JEE Main 2023 · 24 January · evening · Single correct · +4 / −1

Which one amongst the following are good oxidizing agents?
A. Sm²⁺
B. Ce²⁺
C. Ce⁴⁺
D. Tb⁴⁺
Choose the most appropriate answer from the options given below :

  1. (a) C and D only
  2. (b) A and B only
  3. (c) D only
  4. (d) C only
Answer and solution

Answer (a) C and D only

Solution

Chemistry ma’am · 4 steps

For lanthanoids, +3 is home: ions above +3 grab an electron, ions below +3 give one away.

  1. Stable state. For lanthanoids, +3 is the most stable oxidation state. Any ion in another state tends to return to +3.
  2. Which way it goes. A +4 ion returns to +3 by gaining an electron, so it oxidises something else: it is an oxidising agent. A +2 ion returns to +3 by losing an electron, so it is a reducing agent.
    IonOxidation stateChange to reach +3Acts as
    Sm²⁺+2loses an electronreducing agent
    Ce²⁺+2loses an electronreducing agent
    Ce⁴⁺+4gains an electronoxidising agent
    Tb⁴⁺+4gains an electronoxidising agent
  3. Why +4 exists. Ce⁴⁺ (4f⁰) and Tb⁴⁺ (4f⁷) form because empty and half-filled f shells are extra stable. They still prefer +3, and E° for Ce⁴⁺/Ce³⁺ is about +1.74 V, so Ce⁴⁺ is a strong oxidant.
  4. So. Only the +4 ions, C (Ce⁴⁺) and D (Tb⁴⁺), are good oxidising agents. The answer is (a).

The idea to keep. Compare a lanthanoid ion with +3. Above +3 it takes electrons and oxidises others. Below +3 it gives electrons away and reduces others.

Answer from the NTA final answer key. Solution by PrepWiser.

Study d & f Block Elements

Asked about 170 times in JEE Main since 2020. This question is from 2023.