Question

When can an electrochemical cell behave like an electrolytic cell?
a) ${E_{cell}} = 0$
b) ${E_{cell}} > {E_{Ext}}$
c) ${E_{Ext}} > {E_{cell}}$
d) ${E_{cell}} = {E_{Ext}}$

Answer 1

Electrochemical cell converts chemical energy to electrical energy through a spontaneous redox reaction, and electrolytic cell in contrast, converts electrical energy to chemical energy through a non – spontaneous redox reaction.

Complete step by step answer:
For option a), When ${E_{cell}} = 0$, this is the condition in equilibrium, and here, potential at cathode is equal to potential at anode.
For option b), when ${E_{cell}} > {E_{Ext}}$, the reaction of the electrochemical cell will proceed till ${E_{cell}}$ is equal to ${E_{Ext}}$.
For option c), when ${E_{Ext}} > {E_{cell}}$, the reaction will start in the opposite direction. This means that, now the electrochemical cell will convert the electrical energy supplied by the external cell to chemical energy which is the property of an electrolytic cell. So, electrochemical cell will behave like an electrolytic cell.
For option d), when ${E_{cell}} = {E_{Ext}}$, in this situation, the cell reaction stops and no current will flow through the circuit.
Hence, option c) ${E_{Ext}} > {E_{cell}}$ is the correct answer when an electrochemical cell can behave like an electrolytic cell.

Note:
In both electrochemical and electrolytic cells, the oxidation is still at anode and the reduction still occurs at cathode but the polarities of these two electrodes are reversed. Also, an electrochemical cell has $\Delta G < 0$, and an electrolytic cell has $\Delta G > 0$.