JEE Main 2023 · 25 January · evening · Single correct · +4 / −1
Which one among the following metals is the weakest reducing agent?
- (a) K
- (b) Na
- (c) Li
- (d) Rb
Answer and solution
Answer (b) Na
Solution
Chemistry ma’am · 3 steps
Line up the standard reduction potentials: the least negative E° is the weakest reducing agent.
- What decides it. A reducing agent gives away electrons. The more negative the standard reduction potential E°(M⁺/M), the more readily the metal is oxidised, so the stronger it is as a reducing agent.
Metal E°(M⁺/M) Reducing strength Li -3.04 V strongest Rb -2.98 V strong K -2.93 V strong Na -2.71 V, least negative weakest - Why not ionisation energy. E° includes three steps: sublimation, ionisation and hydration of the ion. Small Li⁺ releases a very large hydration enthalpy, which more than makes up for its high ionisation energy. That puts Li at the top, and the order is not a simple trend down the group.
- So. Sodium has the least negative E° of the four, so Na is the weakest reducing agent. The answer is (b).
The idea to keep. In water, judge reducing strength by E°, not by ionisation energy alone. Hydration enthalpy makes Li the strongest alkali metal reductant and leaves Na the weakest.
Answer from the NTA final answer key. Solution by PrepWiser.