Which metal is the most powerful reducing agent in aqueous solution ?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Lithium
The reducing power of a metal in aqueous solution is measured by its standard electrode potential: the more negative the value of E°(M+/M), the more easily the metal loses electrons in water, and the stronger it is as a reducing agent. Lithium has the lowest (most negative) electrode potential, so it is the most powerful reducing agent in aqueous solution:
Li (−3.05 V) < K (−2.93 V) < Ba (−2.90 V) < Na (−2.71 V)
The very small \( \ce{Li+} \) ion has a very high hydration enthalpy, which more than makes up for the high ionisation enthalpy of lithium.
Why the other options are incorrect
Option (1) Potassium — Incorrect. It is more reactive than lithium towards water, but its E° (−2.93 V) is less negative, so in aqueous solution it is a weaker reducing agent.
Option (2) Sodium — Incorrect. E° = −2.71 V, less negative than lithium.
Option (3) Barium — Incorrect. E°(\( \ce{Ba^{2+}} \)/Ba) = −2.90 V, less negative than lithium.
NCERT (2026-27 edition):
Measurement of Electrode Potential – “… Lithium has the lowest electrode potential indicating that lithium ion is the weakest oxidising agent while lithium metal is the most powerful reducing agent in an aqueous solution.”