Why is Ce4+ a good oxidizing agent in aqueous whereas Eu2+ and Yb2+ ions are good reducing agents?

 The oxidizing and reducing behavior of Ce⁴⁺ (Cerium), Eu²⁺ (Europium), and Yb²⁺ (Ytterbium) in aqueous solutions can be explained by analyzing their electronic configurations, stability, and standard electrode potentials.

1. Ce⁴⁺ as a Strong Oxidizing Agent

Cerium in its +4 oxidation state (Ce⁴⁺) readily acts as a strong oxidizing agent in aqueous solutions. This behavior arises due to:

• Electronic Configuration & Stability

  • Ce³⁺: [Xe] 4f¹ (more stable)
  • Ce⁴⁺: [Xe] 4f⁰ (completely empty 4f orbital)
  • The 4f⁰ configuration of Ce⁴⁺ is energetically unstable, making Ce⁴⁺ highly eager to accept an electron and revert to Ce³⁺.

• Standard Electrode Potential (E° value)

  • The redox potential for the reaction: $$\text{Ce⁴⁺} + e⁻ \rightarrow \text{Ce³⁺}$$ is +1.74 V, indicating a strong tendency to gain electrons, thus making Ce⁴⁺ a powerful oxidizing agent.
  • This high E° value means Ce⁴⁺ can easily oxidize other species while itself getting reduced to Ce³⁺.

2. Eu²⁺ and Yb²⁺ as Strong Reducing Agents

Europium (Eu²⁺) and Ytterbium (Yb²⁺) exhibit strong reducing properties because of the relative stability of their respective +2 and +3 oxidation states.

• Electronic Configuration & Stability

  • Eu²⁺: [Xe] 4f⁷ (half-filled 4f orbital, very stable)

  • Eu³⁺: [Xe] 4f⁶ (less stable than Eu²⁺)

  • The half-filled 4f⁷ configuration in Eu²⁺ gives it additional stability, making Eu²⁺ reluctant to lose another electron.

  • As a result, Eu²⁺ easily donates an electron (undergoing oxidation to Eu³⁺), making it a strong reducing agent.

  • Yb²⁺: [Xe] 4f¹⁴ (fully-filled 4f orbital, very stable)

  • Yb³⁺: [Xe] 4f¹³ (less stable than Yb²⁺)

  • Similarly, Yb²⁺ has a fully-filled 4f orbital, which is highly stable. This makes Yb²⁺ reluctant to lose an additional electron, leading to strong reducing behavior.

• Standard Electrode Potentials (E° values)

  • The redox potential for Eu³⁺/Eu²⁺: −0.43 V
  • The redox potential for Yb³⁺/Yb²⁺: −1.14 V
  • Negative E° values indicate that these ions readily donate electrons, thereby behaving as strong reducing agents.

Conclusion

• Ce⁴⁺ is a strong oxidizing agent because its 4f⁰ configuration is unstable, and it has a highly positive E° value (+1.74 V), meaning it readily gains electrons to become Ce³⁺.
• Eu²⁺ and Yb²⁺ are strong reducing agents because they have highly stable 4f⁷ (half-filled) and 4f¹⁴ (fully-filled) configurations, respectively. Their negative E° values indicate a strong tendency to lose electrons and get oxidized to Eu³⁺ and Yb³⁺.