JEE Main 2023 · 29 January · morning · Single correct · +4 / −1
The correct order of hydration enthalpies is
(A) K⁺
(B) Rb⁺
(C) Mg²⁺
(D) Cs⁺
(E) Ca²⁺
Choose the correct answer from the options given below :
- (a) E > C > A > B > D
- (b) C > A > E > B > D
- (c) C > E > A > D > B
- (d) C > E > A > B > D
Answer and solution
Answer (d) C > E > A > B > D
Solution
Chemistry ma’am · 3 steps
Charge first, size second: a small, highly charged ion holds water molecules the tightest.
- What decides it. Hydration enthalpy depends on charge density, that is, charge divided by size. A higher charge or a smaller radius pulls water dipoles in more strongly and releases more energy.
- Charge comes first. The 2+ ions Mg²⁺ and Ca²⁺ are far ahead of the 1+ ions. Within each charge group, the smaller ion wins, and ions grow larger down a group.
Ion Charge Ionic radius (pm) Hydration enthalpy (kJ mol⁻¹) (C) Mg²⁺ +2 72 -1921 (E) Ca²⁺ +2 100 -1577 (A) K⁺ +1 138 -330 (B) Rb⁺ +1 152 -310 (D) Cs⁺ +1 167 -276 - So. Ranked by how much energy is released, the order is Mg²⁺ > Ca²⁺ > K⁺ > Rb⁺ > Cs⁺, which is C > E > A > B > D, option (d).
The idea to keep. To rank hydration enthalpies, sort by charge first, then by size. Down any group the ion gets bigger, so its hydration enthalpy falls.
Answer from the NTA final answer key. Solution by PrepWiser.