JEE Chemistry 2024 Question with Solution

Given: The ions are $\text{Lu}^{3+}$, $\text{Gd}^{3+}$, $\text{Eu}^{2+}$ and $\text{Ce}^{4+}$.

Find: Which species acts as a strong reducing agent.

A reducing agent donates electrons to other substances and itself gets oxidized. So we must identify the ion that can most readily lose an electron.

• $\text{Lu}^{3+}$: Lutetium commonly exists stably in the $+3$ oxidation state, so it does not readily lose electrons further.
• $\text{Gd}^{3+}$: Gadolinium is also stable in the $+3$ oxidation state, so it is not a strong reducing agent.
• $\text{Eu}^{2+}$: Europium in the $+2$ oxidation state can easily lose one electron to form the more stable $+3$ state.
• $\text{Ce}^{4+}$: Cerium in the $+4$ oxidation state tends to accept electrons to form $\text{Ce}^{3+}$, so it behaves as an oxidizing agent.

The relevant oxidation change is

Thus, $\text{Eu}^{2+}$ has a strong tendency to donate an electron and therefore acts as a strong reducing agent.

Therefore, the correct option is C, that is $\text{Eu}^{2+}$.

Given: The question compares lanthanoid ions in different oxidation states.

Find: The strongest reducing agent among them.

The strength of a reducing agent depends on how easily it gets oxidized. Among the given species, $\text{Eu}^{2+}$ is less stable than $\text{Eu}^{3+}$ and therefore readily undergoes oxidation to the $+3$ state.

In contrast, $\text{Lu}^{3+}$ and $\text{Gd}^{3+}$ are already in stable $+3$ states, while $\text{Ce}^{4+}$ more readily gains an electron and acts as an oxidizing agent.

Hence, the strong reducing agent is $\text{Eu}^{2+}$, so the correct option is C.