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

Question

Which of the following is a complex salt? (a) \(\mathrm{KCl} \cdot \mathrm{MgCl}_{2} \cdot 6 \mathrm{H}_{2} \mathrm{O}\) (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 the complex salt.

Step 1: Understand the Question

The problem asks for the identification of a complex salt from the given options. Complex salts have a metal atom or ion bonded to a group of molecules or ions.

Step 2: Examine Option (a)

Option (a) is \( \mathrm{KCl} \cdot \mathrm{MgCl}_{2} \cdot 6 \mathrm{H}_{2} \mathrm{O} \). This is a simple combination of potassium, magnesium chlorides, and water molecules. It does not contain any complex ion.

Step 3: Examine Option (b)

Option (b) is \( \mathrm{FeSO}_{4} \cdot (\mathrm{NH}_{4})_{2} \mathrm{SO}_{4} \cdot 6 \mathrm{H}_{2} \mathrm{O} \). This is commonly known as ammonium iron(II) sulfate or Mohr's salt. However, it does not contain complex ions, just individual ions and water.

Step 4: Examine Option (c)

Option (c) is \( 4 \mathrm{KCN} \cdot \mathrm{Fe} (\mathrm{CN}) \). This compound forms a complex with iron and cyanide ions, making it a complex salt. Fe(CN) within a larger structure typically indicates a complex.

Step 5: Examine Option (d)

Option (d) is \( \mathrm{K}_{2} \mathrm{SO}_{4} \cdot \mathrm{Al}_{2} (\mathrm{SO}_{4})_{3} \cdot 24 \mathrm{H}_{2} \mathrm{O} \). This is a combination of sulfate salts with no indication of complex ions.

Step 6: Conclusion

Among the given options, (c) is the only one with the characteristics of a complex salt, due to the presence of the complex iron-cyanide structure.

Key Concepts

Coordination Chemistry

Coordination chemistry revolves around the study of complex compounds, where a central metal atom or ion is surrounded by groups of molecules or ions known as ligands. These ligands are typically ions or neutral molecules that possess a pair of electrons available for bonding. By forming coordinate covalent bonds with the central metal, they create complex ions.
This area of chemistry is essential for understanding how these structures form, their stability, and their specific geometries. Different complexes can have various coordination numbers, which indicate how many ligand donor atoms are bonded to the central atom or ion.

• The coordination sphere is the central atom or ion and its ligands.
• Complexes can be charged or neutral, forming complex ions in some cases.
• The nature of the ligands and the metal can influence the color, reactivity, and magnetic properties of the complex.

Coordination chemistry is crucial for applications like catalysis, material science, and bioinorganic chemistry, where these complexes play vital roles in biological systems.

Complex Ions

Complex ions form when a central metal ion binds with one or more ligands, creating a highly stable structure. This bonding involves the donation of electrons from the ligand to the metal, forming coordinate covalent bonds.
The formation of complex ions results in various charges on the entire ion, depending on the metal involved and the ligands' nature. These ions can have diverse geometrical shapes such as octahedral, square planar, or tetrahedral, which are dictated by factors like the number of bonds and electronic configuration.

• The charge on a complex ion is determined by the total charges on both the metal and the ligands.
• For example, in the compound 4 KCN · Fe(CN), the Fe(CN) portion forms the complex ion.
• Complex ions can be involved in further chemical reactions, acting as a single entity.

The study of complex ions assists in the understanding of important reactions, such as the catalysis in industrial processes or the binding of metal ions in biological systems.

Chemical Bonding

Chemical bonding is a fundamental concept that describes the forces holding atoms in compounds and molecules. In coordination compounds, these bonds include both ionic and covalent types, but one key feature is the coordinate covalent bond present in complex ions.
A coordinate covalent bond, or dative bond, forms when both electrons in the bond are supplied by one atom, usually a ligand. This results in a stable interaction between the central metal and its surrounding ligands.

• Metal-ligand bonding involves significant electron sharing, influenced by the electronic properties of the metal and ligands.
• Bond strength and stability derive from both the overlap of orbitals and electrostatic interactions.
• The specific nature of bonding affects the color, magnetism, and reactivity of coordination compounds.

Understanding chemical bonding in coordination compounds helps explain their key roles in biological and industrial processes, enabling innovations in areas like drug delivery and material development.