How many of the following species can be reduced by hydrogen gas under standard conditions at 298 K?
Ag+ (E°Ag+/Ag = 0.80 V), Zn2+ (E°Zn2+/Zn = −0.76 V), Cu2+ (E°Cu2+/Cu = 0.34 V), Pb2+ (E°Pb2+/Pb = −0.13 V), Cl2 (E°Cl2/Cl− = 1.36 V), Mg2+ (E°Mg2+/Mg = −2.36 V)
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 3
When hydrogen gas reduces a species, H2 is oxidised to H+, the couple of the standard hydrogen electrode (E° = 0.00 V). The cell made of this couple and the species has
\( \displaystyle E^{\circ}_{cell} = E^{\circ}_{species} - E^{\circ}_{\ce{H+/H2}} = E^{\circ}_{species} \)
The reaction is feasible (\( \Delta_r G^{\circ} = -nFE^{\circ}_{cell} \) negative) only when E° of the species is positive: such a couple is a weaker reducing agent than H+/H2, so H2 can reduce its oxidised form.
Reduced by H2:
Ag+ (0.80 V) → Ag
Cu2+ (0.34 V) → Cu
Cl2 (1.36 V) → Cl−
Not reduced by H2:
Zn2+ (−0.76 V) — E° negative.
Pb2+ (−0.13 V) — E° negative, though small.
Mg2+ (−2.36 V) — E° negative.
Reduced by H2: 3 species.
NCERT (2026-27 edition):
2.2.1 Measurement of Electrode Potential – “…If the standard electrode potential of an electrode is greater than zero then its reduced form is more stable compared to hydrogen gas. Similarly, if the standard electrode potential is negative then hydrogen gas is more stable than the reduced form of the species.”
“1. A negative Eo means that the redox couple is a stronger reducing agent than the H+/H2 couple.”
“2. A positive Eo means that the redox couple is a weaker reducing agent than the H+/H2 couple.”