Problem 25 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following ions is colourless in its aqueous solution? (a) \(\mathrm{Ti}^{3+}\) (b) \(\mathrm{Cu}^{2+}\) (c) \(\mathrm{Ni}^{2+}\) (d) \(\mathrm{Zn}^{2+}\)

Step-by-Step Solution

Answer

(d) \(\mathrm{Zn}^{2+}\) is colourless in aqueous solution.

Step 1: Understand the Problem

The problem asks us to identify which ion among the given options forms a colourless solution in water. This typically depends on the electronic configuration of the ion, particularly in transition metals, which can cause the solution to be colored due to d-d transitions.

Step 2: Analyze Electron Configuration

Examine the electron configurations of each ion to determine their ability for d-d transitions:- **\(\mathrm{Ti}^{3+}:\)** Argon core \(([\mathrm{Ar}]\): 18 electrons) and 1 unpaired electron in \(3d^1\).- **\(\mathrm{Cu}^{2+}:\)** Krypton core \(([\mathrm{Ar}]: 18)\) and \(3d^9\). There is 1 unpaired electron.- **\(\mathrm{Ni}^{2+}:\)** Argon core \(([\mathrm{Ar}]: 18)\) and \(3d^8\). There are 2 unpaired electrons.- **\(\mathrm{Zn}^{2+}:\)** Argon core \(([\mathrm{Ar}]: 18)\) and \(3d^{10}\). All electrons are paired.

Step 3: Determine ion's Colour Property

- **\(\mathrm{Ti}^{3+}\)**, **\(\mathrm{Cu}^{2+}\)**, and **\(\mathrm{Ni}^{2+}\)** have unpaired electrons in their d-orbitals, allowing d-d transitions, often resulting in colored solutions.- **\(\mathrm{Zn}^{2+}\)** lacks unpaired d-electrons (\(3d^{10}\)), hence, no d-d transitions can occur, resulting in a colourless solution.

Step 4: Verify the Conclusion

To confirm, colorless solutions are typically due to the absence of d-d electronic transitions. Since \(\mathrm{Zn}^{2+}\) doesn't undergo these transitions, its solution is indeed colourless.

Key Concepts

d-d transitions

The d-d transitions are a fascinating concept that helps us understand why some transition metal ions exhibit color in their aqueous solutions. When an electron in a transition metal ion gets excited, it can jump from a lower energy d-orbital to a higher energy d-orbital. This process is known as a d-d transition.

These transitions occur because the d-orbitals are split into two different energy levels when the metal ion is in a chemical complex, often due to the surrounding ligands. The energy difference between these split levels falls within the visible light range, leading to absorption of particular light wavelengths. This absorption is what we perceive as color.

Without d-d transitions, visible color is not produced because no particular wavelength of light is absorbed. In essence, only transition metal ions with one or more unpaired electrons in their d-orbitals can undergo these d-d transitions.

electron configuration

Electron configuration describes how electrons are distributed among the orbitals of an atom or ion. It's a foundational concept for understanding atomic behavior and properties. In transition metals, the electron configuration particularly focuses on the distribution in the d-orbitals.

For instance, in the exercise's example, the electron configuration of transition metal ions is crucial in determining their color in solutions. The electron configurations for the given ions are as follows:

• i a href="#"> Ti³⁺: Argon core ([ ext{Ar}]: 18 electrons) and one unpaired electron in 3d¹.
• Cu²⁺: Argon core a href="#">([ ext{Ar}]: 18 electrons) and 3d⁹. There is one unpaired electron.
• Ni²⁺: Argon core ([ ext{Ar}]: 18 electrons) and 3d⁸. There are two unpaired electrons.
• Zn²⁺: Argon core ([ ext{Ar}]: 18 electrons) and 3d¹⁰. All electrons are paired.

An ion with all filled d-orbitals, like Zn²⁺, does not have unpaired electrons to partake in transitions, making it colorless.

colorless ions

Colorless ions are often encountered in chemistry and are characterized by the absence of d-d transitions. The primary reason some ions are colorless is that they have no unpaired electrons in the d-orbitals, specifically observed in transition metal ions.

For example, in our exercise, Zn²⁺ is a colorless ion because, in its electron configuration ( 3d¹⁰), all electrons are paired. With no unpaired electrons, no electronic transitions between d-orbitals happen, thus no light absorption occurs in the visible spectrum, resulting in a colorless solution.

This concept helps us predict whether a certain transition metal ion will display color when dissolved in water, based on its electronic configuration.

unpaired electrons

Unpaired electrons play a crucial role in determining the color of transition metal ions in solutions. In simple terms, they are electrons that do not have a paired counterpart within an atom's orbitals.

An unpaired electron in the d-orbital is necessary for d-d transitions, which, as we learned before, are responsible for the vibrant colors many transition metals exhibit in their aqueous solutions.

In the exercise, the ions i>a href="#">Ti³⁺, Cu²⁺, and Ni²⁺ have unpaired electrons, leading to these d-d transitions and thus producing color.

In contrast, Zn²⁺ does not have any unpaired electrons (with a 3d¹⁰ electron configuration), which is why it remains colorless in solution. Understanding unpaired electrons helps us predict and explain why certain elements are colorful or lack color entirely.