Most metals of the first transition series are oxidised by 1 M H+, bu…

The correct answer is Option (4) → The standard electrode potential, E°(Cu²⁺/Cu), of copper is positive.

• A metal can liberate hydrogen from an acid only if it is a better reducing agent than hydrogen, that is, if its E°(M²⁺/M) is negative.

• Copper is the only metal of the first series with a positive E° (+0.34 V). So H⁺ ions cannot oxidise copper, and no hydrogen is given off.

• Only oxidising acids (nitric acid and hot concentrated sulphuric acid) react with copper, and then the acid itself is reduced.

Option (1) Copper has the lowest enthalpy of atomisation in the 3d series. — Incorrect.

• The lowest enthalpy of atomisation in the series belongs to zinc (126 kJ mol⁻¹).

• The value for copper (339 kJ mol⁻¹) is high, and a high value makes it harder, not easier, to turn the metal into ions.

Option (2) The hydration enthalpy of Cu²⁺ fully balances the energy needed to change Cu(s) into Cu²⁺(aq). — Incorrect. The opposite is true: the high energy needed to form Cu²⁺(aq) from Cu(s) is not balanced by its hydration enthalpy. This is why E° for copper is positive.

Option (3) The second ionisation enthalpy of copper is unusually low. — Incorrect. The second ionisation enthalpy of copper is unusually high, because Cu⁺ has the stable 3d¹⁰ configuration. A low value would make Cu²⁺ easier to form.

Option (4) The standard electrode potential, E°(Cu²⁺/Cu), of copper is positive. — Correct. A positive E° means copper has less tendency to lose electrons than hydrogen, so it cannot displace hydrogen from a non-oxidising acid.

NCERT (2026-27 edition):
4.3.5 Trends in the M²⁺/M Standard Electrode Potentials – “The unique behaviour of Cu, having a positive E^o, accounts for its inability to liberate H₂ from acids. Only oxidising acids (nitric and hot concentrated sulphuric) react with Cu, the acids being reduced. The high energy to transform Cu(s) to Cu²⁺(aq) is not balanced by its hydration enthalpy.”