JEE Main 2025 · 4 April · evening · Single correct · +4 / −1
The incorrect relationship in the following pairs in relation to ionisation enthalpies is :
- (a) Mn²⁺<Fe²⁺
- (b) Mn⁺<Mn²⁺
- (c) Mn⁺<Cr⁺
- (d) Fe²⁺<Fe³⁺
Answer and solution
Answer (a) Mn²⁺<Fe²⁺
Solution
Chemistry ma’am · 4 steps
For each ion, ask which electron leaves next and whether it breaks a half-filled 3d⁵ shell.
- What is compared. The ionisation enthalpy of an ion is the energy needed to pull one more electron off that ion. So write each ion's configuration and see which electron goes.
- Two rules. A higher positive charge holds the remaining electrons more tightly. Removing an electron from a half-filled 3d⁵ shell costs extra energy, because that arrangement is especially stable.
Pair Configurations Approx. energy (kJ mol⁻¹) Given order (a) Mn²⁺ vs Fe²⁺ 3d⁵ vs 3d⁶ 3248 vs 2957 wrong: Mn²⁺ is higher (b) Mn⁺ vs Mn²⁺ 3d⁵ 4s¹ vs 3d⁵ 1509 vs 3248 correct (c) Mn⁺ vs Cr⁺ 3d⁵ 4s¹ vs 3d⁵ 1509 vs 1591 correct (d) Fe²⁺ vs Fe³⁺ 3d⁶ vs 3d⁵ 2957 vs 5290 correct - Why (a) fails. Fe²⁺ (3d⁶) loses its sixth d electron and becomes the stable 3d⁵, so it ionises easily. Mn²⁺ (3d⁵) would have to break its half-filled shell, so its ionisation enthalpy is larger.
- So. The incorrect relationship is (a). The true order is Mn²⁺ > Fe²⁺.
The idea to keep. Ionisation enthalpy is high when the electron must come out of a half-filled or full shell, and low when removing it creates one. Check configurations before trusting nuclear charge.
Answer from the NTA final answer key. Solution by PrepWiser.