Consider the following statements about the standard hydrogen electrode and standard electrode potentials.
(A) The standard hydrogen electrode is assigned a zero potential at all temperatures.
(B) When the standard hydrogen electrode is taken as the anode and another half-cell as the cathode, the emf of the cell gives the oxidation potential of the other half-cell.
(C) The measured emf of the cell Pt(s)|H2(g, 1 bar)|H+(aq, 1 M)||Cu2+(aq, 1 M)|Cu is 0.34 V.
(D) A negative E° means that the redox couple is a weaker reducing agent than the H+/H2 couple.
Which of the statements given above are incorrect? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (B) and (D) only
The potential of a single half-cell cannot be measured. So the standard hydrogen electrode is given a zero potential, and the potential of any other half-cell is found by joining it to the SHE and measuring the emf.
(A) The standard hydrogen electrode is assigned a zero potential at all temperatures. — Correct statement. This zero is a convention, and it holds at all temperatures.
(B) When the standard hydrogen electrode is taken as the anode and another half-cell as the cathode, the emf of the cell gives the oxidation potential of the other half-cell. — Incorrect statement. The other half-cell is the cathode, where reduction occurs, so the emf gives its reduction potential: E° = E°R − 0 = E°R.
(C) The measured emf of the cell Pt(s)|H2(g, 1 bar)|H+(aq, 1 M)||Cu2+(aq, 1 M)|Cu is 0.34 V. — Correct statement. Here the SHE is the anode, so 0.34 V is the standard electrode potential of the Cu2+/Cu half-cell. The positive value shows that Cu2+ ions are reduced more easily than H+ ions.
(D) A negative E° means that the redox couple is a weaker reducing agent than the H+/H2 couple. — Incorrect statement. A negative E° means a stronger reducing agent than the H+/H2 couple; zinc, for example, reduces hydrogen ions.
NCERT (2026-27 edition):
2.2.1 Measurement of Electrode Potential – “…According to convention, a half-cell called standard hydrogen electrode … represented by Pt(s) H2(g) H+(aq), is assigned a zero potential at all temperatures corresponding to the reaction H+(aq) + e– → ½H2(g)”
“At 298 K the emf of the cell, standard hydrogen electrode second half-cell constructed by taking standard hydrogen electrode as anode (reference half-cell) and the other half-cell as cathode, gives the reduction potential of the other half-cell.”
“…The measured emf of the cell: Pt(s) H2(g, 1 bar) H+ (aq, 1 M) Cu2+ (aq, 1 M) Cu is 0.34 V and it is also the value for the standard electrode potential of the half-cell corresponding to the reaction: Cu2+ (aq, 1M) + 2 e– → Cu(s)”
“The positive value of the standard electrode potential in the first case indicates that Cu2+ ions get reduced more easily than H+ ions. … The negative value of the standard electrode potential in the second case indicates that hydrogen ions can oxidise zinc (or zinc can reduce hydrogen ions).”
2.2.1 Measurement of Electrode Potential (point 1 of the text below Table 2.1) – “1. A negative Eo means that the redox couple is a stronger reducing agent than the H+/H2 couple.”