JEE Chemistry 2025 Question with Solution

Given: We must identify the incorrect relation among the listed ionisation enthalpy comparisons.

Find: The option that gives the wrong order.

Ionisation enthalpy depends on electronic configuration, stability, and the extra stability of half-filled orbitals.

From the solution discussion:

$$\text{Cr}^{3+} \text{ has a stable } 3d^3 \text{ / special stability consideration, so its ionisation enthalpy is higher than that of } \text{Mn}^{2+}$$

Hence,

$$\text{Mn}^{2+} < \text{Cr}^{3+}$$

is taken as correct.

For the same element, a higher positive charge generally means higher ionisation enthalpy because of greater effective nuclear charge. Therefore,

$$\text{Mn}^{2+} < \text{Mn}^{3+}$$

is correct.

The solution then points out a discrepancy for iron ions. Although the listed option says

$$\text{Fe}^{2+} < \text{Fe}^{3+}$$

removing an electron from $\text{Fe}^{3+}$ disturbs a relatively stable configuration, so this listed relation is treated as incorrect in the solution.

The source solution explicitly states that the correct option is D. It also notes that options C and D are identical, indicating a likely misprint in the question data.

Therefore, the correct option is D.

Given: Comparisons of ionisation enthalpies for $\text{Mn}^{2+}$, $\text{Cr}^{3+}$, $\text{Mn}^{3+}$, $\text{Fe}^{2+}$ and $\text{Fe}^{3+}$.

Find: Which relation is incorrect.

1. For

$$\text{Mn}^{2+} < \text{Cr}^{3+}$$

the solution attributes the larger ionisation enthalpy of $\text{Cr}^{3+}$ to extra electronic stability, so this is accepted as correct.

2. For

$$\text{Mn}^{2+} < \text{Mn}^{3+}$$

the higher charged ion of the same element usually has greater ionisation enthalpy due to stronger attraction for electrons. So this is correct.

3. For

$$\text{Fe}^{2+} < \text{Fe}^{3+}$$

the second solution notes that removing an electron from $\text{Fe}^{3+}$ would disturb a stable configuration, so this order is considered incorrect.

4. The fourth option repeats the third option exactly. The solution still marks D as the answer.

So, based on the solution, the intended incorrect relation is the repeated iron comparison, and the marked answer is D.

Note: There is a clear duplication/misprint in the options because C and D are identical.