Problem 39 - A Complete Resource Book in Chemistry for JEE Main

Question

In which of the following pairs are both the ions coloured in aqueous solution? (a) \(\mathrm{Sc}^{3+}, \mathrm{Co}^{2+}\) (b) \(\mathrm{Ni}^{2+}, \mathrm{Cu}^{+}\) (c) \(\mathrm{Ni}^{2+}, \mathrm{Ti}^{3+}\) (d) \(\mathrm{Sc}^{3+}, \mathrm{Ti}^{3+}\) (At. no. \(\mathrm{Sc}=21, \mathrm{~T}_{1}=22, \mathrm{Ni}=28, \mathrm{Cu}=29, \mathrm{Co}=27\) )

Step-by-Step Solution

Answer

The pair with both ions coloured is (c) \( \mathrm{Ni}^{2+}, \mathrm{Ti}^{3+} \).

Step 1: Identify Coloured Ions

Coloured ions typically involve transition metals with unpaired electrons in their d-orbitals. Unpaired electrons result in d-d transitions that can absorb specific wavelengths of light, causing the solution to appear coloured.

Step 2: Analyze Each Pair for Coloured Ions

Examine each ion pair to determine which ions can potentially have unpaired electrons.- **(a)** For \( \mathrm{Sc}^{3+} \), the ion has no d-electrons as all 3 are removed, making it colourless. \( \mathrm{Co}^{2+} \) has 7 d-electrons, typically showing colour.- **(b)** \( \mathrm{Ni}^{2+} \) has 8 d-electrons, which can be coloured. \( \mathrm{Cu}^{+} \) loses one d-electron and results in 10 d-electrons, making it colourless.- **(c)** \( \mathrm{Ni}^{2+} \) is already known to be coloured. \( \mathrm{Ti}^{3+} \) has 1 unpaired electron (2 d-electrons after losing 1) and is also coloured.- **(d)** \( \mathrm{Sc}^{3+} \) is colourless based on previous assessment. \( \mathrm{Ti}^{3+} \) is known to be coloured.

Step 3: Determine the Pair with Both Ions Coloured

From the analysis, the only pair where both ions are coloured in aqueous solution is pair **(c)** \( \mathrm{Ni}^{2+} \) and \( \mathrm{Ti}^{3+} \). Both ions have unpaired d-electrons, which allows them to display colour in a solution.

Key Concepts

Transition Metals

Transition metals are unique elements found in the middle of the periodic table. These metals are known for having partially filled d-orbitals. Unlike alkali or alkaline earth metals, transition metals can form multiple oxidation states. This variety in oxidation states is due to the similar energies of their s- and d-electrons.

• The defining feature of transition metals is their ability to form compounds that are often brightly colored. This is attributed to electron transitions between d-orbitals.
• These d-d transitions, where electrons jump between different energy levels within the d-orbitals, absorb certain wavelengths of light and reflect others. This absorption-reflection property is what gives many transition metal compounds their distinct colors.

This multicolor ability is one reason why transition metals are widely used in pigments and dyes. Understanding how these electron transitions work is key to knowing why certain ions in a solution appear colored.

Unpaired Electrons

The presence of unpaired electrons plays a crucial role in the coloration of an element or compound. When an atom or ion has unpaired electrons in its d-orbitals, these electrons can engage in what is known as d-d transitions.

• An unpaired electron is one that does not have a corresponding partner in an orbital. This uniqueness leads to various electronic and magnetic properties.
• For a transition metal ion to be colored, usually, it must have some unpaired electrons in its d-orbital. These unpaired electrons transition between different d-orbitals by absorbing specific light wavelengths, which results in the phenomenon of color.

The ability of unpaired electrons to produce color is utilized in various applications, such as the use of cobalt compounds in blue glass or nickel complexes in green ceramics. Knowing which ions have unpaired electrons helps predict whether a solution will exhibit color.

Aqueous Solutions

An aqueous solution is simply a solution where water is the solvent. A very common type of solution in chemistry, it can be colorless or colored depending on the solutes dissolved in it.

• When discussing colored ions, the solute is typically a transition metal ion that is dissolved in the water. These ions interact with water molecules in the solution, affecting how light is absorbed and emitted.
• The nature of the aqueous environment and the presence of water molecules can sometimes lead to changes in the oxidation state or coordination number of the ions, directly influencing the colors observed.

In a lab setting, observing the color of an aqueous solution can provide insights into the chemical identity or concentration of its components. Scientists utilize this principle in chemical analysis and spectroscopy to identify various substances based on their unique color signatures in solution.