Problem 11 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Given \(\mathrm{E}_{\mathrm{Cl}_{2} / \mathrm{Cl}^{-}}=1.36 \mathrm{~V}, \mathrm{E}_{\mathrm{Cr}^{3+} / \mathrm{Cr}}^{\circ}=-0.74 \mathrm{~V}\) \(\mathrm{E}_{\mathrm{Cr}_{2} / \mathrm{O}_{7}^{2-} / \mathrm{Cr}^{3+}}=1.33 \mathrm{~V}, \mathrm{E}_{\mathrm{MnO}_{4}^{\circ} / \mathrm{Mn}^{2+}}=1.51 \mathrm{~V}\) Among the following, the strongest reducing agent is (a) \(\mathrm{Cr}\) (b) \(\mathrm{Mn}^{2+}\) (c) \(\mathrm{Cr}^{3+}\) (d) \(\mathrm{Cl}^{-}\)

Step-by-Step Solution

Answer

The strongest reducing agent is (a) Cr.

Step 1: Understand Reducing Agents

Reducing agents are substances that lose electrons easily in a redox reaction, causing another substance to be reduced.

Step 2: Standard Electrode Potential (E°) Interpretation

The electrode potential (E°) determines the ability of a species to gain electrons. A negative E° means a species is more likely to lose electrons and act as a reducing agent.

Step 3: Review Given Potentials

We have `E_{Cr^3+/Cr}^{ ext{°}} = -0.74 V`, `E_{Cl_2/Cl^-} = 1.36 V`, `E_{Cr_2O_7^{2-}/Cr^{3+}} = 1.33 V` and `E_{MnO_4^{ ext{°}}/Mn^{2+}} = 1.51 V`.

Step 4: Identify Reduction Potentials for Reducing Agents

For reducing agents, look at the reduction potentials: `Cr^3+/Cr` is `-0.74 V`, `Cl^-` in `Cl2` has `1.36 V`, `Cr^3+` has `1.33 V`, `Mn^{2+}` has `1.51 V`.

Step 5: Determine Reducing Strength

From the potentials: `Cr^3+/Cr` is `-0.74 V`, which is the most negative. This means `Cr` is the strongest reducing agent as it is most ready to lose electrons.

Key Concepts

Standard Electrode Potential

In electrochemistry, the standard electrode potential, often denoted as \( E^{°} \), is a measure of the tendency of a chemical species to acquire electrons and be reduced. It is measured in volts and provides a reference point for determining the direction of redox reactions. The standard electrode potential values are measured under standard conditions, which usually means a temperature of 298 K, a pressure of 1 atm, and a concentration of 1 M for each ion.

When the \( E^{°} \) value is positive, it indicates a strong tendency for a species to gain electrons and act as an oxidizing agent. Conversely, a negative \( E^{°} \) suggests that a species is more likely to lose electrons, functioning as a reducing agent. It’s important to note that these potentials are calculated based on a comparison with the standard hydrogen electrode, which is arbitrarily assigned a potential of 0 V.

• The higher the \( E^{°} \) value, the stronger the oxidizing power of the element or compound.
• A negative \( E^{°} \) indicates a good reducing agent as it suggests a readiness to lose electrons.

Understanding standard electrode potentials can accurately predict the direction in which a redox reaction can occur and, in the context of deciding which species acts as a reducing agent, a negative value provides a quick clue.

Reducing Agent

In the world of redox reactions, a reducing agent, also known as a reductant, is a substance that donates electrons to another substance. By losing electrons, the reducing agent itself is oxidized in the process. Therefore, reducing agents play a pivotal role in various chemical reactions, allowing other molecules to gain electrons and undergo reduction.

A good reducing agent is typically characterized by:

• Its ability to easily lose electrons.
• A lower or more negative standard electrode potential \( (E^{°}) \).
• Being comprised of elements that have high electron density, which they can readily donate.

In the exercise you mentioned, chromium (\( Cr \)) with \( E_{Cr^{3+}/Cr}^{°} = -0.74 \, V \) emerges as the strongest reducing agent compared to the others provided. It holds the most negative \( E^{°} \) value, implying it loses electrons more readily than others, thus highlighting its efficiency as a reducing agent.

Redox Reactions

Redox reactions, short for reduction-oxidation reactions, are chemical processes in which the oxidation state of elements changes due to the transfer of electrons. In every redox reaction, one substance undergoes reduction by gaining electrons, while another undergoes oxidation by losing electrons. These reactions are fundamental to numerous biological and chemical processes, including cellular respiration and corrosion.

Key components of redox reactions include:

• Reduction: The gain of electrons by a molecule, atom, or ion, resulting in a decrease in oxidation state.
• Oxidation: The loss of electrons, leading to an increase in oxidation state.
• Oxidizing Agent: The substance that gets reduced by gaining electrons.
• Reducing Agent: The substance that gets oxidized by losing electrons.

In the context of the problem given, understanding how each element and compound acts in their respective redox reactions helps us determine their roles as either reducing or oxidizing agents. For instance, in the given potentials, the differences in standard electrode potentials highlight which species are better suited as reducing agents, aiding in identifying the strongest one among them, like chromium \( Cr \).