JEE Main 2024 · 1 February · evening · Single correct · +4 / −1
Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R) .
Assertion (A) : In aqueous solutions Cr²⁺ is reducing while Mn³⁺ is oxidising in nature.
Reason (R) : Extra stability to half filled electronic configuration is observed than incompletely filled electronic configuration.
In the light of the above statements, choose the most appropriate answer from the options given below :
- (a) (A) is true but (R) is false
- (b) Both (A) and (R) are true and (R) is the correct explanation of (A)
- (c) Both (A) and (R) are true but (R) is not the correct explanation of (A)
- (d) (A) is false but (R) is true
Answer and solution
Answer (b) Both (A) and (R) are true and (R) is the correct explanation of (A)
Solution
Chemistry ma’am · 3 steps
Both ions are 3d⁴. Each one gains or loses an electron to reach a half-filled, extra-stable set.
- Same starting point. Cr²⁺ and Mn³⁺ are both 3d⁴. What decides their behaviour is which neighbouring configuration is more stable.
Ion Configuration Change in water Product configuration Behaviour Cr²⁺ 3d⁴ loses one electron to give Cr³⁺ 3d³, i.e. t_(2g)³, a half-filled t_(2g) set reducing Mn³⁺ 3d⁴ gains one electron to give Mn²⁺ 3d⁵, a half-filled d subshell oxidising - Check each statement. (A) is true: Cr²⁺ gives away an electron and Mn³⁺ takes one. (R) is true: half-filled configurations carry extra stability.
- So. Both ions change in the direction of a half-filled configuration, so (R) is exactly the reason behind (A). The answer is (b).
The idea to keep. To judge whether a transition metal ion is reducing or oxidising, ask which way it reaches extra stability: d⁵, or t_(2g)³ in water. It moves towards that side.
Answer from the NTA final answer key. Solution by PrepWiser.