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) \(2 \mathrm{Cr}_{2}{\underline{\phantom{xx}}}^{3+}+7 \mathrm{H}_{2} \mathrm{O} \longrightarrow \mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}+14 \mathrm{H}^{+}+6 \mathrm{e}^{-}\) (d) \(\mathrm{Fe}^{2+} \longrightarrow \mathrm{Fe}^{3+}+\mathrm{e}^{-}\)

Step-by-Step Solution

Answer

Reactions (c) and (d) are possible at the anode.

Step 1: Understand Anode Reaction

At the anode, oxidation occurs. This means electrons are lost during the reaction. To identify which reaction is possible at the anode, we look for the reaction where electrons are a product rather than a reactant.

Step 2: Analyze Each Reaction

Let's examine each option to determine if oxidation occurs.(a) \ \(\mathrm{F}_{2}+2 \mathrm{e}^{-} \longrightarrow 2 \mathrm{~F}^{-}\) - This is a reduction reaction as it involves the gain of electrons.(b) \ \(2 \mathrm{H}^{+}+\frac{1}{2} \mathrm{O}_{2}+2 \mathrm{e}^{-} \longrightarrow \mathrm{H}_{2} \mathrm{O}\) - This is a reduction reaction.(c) \ \(2 \mathrm{Cr}_{2}{\underline{\phantom{xx}}}^{3+}+7 \mathrm{H}_{2} \mathrm{O} \longrightarrow \mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}+14 \mathrm{H}^{+}+6 \mathrm{e}^{-}\) - This is an oxidation reaction as electrons are released.(d) \ \(\mathrm{Fe}^{2+} \longrightarrow \mathrm{Fe}^{3+}+\mathrm{e}^{-}\) - This is an oxidation reaction.

Step 3: Identify Possible Anode Reactions

From Step 2, reactions (c) and (d) are oxidation reactions. This means they release electrons, making them possible at the anode.

Key Concepts

Anode Reaction

In electrochemistry, an anode is a crucial component where oxidation occurs. An anode reaction is characterized by the loss of electrons from a substance, which means electrons are emitted as products in these reactions. At the anode, a component undergoes oxidation, losing electrons and subsequently increasing its oxidation state. Therefore, when identifying an anode reaction, it’s essential to observe if electrons appear as a product.
For example, in the reaction

1. \( ext{Fe}^{2+} \rightarrow ext{Fe}^{3+} + ext{e}^{-} \)
2. \( ext{Cr}_2^{3+} + 7 ext{H}_2 ext{O} \rightarrow ext{Cr}_2 ext{O}_7^{2-} + 14 ext{H}^+ + 6 ext{e}^{-} \)

we see electrons emerging as products. Thus, these are considered anode reactions. It’s important to remember that oxidation cannot occur without a simultaneous reduction in the overall redox process. This concept is foundational to understanding electrochemical reactions.
At the anode:

• Oxidation occurs.
• Electrons are a product.
• The oxidation state of a substance increases.

Oxidation Reaction

Oxidation is a fundamental concept in electrochemistry and is defined as the process where a species loses electrons. This event leads to an increase in the oxidation state of the element involved. In any oxidation reaction, electrons are released; hence, these reactions are typically represented as having electrons on the product side of the equation.
For instance, observe the reactions:

• \( ext{Fe}^{2+} \rightarrow ext{Fe}^{3+} + ext{e}^{-} \)
• \( 2 ext{Cr}_{2}^{3+} + 7 ext{H}_2 ext{O} \rightarrow ext{Cr}_2 ext{O}_7^{2-} + 14 ext{H}^+ + 6 ext{e}^{-} \)

In each case, electrons are lost, signifying oxidation. The oxidation process is a key part of redox reactions and occurs at the anode during an electrochemical cell operation.
Understanding oxidation helps in:

• Predicting how molecules will react.
• Determining the flow of electrons in a chemical process.
• Balancing redox reactions by recognizing electron loss.

Redox Reactions

Redox reactions are a type of chemical reaction that involve the transfer of electrons between two species. The term 'redox' is a portmanteau of 'reduction' and 'oxidation'. This simultaneous occurrence means while one substance undergoes oxidation by losing electrons, another undergoes reduction by gaining those electrons.
For example, consider a reaction in an electrochemical cell:
- The anode reaction could be: \( ext{Fe}^{2+} \rightarrow ext{Fe}^{3+} + ext{e}^{-} \), representing oxidation.- The cathode counterpart might be: \( ext{O}_2 + 4 ext{e}^{-} + 4 ext{H}^+ \rightarrow 2 ext{H}_2 ext{O} \), representing reduction.
In redox processes:

• Electrons are transferred from the reducing agent to the oxidizing agent.
• There is always a paired reaction consisting of one oxidation and one reduction.
• It is fundamental in energy transfer processes such as cellular respiration and battery operation.

Fully grasping redox reactions enables better comprehension of energy and chemical dynamics across various contexts, from biological systems to industrial applications.