Chapter 21, Problem 5STP

Interpretation Introduction

Interpretation: The correct option for a positive cell potential for a redox reaction needs to be identified.

Concept Introduction: In an electrochemical cell, a redox reaction takes place. In a redox reaction, oxidation and reduction take place simultaneously. There are two electrodes; anode and cathode. Oxidation occurs at the anode and reduction at the cathode.

Explanation of Solution

Reason for the correct option:

The cell potential of an electrochemical cell is related to Gibbs free energy as follows:

  $\Delta G=-nFE$

Here, $\Delta G<0$ for spontaneous reactions and $\Delta G<0$ for non-spontaneous reactions.

For the value of $\Delta G$ to be negative or less than zero, the value of E or cell potential must be positive. Similarly, for the value of $\Delta G$ to be positive or more than zero, the value of E or cell potential must be negative.

Therefore, the cell potential values must be positive for a spontaneous reaction and it must be negative for a non-spontaneous reaction.

Therefore, option (A) is correct.

Conclusion

Reason for incorrect option:

If the value of cell potential is negative (positive change in Gibbs free energy), a redox reaction is not spontaneous thus, option (B) is incorrect.

In electrolysis, non-spontaneous redox reactions take place and for such reactions, cell potential is negative thus, option (C) is incorrect.