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 is the amount of energy released when ions are surrounded by water molecules. It is inversely related to the size of the ion; smaller ions have higher hydration enthalpies because they can be more effectively surrounded by water molecules.

Let's analyze the given ions:

• K^+ (Potassium ion)
• Rb^{2+} (Rubidium ion, presuming this is a typo since Rubidium typically forms Rb^+)
• Mg^{2+} (Magnesium ion)
• Cs^+ (Cesium ion)
• Ca^{2+} (Calcium ion)

The general trend for hydration enthalpies is that divalent ions (+2 charge) tend to have larger hydration enthalpies than monovalent ions (+1 charge) due to their higher charge density. Among elements in the same charge state, smaller ionic radius results in higher hydration enthalpy.

1. Both Mg^{2+} and Ca^{2+} have a higher charge of +2 compared to the monovalent ions, increasing their hydration enthalpy.
2. Among Mg^{2+} and Ca^{2+}, Mg^{2+} is smaller than Ca^{2+}, thus Mg^{2+} has the highest hydration enthalpy.
3. Following this, we consider Ca^{2+}.
4. Among the monovalent ions, K^+, Rb^+, \text{and } Cs^+, the size increases down the group in the periodic table.
5. Therefore, K^+ will have higher hydration enthalpy than Rb^+ and Cs^+ because it is smaller.
6. Finally, Cs^+ has the lowest hydration enthalpy due to its larger size.

Thus, arranging them in decreasing order of hydration enthalpies, we get:

C \gt E \gt A \gt B \gt D

Therefore, the correct answer is the option: C > E > A > B > D.