Problem 65 - 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 aqueous solution of \( \mathrm{Co(NO}_{3})_{2}\) will be coloured.

Step 1: Identify Colour Formation in Aqueous Solutions

To determine if the aqueous solution of a salt is coloured, we must identify if it contains a transition metal with unpaired d electrons since these transition metals often form coloured solutions.

Step 2: Check Each Salt for Transition Metals

Let's check each given salt:(a) \(\mathrm{Zn(NO}_{3})_{2}\) contains Zn which is a transition metal but has fully paired d electrons and typically forms colourless solutions.(b) \(\mathrm{LiNO}_{3}\) contains Li from Group 1, which is not a transition metal, hence forms colourless solutions.(c) \(\mathrm{Co(NO}_{3})_{2}\) contains Co, a transition metal with unpaired d electrons and typically forms coloured solutions.(d) Potash alum, \(\mathrm{K}_{2} ext{SO}_{4} ullet ext{Al}_{2}( ext{SO}_{4})_{3} ullet 24 ext{H}_{2} ext{O}\), contains Al which is not a transition metal and forms colourless solutions.

Step 3: Determine Which Solutions are Coloured

Among the given salts, only \(\mathrm{Co(NO}_{3})_{2}\) forms a coloured solution because cobalt ions in solution have unpaired d electrons that can absorb visible light, resulting in a coloured solution.

Key Concepts

Transition Metals

Transition metals reside in the d-block of the periodic table, often in groups 3 to 12. These metals are fascinating because of their ability to form compounds with distinctive colours. This unique characteristic primarily arises due to their electron configurations. Transition metals have partially filled d orbitals, which allow them to form various oxidation states. These variable oxidation states facilitate the formation of colored solutions by absorbing specific wavelengths of light.
Transition metal ions can exhibit magnetic properties too, depending on the number of unpaired electrons in their d orbitals. Furthermore, they often form complex ions, surrounded by molecules or ions bound to the metal center. These complexes are pivotal to many biological processes and industrial applications. Transition metals like copper, iron, and cobalt are particularly known for their vibrant hues in solution form.

d Electrons

The concept of d electrons is central to explaining the coloration of transition metal solutions. D electrons refer to electrons located in the d subshell of an atom’s electron shell. In transition metals, these electrons are situated in what is known as partially filled d orbitals.
The behavior of d electrons in transition metals is a crucial factor in determining the color of these metal compounds. When light hits a transition metal compound, d electrons absorb some wavelengths of light to get promoted to a higher energy state. The specific wavelengths absorbed determine the color the compound will display, as the unabsorbed wavelengths are the ones we see.

• Unpaired d electrons: Key to maintaining multiple oxidation states and causing colour changes.
• Crystal field splitting: The interaction between ligands and d electrons affects the absorption of light, leading to vibrant colors.

This interaction between light and d electrons in transition metals is the secret behind their colorful displays.

Cobalt Nitrate

Cobalt nitrate, \(\mathrm{Co(NO}_{3})_{2}\), serves as a classic example where the concept of d electrons and transition metals results in color formation. Cobalt is a transition metal with the chemical symbol Co and an atomic number of 27. It has partially filled d orbitals, specifically five d electrons (d\(^7\) configuration in common oxidation state) that significantly influence its chemistry and color attributes.
In an aqueous solution, cobalt ions create a pink colour due to the presence of unpaired d electrons. The d-d transition, where d electrons jump between energy levels upon absorbing certain wavelengths of light, typically causes this pink hue. Cobalt compounds, like cobalt nitrate, are often used in creating pigments for ceramics and glasses due to their striking colors.

Colour Formation in Solutions

Colour formation in solutions, especially with transition metals, is a captivating subject combining chemistry and light physics. When a metal compound like cobalt nitrate dissolves in water, the cobalt ions are surrounded by water molecules, forming a complex ion. This complex can absorb visible light as a result of electronic transitions within the metal's d orbitals.
The absorption occurs when unpaired d electrons are excited to higher energy states. The specific energy absorption corresponds to particular wavelengths of light. The remaining wavelengths, which are not absorbed, are what we perceive as color.

• Visible light absorption: Different d electron transitions absorb specific light wavelengths, leading to various colors.
• Ligand Field Theory: Offers an explanation for color variations based on the nature and arrangement of ligands.

Thus, colour in solutions arises from the intricate dance of light with electrons, offering both aesthetic pleasure and insights into chemical structures.