Problem 66 - A Complete Resource Book in Chemistry for JEE Main

Question

The aqueous solution of the following salts will be coloured in the case of (a) \(\mathrm{Zn}\left(\mathrm{NO}_{3}\right)_{2}\) (b) \(\mathrm{LiNO}_{3}\) (c) \(\mathrm{Co}\left(\mathrm{NO}_{3}\right)_{2}\) (d) potash alum

Step-by-Step Solution

Answer

The solution containing \( \mathrm{Co}(\mathrm{NO}_3)_2 \) will be colored.

Step 1: Identify the metal ions involved

For each salt given, identify the metal ion it contains. - (a) \( \mathrm{Zn} \left(\mathrm{NO}_{3}\right)_{2} \) contains \( \text{Zn}^{2+} \) ions.- (b) \( \text{LiNO}_3 \) contains \( \text{Li}^{+} \) ions.- (c) \( \mathrm{Co} \left( \mathrm{NO}_{3} \right)_{2} \) contains \( \text{Co}^{2+} \) ions.- (d) Potash alum, \( K_2\text{SO}_4\cdot\text{Al}_2\left(\text{SO}_4\right)_3\cdot24\text{H}_2\text{O} \), contains \( \text{Al}^{3+} \) ions.

Step 2: Understand the color properties of the ions

Transition metal ions typically form colored solutions due to d-d electron transitions, whereas alkali and alkaline earth metals are usually colorless in solutions.- The \( \mathrm{Zn}^{2+} \) ion does not have unpaired d-electrons and typically forms colorless solutions.- The \( \mathrm{Li}^{+} \) ion is an alkali metal ion and usually forms colorless solutions.- The \( \mathrm{Co}^{2+} \) ion is a transition metal ion and is known for forming colored solutions (pink).- The \( \mathrm{Al}^{3+} \) ion also typically forms colorless solutions.

Step 3: Conclude which salts will have colored solutions

Based on the properties of the ions:- \( \mathrm{Zn}(\mathrm{NO}_3)_2 \) will form a colorless solution.- \( \mathrm{LiNO}_3 \) will form a colorless solution.- \( \mathrm{Co}(\mathrm{NO}_3)_2 \) will form a colored solution (pink) due to \( \mathrm{Co}^{2+} \). - Potash alum will form a colorless solution.

Key Concepts

Aqueous Solutions

An aqueous solution is a solution in which water is the solvent. In such solutions, substances dissolve and dissociate into ions or molecules. When salts dissolve in water, they interact with the water molecules. These interactions often determine whether the solution will be colored or not.

In the context of transition metals, when they dissolve in water, they form complexes where water molecules coordinate around the metal ion. These complexes can exhibit vivid colors due to electronic transitions within the metal ion.

Important points about aqueous solutions include:

• Water as a universal solvent can dissolve many ionic compounds by breaking them into their constituent ions.
• The physical properties of the resulting solution, such as color, are determined by the ions present.
• Not all ions affect the color of the solution; typically, transition metal ions are responsible for colored solutions.

Transition Metals

Transition metals are elements found in the d-block of the periodic table. They are known for their ability to form various oxidation states and colored compounds.

Characteristics of transition metals include:

• Presence of partially filled d orbitals which allow for various electronic arrangements.
• Ability to form complex ions with other molecules, often involving d-d transitions.
• Exhibit a range of colors in their compounds, depending on the metal and its oxidation state.

Transition metals like cobalt ( l{Co}^{2+} ) are notable for forming colorful solutions when dissolved in water, primarily because of the electronic transitions between the d-orbitals. The specific color observed is due to the energy difference between these d-orbitals, influenced by the ligands surrounding the metal ion.

d-d Electron Transitions

d-d electron transitions are phenomena that occur in transition metal ions due to their partially filled d orbitals. When these ions are in aqueous solutions, they interact with surrounding water molecules to form coordination complexes.

These complexes can have different energy levels, defined by the layout of the d orbitals. When light hits the complex, electrons can absorb energy and jump from a lower-energy d orbital to a higher-energy d orbital. This process is called a "d-d transition" and typically results in the absorption of certain wavelengths of light. The remaining light gives the solution its characteristic color.

Important aspects of d-d transitions:

• The transition occurs internally, within the d orbitals of the metal ion.
• The wavelengths (or colors) absorbed depend on the energy gaps between orbitals.
• The geometry and nature of ligands also affect the color as they influence the energy levels.

These transitions are why compounds of ions like l{Co}^{2+} exhibit colors, such as the pink color observed in cobalt nitrate solutions.