Question

Statement-I : Consider the following pairs of ions $(\text{Sc}^{3+}, \text{Ti}^{3+}), (\text{Ti}^{4+}, \text{Ni}^{2+}), (\text{Cu}^{2+}, \text{Zn}^{2+})$ and $(\text{Cr}^{3+}, \text{Mn}^{3+})$. Out of these pairs three pairs consist of ions that are both coloured :
Statement-II : Among the lanthanide ions $\text{Eu}^{2+}$, $\text{Gd}^{3+}$, $\text{Ce}^{4+}$ and $\text{Tb}^{4+}$, the ion $\text{Tb}^{4+}$ is the strongest reducing agent.
Choose the correct option.

• A. Both statements are true
• B. Both statements are false
• C. Statement-I is true and statement-II is false
• D. Statement-I is false and statement-II is true

Hint:

Ions with empty ($d^0$) or full ($d^{10}$) d-orbitals are colorless. In lanthanides, $+4$ ions are oxidizers and $+2$ ions are reducers as they strive for the $+3$ state.

Answer 1

The Correct Option is B

Check Statement I:
1. $(\text{Sc}^{3+}, \text{Ti}^{3+})$: $\text{Sc}^{3+}$ ($3d^0$, colorless), $\text{Ti}^{3+}$ ($3d^1$, colored). Not both colored.
2. $(\text{Ti}^{4+}, \text{Ni}^{2+})$: $\text{Ti}^{4+}$ ($3d^0$, colorless), $\text{Ni}^{2+}$ ($3d^8$, colored). Not both colored.
3. $(\text{Cu}^{2+}, \text{Zn}^{2+})$: $\text{Cu}^{2+}$ ($3d^9$, colored), $\text{Zn}^{2+}$ ($3d^{10}$, colorless). Not both colored.
4. $(\text{Cr}^{3+}, \text{Mn}^{3+})$: $\text{Cr}^{3+}$ ($3d^3$, colored), $\text{Mn}^{3+}$ ($3d^4$, colored). Both colored.
Only one pair consists of ions that are both colored. Statement I says "three pairs", which is False.
Check Statement II:
Lanthanides are most stable in the $+3$ oxidation state.
$\text{Tb}^{4+}$ ($4f^7$) tends to gain an electron to become $\text{Tb}^{3+}$ ($4f^8$). Since it gains electrons, it acts as an oxidizing agent.
$\text{Eu}^{2+}$ ($4f^7$) tends to lose an electron to become $\text{Eu}^{3+}$ ($4f^6$). Since it loses electrons, it acts as a reducing agent.
$\text{Tb}^{4+}$ is an oxidizing agent, not a reducing agent. Statement II is False.
Both statements are false.