JEE Main 2021 · 16 March · morning · Single correct · +4 / −1
Given below are two statements :
Statement I : The Eᵒ value for Ce⁴⁺ / Ce³⁺ is + 1.74 V.
Statement II : Ce is more stable in Ce⁴⁺ state than Ce³⁺ state.
In the light of the above statements, choose the most appropriate answer from the options given below :
- (a) Statement I is incorrect but statement II is correct
- (b) Both statement I and statement II are correct
- (c) Both statement I and statement II are incorrect
- (d) Statement I is correct but statement II is incorrect
Answer and solution
Answer (d) Statement I is correct but statement II is incorrect
Solution
Chemistry ma’am · 4 steps
A large positive E° means Ce⁴⁺ grabs an electron, so +3 is cerium's stable state.
- Statement I. The standard reduction potential of the Ce⁴⁺/Ce³⁺ couple is +1.74 V, the value quoted in NCERT. So statement I is correct.
Ce⁴⁺ + e⁻ → Ce³⁺ E° = +1.74 V
- Read the sign. A large positive E° means this reduction is strongly favoured. Ce⁴⁺ readily accepts an electron and becomes Ce³⁺, so Ce⁴⁺ is a strong oxidising agent. This is why ceric salts are used as oxidants in titrations.
- Statement II. Ce⁴⁺ exists because it has the noble-gas 4f⁰ configuration. But it readily falls back to Ce³⁺, and +3 is the characteristic oxidation state of all lanthanoids. So cerium is more stable as Ce³⁺, and statement II is incorrect.
- So. Statement I is correct and statement II is incorrect, which is option (d).
The idea to keep. For lanthanoids, +3 is home. If a +4 ion has a large positive E° for reduction to +3, it is a strong oxidant, which tells you +4 is the less stable state.
Answer from the NTA final answer key. Solution by PrepWiser.