Statement I: A negative value of the standard reduction potential of an electrode means that reduction takes place at this electrode with reference to the standard hydrogen electrode.
Statement II: The standard electrode potential of a half-cell has a fixed value at a given temperature.
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Statement I is incorrect but Statement II is correct.
The standard electrode potential E° of a half-cell is its reduction potential when all the species are at unit concentration, measured against the standard hydrogen electrode (E° = 0.00 V).
Statement I: A negative value of the standard reduction potential of an electrode means that reduction takes place at this electrode with reference to the standard hydrogen electrode. — False. A negative E° means the couple is a stronger reducing agent than the \( \ce{H^+}/\ce{H2} \) couple: its reduced form gives up electrons more readily than hydrogen gas does. Joined to the standard hydrogen electrode, this electrode is oxidised (it is the anode) and \( \ce{H^+} \) ions are reduced. For example, for \( \ce{Zn^2+}/\ce{Zn} \) (E° = −0.76 V), zinc dissolves as \( \ce{Zn^2+} \).
Statement II: The standard electrode potential of a half-cell has a fixed value at a given temperature. — True. E° is measured under fixed standard conditions (unit concentration of every species), so at a given temperature each half-cell has one value; standard values are listed at 298 K.
NCERT (2026-27 edition):
Table 2.1: Standard Electrode Potentials at 298 K – “1. A negative E° means that the redox couple is a stronger reducing agent than the H+/H2 couple.”
2.2 Galvanic Cells – “… When the concentrations of all the species involved in a half-cell is unity then the electrode potential is known as standard electrode potential.”