Problem 1 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is a complex salt? (a) \(\mathrm{KCl} . \mathrm{MgCl}_{2} \cdot 6 \mathrm{H}_{2} \mathrm{O}\) (b) \(\mathrm{FeSO}_{4} \cdot\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4} \cdot 6 \mathrm{H}_{2} \mathrm{O}\) (c) \(4 \mathrm{KCN} \cdot \mathrm{Fe}(\mathrm{CN})\) (d) \(\mathrm{K}_{2} \mathrm{SO}_{4} \cdot \mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3} \cdot 24 \mathrm{H}_{2} \mathrm{O}\)

Step-by-Step Solution

Answer

Option (c) is a complex salt.

Step 1: Understand Complex Salts

A complex salt is a compound in which one of the cations or anions forms a coordination complex. Look for a compound that includes complex ions like [Fe(CN)6] in the structure.

Step 2: Analyze Option (a)

KCl . MgCl₂ . 6H₂O is a simple salt and does not contain any complex ions. Hence, it is not a complex salt.

Step 3: Analyze Option (b)

FeSO₄.(NH₄)₂SO₄.6H₂O, known as Mohr's salt, is a double salt. Upon dissolution, it dissociates into its constituent ions rather than maintaining a complex structure, so it is not a complex salt.

Step 4: Analyze Option (c)

4 KCN . Fe(CN)₆ contains the complex ion [Fe(CN)₆]⁴⁻, making it a complex salt. The Fe(CN)₆ forms a coordination complex with the cyanide ions.

Step 5: Analyze Option (d)

K₂SO₄ . Al₂(SO₄)₃ . 24H₂O, also known as alum, is a double salt and upon dissolution, it dissociates into its individual ions rather than maintaining a complex structure, hence it is not a complex salt.

Key Concepts

Coordination Complexes

Coordination complexes are fascinating structures in chemistry where a central atom or ion, often a metal, is surrounded by molecules or ions, known as ligands. These ligands form coordinate covalent bonds with the central atom. This structure creates a distinctive 3D orientation that affects the chemical properties of the complex.
For example, in the complex \([Fe(CN)_6]^{3-}\), iron is the central metal atom, and the cyanide ions (CN⁻) act as ligands, donating their lone pairs of electrons to form bonds with iron.

• Central atom: typically a metal, essential for the complex's structure
• Ligands: can be ions or neutral molecules
• Coordinate bonds: formed by the donation of an electron pair from the ligand to the metal

Coordination complexes are key in understanding the behavior of complex salts and have applications in various fields such as catalysis, material science, and more.

Double Salts

A double salt is a compound formed from two different salts that crystallize together in a definite proportion. These salts dissociate completely into their constituent ions when dissolved in water. This property makes them structurally different from complex salts, which maintain their coordinated structure even in solution.
For instance, Mohr's salt \(FeSO_4.(NH_4)_2SO_4.6H_2O\) is a classic example. When dissolved, it fully dissociates into \(Fe^{2+}\), \(NH_4^+\), and \(SO_4^{2-}\) ions.

• Consists of two or more salts
• Crystallizes in a set ratio
• Dissociates completely in solution

Double salts are important in chemistry and industry for their reactivity and solubility properties.

Complex Ions

Complex ions are ions that consist of a metal atom or ion bonded to one or more ligands. They can carry an overall positive, negative, or neutral charge, depending on the charge of the metal and the ligands involved. The arrangement and properties of these bonds contribute to the overall characteristics of the ion.
For example, in \([Fe(CN)_6]^{4-}\), the metal ion is iron, and it is surrounded by six cyanide ions that contribute to the ion's structural configuration and charge.

• Formed by a central metal atom or ion
• Connected to ligands which can be neutral or charged species
• Formal charge depends on the combination of metal and ligands

Understanding complex ions is essential, as they influence the reactivity and solubility of the complex salts.

Dissociation of Salts

Dissociation is the process by which salts break down into their constituent ions when dissolved in a solvent, usually water. This process differs significantly between double salts and complex salts.
Double salts dissociate fully into their individual ions, losing any complex structural integrity they had when solid. For example, alum \(K_2SO_4.Al_2(SO_4)_3.24H_2O\) completely dissociates. On the other hand, complex salts, such as those that contain coordination complexes like \([Fe(CN)_6]^{4-}\), maintain their coordination structure even when dissolved, often affecting the solution's properties differently.

• A process affecting both complex and double salts
• Impacts the stability of salts in solution
• Varies based on the nature of the salt

Dissociation is crucial for understanding the practical applications and reactions of different salts in solutions.