Question

An electrochemical cell can behave like an electrolytic cell when _______.

• A. $E_{cell}>E_{ext}$
• B. $E_{cell}=0$
• C. $E_{ext}>E_{cell}$
• D. $E_{cell}=E_{ext}$

Answer 1

The correct option is C $E_{ext}>E_{cell}$

Understanding galvanic & electrolytic cells

Galvanic cell: In this cell electrical energy is produced by spontaneous chemical reactions i.e., simultaneous oxidation and reduction reactions.

Electrolytic cell: In this cell electrical energy is used to facilitate a non-spontaneous redox reaction. Here an external voltage has to be supplied.

An electrochemical cell can behave like an electrolytic cell when there is an application of an external opposite potential on the galvanic cell and reaction is not inhibited until the opposing voltage reaches the value $1.1V$

Let’s analyse the options:

Option (a):

$E_{cell}=0$ implies that reaction is in equilibrium wherein it represents a concentration cell. So, this is not the answer.

Option (b):

$E_{cell}>E_{ext}$

When external opposite potential $E_{ext}$ is applied on galvanic cell and increased slowly, the reaction continues to take place till voltage reaches the value of $E_{cell}$.


Option (c):

$E_{ext}>E_{cell}$

When we further increase external potential beyond the value of cell potential, the reaction starts to proceed in opposite direction.

Instead of producing electrical energy by chemical reaction, the cell starts to facilitate non-spontaneous reaction.

Reaction will function in the opposite direction on increasing the external potential any further.

Option (d):

$E_{ext}=E_{cell}$

When external potential becomes equal to cell potential $\left(E_{cell}\right)$, no current flows through the cell and reaction stops completely.

Therefore, when $E_{ext}>E_{cell}$, an electrochemical cell can behave like an electrolytic cell.

Hence, correct answer is option (c).