The correct order of hydration enthalpies is
(A) `K^(+),` (B) `Rb^(+),` (C) `Mg^(2+),` (D) `Cs^(+),` (E) `Ca^(2+)`
Choose the correct answer from the options given below:

Options

A. `CgtEgtAgtBgtD`
B. `CgtEgtAgtDgtB`
C. `CgtAgtEgtBgtD`
D. `EgtCgtAgtBgtD]`

Answer

To determine the correct order of hydration enthalpies for the given cations, we need to consider the factors that influence hydration enthalpy. Hydration enthalpy is the energy released when ions are surrounded by water molecules. This property is influenced primarily by the size of the ions: smaller ions typically have higher hydration enthalpies due to their greater charge density, which allows them to attract water molecules more effectively.

### Step-by-Step Solution:

1. **Identify the Cations and Their Sizes**:
- The cations given are: K⁺ (Potassium), Rb⁺ (Rubidium), Mg²⁺ (Magnesium), Cs⁺ (Cesium), and Ca²⁺ (Calcium).
- In terms of ionic size, the order of these cations from smallest to largest is:
- Mg²⁺ < Ca²⁺ < K⁺ < Rb⁺ < Cs⁺

2. **Understanding Hydration Enthalpy**:
- Hydration enthalpy is inversely proportional to the size of the ion. This means that smaller ions will have higher hydration enthalpy because they can attract water molecules more effectively.

3. **Determine the Order of Hydration Enthalpy**:
- Since Mg²⁺ is the smallest ion, it will have the highest hydration enthalpy.
- Following Mg²⁺, Ca²⁺ will have the next highest hydration enthalpy, followed by K⁺, Rb⁺, and finally Cs⁺, which has the largest size and thus the lowest hydration enthalpy.
- Therefore, the order of hydration enthalpies from highest to lowest is:
- Mg²⁺ > Ca²⁺ > K⁺ > Rb⁺ > Cs⁺

4. **Assign the Cations to the Options**:
- Based on the order derived:
- C (Mg²⁺) > E (Ca²⁺) > A (K⁺) > B (Rb⁺) > D (Cs⁺)
- This corresponds to the order CEABD.

5. **Conclusion**:
- The correct order of hydration enthalpies is CEABD.