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JEE Main 2023 · 29 Jan · Shift 1

JEE Main 2023 Chemistry question on Hydrogen and s-Block Elements

The correct order of hydration enthalpies is

(A) K+\text{K}^+
(B) Rb+\text{Rb}^+
(C) Mg2+\text{Mg}^{2+}
(D) Ca+\text{Ca}^+
(E) Ca2+\text{Ca}^{2+}

Choose the correct answer from the options given below:
  1. A.
    E > C > A > B > D
  2. B.
    C > E > A > D > B
  3. C.
    C > A > E > B > D
  4. D.
    C > E > A > B > D
Show answer and solution

Correct option: D

Solution

Hydration enthalpy increases with higher charge density (charge/size ratio). Mg²⁺ has the smallest size among the doubly charged ions and thus the highest charge density, so it has the greatest hydration enthalpy. Ca²⁺ is also doubly charged but larger than Mg²⁺, so it comes next. Among the singly charged ions, K⁺ is smaller than Rb⁺, giving K⁺ a higher hydration enthalpy than Rb⁺. Ca⁺ (singly charged, large radius) has the lowest hydration enthalpy among all. This gives the order: Mg²⁺ (C) > Ca²⁺ (E) > K⁺ (A) > Rb⁺ (B) > Ca⁺ (D).

[JEE Main 2023 · 29 Jan · Shift 1]

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