Problem 175 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following reaction is possible at anode? [2002] (a) \(\mathrm{F}_{2}+2 \mathrm{e}^{-} \longrightarrow 2 \mathrm{~F}^{-}\) (b) \(2 \mathrm{H}^{+}+1 / 2 \mathrm{O}_{2}+2 \mathrm{e}^{-} \longrightarrow \mathrm{H}_{2} \mathrm{O}\) (c) \(2 \mathrm{Cr}_{2}^{3+}+7 \mathrm{H}_{2} \mathrm{O} \longrightarrow \mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}+14 \mathrm{H}^{+}+6 \mathrm{e}^{-}\) (d) \(\mathrm{Fe}^{2+} \longrightarrow \mathrm{Fe}^{3+}+\mathrm{e}^{-}\)

Step-by-Step Solution

Answer

Option (d): \(\mathrm{Fe}^{2+} \longrightarrow \mathrm{Fe}^{3+}+\mathrm{e}^{-}\) is possible at the anode.

Step 1: Introduction to Anodic Reactions

In electrochemical cells, the anode is the electrode where oxidation occurs. This means that at the anode, electrons are lost by the species.

Step 2: Examining the Reactions

Go through each option and determine whether oxidation or reduction is taking place.- Option (a): \(\mathrm{F}_{2}+2 \mathrm{e}^{-} \longrightarrow 2 \mathrm{~F}^{-}\) represents reduction as electrons are gained by \(\mathrm{F}_2\).- Option (b): \(2 \mathrm{H}^{+}+1/2 \mathrm{O}_{2}+2 \mathrm{e}^{-} \longrightarrow \mathrm{H}_{2} \mathrm{O}\) is also a reduction reaction.- Option (c): \(2 \mathrm{Cr}_{2}^{3+}+7 \mathrm{H}_{2} \mathrm{O} \longrightarrow \mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}+14 \mathrm{H}^{+}+6 \mathrm{e}^{-}\) is an oxidation reaction since electrons are produced.- Option (d): \(\mathrm{Fe}^{2+} \longrightarrow \mathrm{Fe}^{3+}+\mathrm{e}^{-}\) is oxidation as \(\mathrm{Fe}^{2+}\) loses an electron.

Step 3: Identify the Correct Reaction

From the examination, options (c) and (d) are oxidation reactions. However, the correct or most common anodic reaction usually involves a simple one-electron transfer. Therefore, \(\mathrm{Fe}^{2+} \longrightarrow \mathrm{Fe}^{3+}+\mathrm{e}^{-}\) is more likely to be possible at the anode.

Key Concepts

Anodic Reactions

In electrochemical cells, the **anode** is the electrode where the process of **oxidation** takes place. Understanding anodic reactions begins with knowing that at the anode, molecules or ions lose electrons. This loss of electrons is a distinctive feature that separates oxidation from reduction. During anodic reactions:

• One or more reactants lose electrons.
• The overall charge of the reactants increases.

These reactions are crucial because they provide the electrons needed for reduction reactions at the cathode, completing the circuit of an electrochemical cell.

Oxidation and Reduction

**Oxidation and reduction** are two fundamental chemical reactions that occur together and are often referred to as **redox reactions**. Understanding these processes is key to mastering electrochemistry. **Oxidation** involves:

• Loss of electrons by a chemical species.
• An increase in oxidation state.

**Reduction**, on the other hand, involves:

• Gain of electrons by a chemical species.
• A decrease in oxidation state.

Both processes happen simultaneously—when one species is oxidized, another is reduced. This electron flow between the two reactions is what allows electrochemical cells to function.

Electron Transfer

The transfer of **electrons** is a fundamental part of redox reactions and electrochemical cell operation. It describes how electrons move from one species to another, facilitating the conversion of chemical energy into electrical energy. **Key aspects of electron transfer:**

• Electrons move from a higher energy state to a lower one.
• The path traveled by electrons is from the anode to the cathode in a cell.
• Electron transfer ensures the completion of the electrochemical circuit.

By understanding electron transfer, one can grasp how oxidation at the anode and reduction at the cathode are interconnected processes driving the cell's operation.

Reaction Mechanisms

**Reaction mechanisms** provide a step-by-step description of how chemical reactions proceed. They are essential in understanding how anodic reactions occur and what factors influence them. Important points about reaction mechanisms:

• They illustrate the detailed pathway from reactants to products.
• Show intermediate structures that exist during the reaction.
• Help predict the rate and feasibility of the reaction based on electron transfer.

Through reaction mechanisms, chemists can deduce which species are more likely to undergo oxidation or reduction, and predict where and how anodic and cathodic processes will take place in an electrochemical cell.