Problem 108 - A Complete Resource Book in Chemistry for JEE Main

Question

Given the standard reduction potentials \(\mathrm{Zn}^{2+} / \mathrm{Zn}=\) \(-0.74 \mathrm{~V}, \mathrm{Cl}_{2} / \mathrm{Cl}^{-}=1.36 \mathrm{~V}, \mathrm{H}^{+} 1 / 2 \mathrm{H}_{2}=0 \mathrm{~V}\) and \(\mathrm{Fe}^{2+} / \mathrm{Fe}^{3+}\) \(=0.77 \mathrm{~V}\). The order of increasing strength as reducing agent is (a) \(\mathrm{Zn}, \mathrm{H}_{2}, \mathrm{Fe}^{2+}, \mathrm{Cl}^{-}\) (b) \(\mathrm{H}_{2}, \mathrm{Zn}, \mathrm{Fe}^{2+}, \mathrm{Cl}^{-}\) (c) \(\mathrm{Cl}^{-}, \mathrm{Fe}^{2+}, \mathrm{Zn}, \mathrm{H}_{2}\) (d) \(\mathrm{Cl}^{-} \mathrm{Fe}^{2+}, \mathrm{H}_{2}, \mathrm{Zn}\)

Step-by-Step Solution

Answer

The correct order is (d): \(\mathrm{Cl}^{-}, \mathrm{Fe}^{2+}, \mathrm{H}_{2}, \mathrm{Zn}\).

Step 1: Understanding Reducing Agents

Reducing agents are substances that donate electrons in chemical reactions, thereby reducing other substances. Their strength as reducing agents is inversely related to their reduction potentials: lower reduction potential means stronger reducing agent, as it prefers to lose electrons.

Step 2: List Standard Reduction Potentials

Given the standard reduction potentials:- Zn: \(-0.74\, \text{V}\)- Cl\(^-\): \(1.36\, \text{V}\)- H\(^{+}\)/H\(_2\): \(0\, \text{V}\)- Fe\(^{2+}/\text{Fe}^{3+}\): \(0.77\, \text{V}\)

Step 3: Compare Reduction Potentials

The given potentials are for reduction reactions. A more negative potential means the substance is more willing to lose electrons and thus is a stronger reducing agent. Compare the reduction potentials: - Zn has the most negative potential (\(-0.74\, \text{V}\)), so it's the strongest reducing agent.- H\(_2\) is next (\(0\, \text{V}\)).- Fe\(^{2+}\) comes after (\(0.77\, \text{V}\)).- Cl\(^-\) having the highest potential (\(1.36\, \text{V}\)) is the weakest reducing agent.

Step 4: Rank Reducing Agents by Strength

Order the reducing agents from weakest to strongest based on their reduction potentials:- Weakest: Cl\(^-\) - Next: Fe\(^{2+}\)- Next: H\(_2\)- Strongest: Zn

Key Concepts

Standard Reduction Potentials

Standard reduction potentials are crucial in understanding how substances behave in oxidation-reduction reactions. These potentials determine the tendency of a substance to gain electrons and thus be reduced. The more positive the standard reduction potential, the greater the substance's affinity for electrons, making it more likely to be reduced. Conversely, a more negative potential indicates a greater propensity to lose electrons, making the substance a better reducing agent.

By referring to standard reduction potentials, we can predict the outcome of a redox reaction. For instance, a substance with a higher reduction potential can oxidize one with a lower potential. Exploring these values helps us rank the strength and direction of redox partners. The exercise asks us to interpret these electrodes' potentials to identify the most potent reducing agents based on these values.

Reducing Agents

Reducing agents, also known as reductants, play a significant role in redox reactions by donating electrons to other substances. The term 'reducing' can often be misleading as the reductant itself gets oxidized in the process.

A strong reducing agent is characterized by its ability to lose electrons easily. This strength is inversely related to its standard reduction potential: the lower the potential, the stronger the reducing agent. For example, zinc (Zn) with a reduction potential of e{-0.74 ext{V}} is a much stronger reducing agent than iron (Fe) or chlorine (Cl ext{-}). Always keep in mind that the role of reducing agents is pivotal in both electrochemical cells and various industrial applications.

Electron Transfer

Electron transfer is the heart of any oxidation-reduction (redox) reaction. It involves the movement of electrons from one element to another, leading to changes in oxidation states.

In redox reactions, the reducing agent donates electrons, which are accepted by the oxidizing agent. This electron transfer leads to the reduction of the oxidizing agent and oxidation of the reducing agent. It's essential to understand these electron transfers to grasp the mechanism of redox reactions. Without electron transfer, the transformation and interaction between molecules wouldn’t occur, abolishing the reaction entirely. Tracking electrons enables chemists to predict reaction paths, products, and the overall feasibility of reactions.

Oxidation-Reduction Principles

The principles of oxidation and reduction are foundational in chemistry. They involve a paired process where one species is oxidized, and another is reduced. Oxidation refers to the loss of electrons, while reduction pertains to the gain. Together, they must occur simultaneously in an oxidation-reduction reaction.

Understanding these principles allows us to balance redox reactions by conserving charge and mass in a chemical equation. It is also used to determine electron flow in electrochemical cells and other chemical systems. The principles also help predict reaction spontaneity: if the sum of standard reduction potentials for a system is positive, the redox reaction can proceed spontaneously. Oxidation-reduction principles are not only key in academic research but also in practical applications such as energy production and metal extraction.