Problem 52 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

While \(\mathrm{Fe}^{3+}\) is stable, \(\mathrm{Mn}^{3+}\) is not stable in acid solution because (a) \(\mathrm{O}_{2}\) oxideses \(\mathrm{Mn}^{2+}\) to \(\mathrm{Mn}^{3+}\) (b) \(\mathrm{O}_{2}\) oxideses both \(\mathrm{Mn}^{2+}\) to \(\mathrm{Mn}^{3+}\) and \(\mathrm{Fe}^{2+}\) to \(\mathrm{Fe}^{3+}\) (c) \(\mathrm{Fe}^{3+}\) oxideses \(\mathrm{H}_{2} \mathrm{O}\) to \(\mathrm{O}_{2}\) (d) \(\mathrm{Mn}^{3+}\) oxideses \(\mathrm{H}_{2} \mathrm{O}\) to \(\mathrm{O}_{2}\)

Step-by-Step Solution

Answer

Option (d) is the reason: \( \mathrm{Mn}^{3+} \) oxidizes \( \mathrm{H}_{2} \mathrm{O} \) to \( \mathrm{O}_{2} \), leading to its instability.

Step 1: Understanding the stability of ions

To understand why \( \mathrm{Fe}^{3+} \) is stable and \( \mathrm{Mn}^{3+} \) is not in an acidic solution, we need to consider the electronic configurations. \( \mathrm{Fe}^{3+} \) has a half-filled \( 3d^5 \) configuration, providing additional stabilization. \( \mathrm{Mn}^{3+} \) would have a configuration \( 3d^4 \) which is less stable.

Step 2: Standard electrode potentials

Consider the standard electrode potentials for Mn and Fe ions. The reduction potential for \( \mathrm{Mn}^{3+}/\mathrm{Mn}^{2+} \) is about +1.51 V, whereas for \( \mathrm{Fe}^{3+}/\mathrm{Fe}^{2+} \) it is +0.77 V. The higher positive value for Mn means it's a stronger oxidizing agent and less stable, leading to its reduction.

Step 3: Analyzing the given options

Each option suggests a different mechanism. For \( \mathrm{Mn}^{3+} \) instability, we need to consider potential reactions in acidic conditions and available oxidizing agents in the options such as \( \mathrm{O}_{2} \) and \( \mathrm{H}_{2} \mathrm{O} \).

Step 4: Evaluation against options

Option (d) states that \( \mathrm{Mn}^{3+} \) can oxidize \( \mathrm{H}_{2} \mathrm{O} \) to \( \mathrm{O}_{2} \). Given the strong oxidizing nature of \( \mathrm{Mn}^{3+} \), this reaction is feasible, making \( \mathrm{Mn}^{3+} \) an unstable ion in acidic solutions, as it will attempt to reduce itself by interacting with water.

Key Concepts

Electrode Potentials

In the world of chemistry, electrode potentials tell us how likely a species is to gain or lose electrons. Essentially, they give us a hint about the stability of various ions in a given solution.
For the ions in our exercise, \[\mathrm{Fe}^{3+}/\mathrm{Fe}^{2+}\] has an electrode potential of about +0.77 V. This indicates a moderate tendency to be reduced, meaning it doesn't dramatically encourage other species to give up electrons.
On the other hand, the \[\mathrm{Mn}^{3+}/\mathrm{Mn}^{2+}\] system has a higher electrode potential at approximately +1.51 V. A higher value signifies a stronger pull towards reduction, meaning \[\mathrm{Mn}^{3+}\] is more eager to gain electrons and is less stable in its oxidized form.
**Key points**:

• Higher electrode potential = Stronger oxidizing agent.
• Less stable = More likely to gain electrons rapidly.

Understanding this is crucial because it explains why \[\mathrm{Mn}^{3+}\] doesn't hang around for long in solutions; it wants nothing more than to become more stable by grabbing some electrons.

Oxidizing Agents

Oxidizing agents are chemical species that snag electrons from other compounds. In this process, they themselves get reduced. When examining \[\mathrm{Fe}^{3+}\] and \[\mathrm{Mn}^{3+}\], we can identify their differing roles as oxidizing agents in acidic solutions.
**Mn as an Oxidizing Agent**: The high electrode potential of \[\mathrm{Mn}^{3+}\] reveals its role as an eager oxidizer. It prefers to accept electrons, driving it to react readily in attempts to become more stable, reducing to \[\mathrm{Mn}^{2+}\]. This inherently makes it a less stable ion unless it can successfully nab some electrons from its surroundings before degrading.
**Fe as a Milder Oxidizing Agent**: In contrast, \[\mathrm{Fe}^{3+}\] is more selective with its interactions because it's comparatively a weaker oxidizing agent. Its successful electron configuration allows it more stability, which explains why it is comfortably stable in acidic solutions. It does not have an overwhelming need to accept electrons from neighboring ions or molecules.

Electronic Configuration

Electronic configuration refers to the arrangement of electrons in an atom or ion. This distribution is vital for predicting the chemical behavior of elements, including ion stability and reactivity.
**Iron and Manganese Configurations**:

• The configuration for \[\mathrm{Fe}^{3+}\] is \[\left[ \mathrm{Ar} \right] 3d^5\], featuring a half-filled 'd' subshell. Each of the five 3d orbitals contains one electron, creating an energetically stable state known as the half-filled stability phenomenon.
• Conversely, \[\mathrm{Mn}^{3+}\]'s configuration is \[\left[ \mathrm{Ar} \right] 3d^4\]. This arrangement does not benefit from the same stability since the 3d subshell lacks the symmetry and added stability of being half-filled.

Chemical stability often stems from achieving filled or half-filled subshells. This stability means the ions can resist changes in their electronic arrangements, in acid or any other type of solution.