Question

Hydration enthalpy of which of the following ions is highest?

• A. Mg\(^{2+}\)
• B. Na\(^{+}\)
• C. Ca\(^{2+}\)
• D. K\(^{+}\)

Hint:

Hydration enthalpy \(\propto \frac{\text{charge}^2}{\text{ionic radius}}\). Small, highly charged ions have the highest values.

Answer 1

The Correct Option is A

Step 1: Use charge density as the key trend.
Hydration enthalpy increases with higher ionic charge and smaller ionic radius (higher charge density). Among Na\(^+\), K\(^+\), Ca\(^{2+}\), Mg\(^{2+}\): the +2 ions are far more hydrated than +1 ions.

Step 2: Compare Mg\(^{2+}\) vs Ca\(^{2+}\).
Mg\(^{2+}\) has a smaller radius than Ca\(^{2+}\), giving it the highest charge density and the greatest hydration enthalpy.
\[ \boxed{Mg^{2+}} \]