Problem 121 - A Complete Resource Book in Chemistry for JEE Main

Question

The standard reduction potentials at \(298 \mathrm{~K}\) for the following half- reactions are given against each \(\mathrm{Zn}^{21}(\mathrm{aq})+2 \mathrm{e}=\mathrm{Zn}(\mathrm{s})-0.762\) \(\mathrm{Cr}^{3+}(\mathrm{aq})+2 \mathrm{e} \cdots \mathrm{Cr}(\mathrm{s}) \quad-0.740\) \(2 \mathrm{H}^{+}(\mathrm{aq})+2 \mathrm{e} \rightleftharpoons \mathrm{H}_{2}(\mathrm{~g}) \quad 0.000\) \(\mathrm{Fe}^{3+}(\mathrm{aq})+2 \mathrm{e}=\mathrm{Fe}^{2+}\) (aq) \(0.770\) Which is the strongest reducing agent? (a) \(\mathrm{H}_{2}(\mathrm{~g})\) (b) \(\mathrm{Cr}(\mathrm{s})\) (c) \(\mathrm{Zn}(\mathrm{s})\) (d) \(\mathrm{Fe}^{2+}(\mathrm{aq})\)

Step-by-Step Solution

Answer

The strongest reducing agent is \( \mathrm{Zn} (s) \).

Step 1: Understanding Standard Reduction Potential

The standard reduction potential is a measure of the tendency of a chemical species to acquire electrons and be reduced. The more negative the standard reduction potential, the stronger is the reducing agent because it loses electrons more easily.

Step 2: Listing Given Potentials

The given reduction potentials are for: \( \mathrm{Zn}^{2+} + 2e^- \rightarrow \mathrm{Zn} \) with \(-0.762\, \mathrm{V}\), \( \mathrm{Cr}^{3+} + 2e^- \rightarrow \mathrm{Cr} \) with \(-0.740\, \mathrm{V}\), \( \mathrm{H}^+ + 2e^- \rightleftharpoons \mathrm{H}_2 \) with \(0.000\, \mathrm{V}\), and \( \mathrm{Fe}^{3+} + 2e^- \rightarrow \mathrm{Fe}^{2+} \) with \(0.770\, \mathrm{V}\).

Step 3: Identifying the Reducing Agents

The reducing agents in this set of reactions are the substances that are oxidized. These include \( \mathrm{Zn}, \mathrm{Cr}, \mathrm{H}_2, \text{ and } \mathrm{Fe}^{2+}\).

Step 4: Comparing Potentials to Find the Strongest Reducing Agent

Since the strongest reducing agent corresponds to the reaction with the lowest (most negative) reduction potential, compare the values: \(-0.762\) for \( \mathrm{Zn} \), \(-0.740\) for \( \mathrm{Cr} \), and one positive and a zero potential for the others. \( \mathrm{Zn} \) has the most negative potential, meaning it is the strongest reducing agent.

Key Concepts

Reducing Agents

Reducing agents are substances that donate electrons in a chemical reaction. When they give away electrons, they are themselves oxidized. This is a key concept in redox reactions where the transfer of electrons occurs simultaneously. Reducing agents are also referred to as electron donors.
In electrochemistry, the strength of a reducing agent is determined by its standard reduction potential. A more negative standard reduction potential indicates a stronger reducing agent. This means the substance can more readily lose electrons. For example, among common metals, zinc is a strong reducing agent due to its negative standard reduction potential of \(-0.762\,\text{V}\).
Understanding which substance is the strongest reducing agent can help in predicting the direction and feasibility of a chemical reaction. When given multiple half-reactions with their standard reduction potentials, you can identify the strongest reducing agent by comparing these values.

Electrochemistry

Electrochemistry is the branch of chemistry that deals with the interplay of chemical changes and electrical energy. At its core, it involves redox reactions, where electron transfer occurs.
Standard reduction potential is a crucial concept in electrochemistry. It provides a way to measure the tendency of a substance to be reduced. The standard hydrogen electrode, assigned a potential of 0.000 V, acts as a reference point for measuring other substances' potentials.

• Positive reduction potentials indicate a greater likelihood of accepting electrons, meaning the substance can be reduced more easily.
• Negative reduction potentials, on the other hand, mean a substance is more likely to lose electrons and act as a reducing agent.

In electrochemical cells, these potentials determine the flow of electrons. A common application of electrochemistry is in batteries, where these principles are used to convert chemical energy into electrical energy.

Oxidation-Reduction Reactions

Oxidation-reduction reactions, or redox reactions, are an essential part of chemistry where electron transfer between substances occurs. These reactions impact a variety of processes, from basic metabolic functions to industrial chemical processes.
In redox reactions:

• Oxidation involves the loss of electrons by a substance.
• Reduction involves the gain of electrons by a substance.

These two processes are always paired because, when one substance loses electrons, another must gain them. This interconnectedness is often depicted in half-reactions, which show either the oxidation or reduction separately.
In electrochemistry, these paired processes are harnessed in electrochemical cells. For a reaction to occur spontaneously, the overall cell potential needs to be positive. By comparing standard reduction potentials, chemists can predict which substances will be oxidized or reduced in a reaction, thus understanding and manipulating chemical processes.