Given below are reduction potentials:
(A) \(E^o_{Cu^{2+}/Cu} = 0.34 V\)
(B) \(E^o_{Sn^{2+}/Sn} = -0.14 V\)
(C) \(E^o_{Zn^{2+}/Zn} = -0.76 V\)
(D) \(E^o_{K^+/K} = -2.93 V\)
The strongest reducing agent from the above is:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → K
A reducing agent gives away electrons and is itself oxidised. The standard reduction potential E° tells how easily the oxidised form takes up electrons; the more negative the E°, the less the ion wants electrons and the more readily the metal gives them up. So the metal with the most negative E° is the strongest reducing agent.
Arranging the given couples:
\( E^\circ: \ \ce{K^+/K} \ (-2.93\,\mathrm{V}) \lt \ce{Zn^2+/Zn} \ (-0.76\,\mathrm{V}) \lt \ce{Sn^2+/Sn} \ (-0.14\,\mathrm{V}) \lt \ce{Cu^2+/Cu} \ (+0.34\,\mathrm{V}) \)
Reducing power: K > Zn > Sn > Cu. Potassium is the strongest reducing agent.
Why the other options are incorrect
Option (1) Cu — Incorrect. Its E° is positive (+0.34 V); Cu is a weaker reducing agent than \( \ce{H2} \), and \( \ce{Cu^{2+}} \) is the best oxidising agent of the four.
Option (2) Zn — Incorrect. A good reducing agent (−0.76 V), but its E° is much less negative than that of K.
Option (3) Sn — Incorrect. Its E° (−0.14 V) is only slightly negative, so tin is a weak reducing agent.
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.”