Which of the following statements about the action of acids on copper under standard conditions is correct?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → In nitric acid, copper is oxidised by the nitrate ion and not by the hydrogen ion.
The standard electrode potential of the Cu2+/Cu electrode is +0.34 V. The positive value shows that Cu2+ ions are reduced more easily than H+ ions. So the reverse change, H+ ions oxidising copper, cannot occur under standard conditions. Copper still dissolves in nitric acid, because there it is oxidised by the nitrate ion.
Why the other options are incorrect
Option (1) Hydrogen ions oxidise copper, so copper dissolves in hydrochloric acid. — Incorrect. Hydrogen ions cannot oxidise copper, so copper does not dissolve in hydrochloric acid.
Option (2) In nitric acid, copper is oxidised by the hydrogen ion and not by the nitrate ion. — Incorrect. It is the other way round: the hydrogen ion of nitric acid cannot oxidise copper, but the nitrate ion can.
Option (4) Hydrogen gas cannot reduce copper ions, as the E° of the Cu2+/Cu electrode is positive. — Incorrect. The positive E° means just the opposite: hydrogen gas can reduce copper ions to copper.
NCERT (2026-27 edition):
2.2.1 Measurement of Electrode Potential – “The positive value of the standard electrode potential in the first case indicates that Cu2+ ions get reduced more easily than H+ ions. The reverse process cannot occur, that is, hydrogen ions cannot oxidise Cu (or alternatively we can say that hydrogen gas can reduce copper ion) under the standard conditions described above. Thus, Cu does not dissolve in HCl. In nitric acid it is oxidised by nitrate ion and not by hydrogen ion.”