.
• In laboratory settings: They are often used to synthesize new compounds and to test the reactivity of different chemical species.