The standard reduction potential of \(Mg^{2+}/Mg, Pb^{2+}/Pb\) and \(Sn^{2+}/Sn\) are -2.36, -0.13 and -0.14 V respectively. The reaction \(X+Y^{2+}\rightarrow X^{2+}+Y\) will be spontaneous when:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \(X=Mg, Y=Pb\)
In the reaction X + \( \ce{Y^{2+}} \) → \( \ce{X^{2+}} \) + Y, the metal X is oxidised (anode) and \( \ce{Y^{2+}} \) is reduced (cathode). The reaction is spontaneous when
E°cell = E°\( \ce{Y^{2+}} \)/Y − E°\( \ce{X^{2+}} \)/X > 0
that is, when X has the more negative reduction potential (stronger reducing agent) than Y.
For X = Mg, Y = Pb: E°cell = (−0.13) − (−2.36) = +2.23 V, positive, so Mg displaces Pb from \( \ce{Pb^{2+}} \) spontaneously.
Why the other options are incorrect
Option (2) \(X=Pb, Y=Sn\) — Incorrect. E°cell = (−0.14) − (−0.13) = −0.01 V; negative, so Pb cannot reduce \( \ce{Sn^{2+}} \).
Option (3) \(X=Pb, Y=Mg\) — Incorrect. E°cell = (−2.36) − (−0.13) = −2.23 V; Pb is far too weak a reducing agent to reduce \( \ce{Mg^{2+}} \).
Option (4) \(X=Sn, Y=Mg\) — Incorrect. E°cell = (−2.36) − (−0.14) = −2.22 V; negative, not spontaneous.
NCERT (2026-27 edition):
Measurement of Electrode Potential – “… 1. A negative E⊖ means that the redox couple is a stronger reducing agent than the H+/H2 couple.
2. A positive E⊖ means that the redox couple is a weaker reducing agent than the H+/H2 couple.”