AIIMS 2006 Chemistry Electrochemical Cells MCQ Question

To understand the question regarding the Daniell cell, let's break down the concepts involved.

Explanation of the Daniell Cell

The Daniell cell is a type of electrochemical cell that consists of two half-cells: one containing zinc and the other containing copper. The cell can be represented as:

$\text{Zn} | \text{Zn}^{2+} || \text{Cu}^{2+} | \text{Cu}$

Here, zinc (Zn) serves as the anode, and copper (Cu) serves as the cathode. The standard cell potential for this setup is given as $E_{\text{cell}} = 1.1 \, \text{V}$.

Assertion Explanation

When the Daniell cell operates spontaneously, electrons flow from the anode (Zn) to the cathode (Cu) through an external circuit. This flow of electrons is due to the reduction of copper ions at the cathode and the oxidation of zinc at the anode:

• At the anode (oxidation): $\text{Zn} \rightarrow \text{Zn}^{2+} + 2\text{e}^-$

• At the cathode (reduction): $\text{Cu}^{2+} + 2\text{e}^- \rightarrow \text{Cu}$

Opposite Potential Greater than 1.1 V

When an external potential greater than 1.1 V is applied in the opposite direction, the electrochemical reaction can be reversed. This means that instead of Zn being oxidized and Cu being reduced (as in spontaneous operation), we effectively force the system to do the opposite:

1. Electrons will flow from the cathode to the anode (the opposite of normal operation).
2. Zn will be deposited at the anode, meaning that zinc ions will gain electrons and get reduced to solid zinc.
3. Cu will dissolve at the cathode, meaning that solid copper will lose electrons and be oxidized to copper ions.

Correct Answer Justification

Option B: Zn is deposited at anode and Cu is dissolved at cathode. This statement is correct because applying a potential greater than the cell potential forces the electrons to move in the opposite direction, facilitating the deposition of Zn at the anode and dissolution of Cu at the cathode.

Incorrect Option Clarification

Option A: Electron from cathode to anode. This statement is misleading. While it is true that in the reverse process, electrons move from the cathode to the anode, the important concept here is that the electrons flowing in that direction are not a result of the normal spontaneous reaction but rather the application of an external potential that drives the reaction backward. Thus, the focus on electron flow without considering the overall chemical processes makes this option incomplete.

Summary

To summarize, the application of an external potential greater than 1.1 V in a Daniell cell forces the cell to operate in reverse, leading to the deposition of zinc at the anode and the dissolution of copper at the cathode. Hence, the correct answer is B.

This situation can be further analyzed using the Nernst equation, which describes how the cell potential changes with concentration. However, for the sake of this explanation, our focus is on the fundamental processes occurring due to the applied external potential rather than the detailed electrochemical calculations.