Which of the following describes the overpotential in electrolysis?

The correct answer is Option (4) → The extra potential needed to make a feasible but kinetically slow electrode process take place

• The products of electrolysis are decided by the standard electrode potentials of the species present, but also by how fast each electrode process is. Some processes are possible but so slow that they do not seem to take place at lower voltages; the extra potential they need is the overpotential.

• For example, in the electrolysis of aqueous NaCl, the overpotential of oxygen makes chlorine, not oxygen, come off at the anode.

Why the other options are incorrect

Option (1) The potential difference that develops between an electrode and the electrolyte around it — Incorrect. This is the electrode potential, set up by the tendency of metal ions to deposit and of metal atoms to go into solution.

Option (2) The potential of a half-cell in which the concentrations of all the species are unity — Incorrect. This is the standard electrode potential.

Option (3) The opposing external potential at which the reaction in a galvanic cell stops altogether — Incorrect. This external potential is equal to the cell potential (1.1 V for the Daniell cell); above it the cell starts to work as an electrolytic cell.

NCERT (2026-27 edition):
2.5.1 Products of Electrolysis – “…Moreover, some of the electrochemical processes although feasible, are so slow kinetically that at lower voltages these do not seem to take place and extra potential (called overpotential) has to be applied, which makes such process more difficult to occur.”