Which is a stronger reducing agent or and why?
Cr²⁺ is a stronger reducing agent than Fe²⁺.
When Cr²⁺ (d⁴ configuration) gets oxidised to Cr³⁺, it attains a d³ configuration. This d³ configuration has a half-filled t₂g level which provides extra stability to the Cr³⁺ ion.
On the other hand, when Fe²⁺ (d⁶ configuration) gets oxidised to Fe³⁺, it attains a d⁵ configuration. In an aqueous medium, the d³ configuration is more stable compared to d⁵ configuration based on Crystal Field Stabilization Energy (CFSE). This greater stability of the product (Cr³⁺) makes Cr²⁺ a stronger reducing agent as it more readily undergoes oxidation.
Explanation
The textbook directly states that Cr²⁺ is a stronger reducing agent than Fe²⁺. The reason lies in the electronic configurations and their stability. Cr²⁺ has d⁴ configuration which changes to d³ (Cr³⁺) upon oxidation. The d³ configuration has a half-filled t₂g level, making it very stable. Fe²⁺ has d⁶ configuration which changes to d⁵ (Fe³⁺) upon oxidation. In aqueous medium, d³ is more stable than d⁵ configuration based on CFSE values. This stability difference explains why Cr²⁺ is a stronger reducing agent.