Question

The correct order of hydration enthalpies is 
(A) K⁺ 
(B) Rb²⁺ 
(C) Mg²⁺ 
(D) Cs⁺ 
(E) Ca²⁺ 
Choose the correct answer from the options given below:

• A. E > C > A > B > D
• B. C > E > A > D > B
• C. C > A > E > B > D
• D. C > E > A > B > D

Hint:

For hydration enthalpy trends:
• Smaller ions with higher charges have greater hydration enthalpies.
• Compare ionic radii and charges to establish trends within groups and periods.

Answer 1

The Correct Option is D

Hydration enthalpy depends on the charge density of ions, which is determined by the charge on the ion and its ionic radius.
Smaller ions with higher charges have higher hydration enthalpies due to stronger electrostatic interactions with water molecules.
For alkali metal ions:
   \[\text{K}^+ > \text{Rb}^+ > \text{Cs}^+ \quad \text{(A > B > D)}.\]
   This is because \(\text{K}^+\) is smaller than \(\text{Rb}^+\) and \(\text{Cs}^+\).
For alkaline earth metal ions:
   \[\text{Mg}^{2+} > \text{Ca}^{2+} \quad \text{(C > E)}.\]
   \(\text{Mg}^{2+}\) has a smaller radius and higher charge density than \(\text{Ca}^{2+}\).
Final Order:
\[\text{Mg}^{2+} > \text{Ca}^{2+} > \text{K}^+ > \text{Rb}^+ > \text{Cs}^+ \quad \text{(C > E > A > B > D)}.\]
Final Answer: \((4)\) \(\mathbf{C > E > A > B > D}\).