Question

The correct order of hydration enthalpies of alkali metal ions is -

• A. \(\c{ Li^{+} > Na^{+} > K^{+} > Rb^{+} > Cs^{+} }\)
• B. \(\c{ Li^{+} > Na^{+} > K^{+} > Cs^{+} > Rb^{+} }\)
• C. \(\c{ Na^{+} >Li^{+} > K^{+} > Rb^{+} > Cs^{+} }\)
• D. \(\c{ Na^{+} > Li^{+} >K^{+} > Cs^{+} > Rb^{+} }\)

Answer 1

The Correct Option is A

The hydration enthalpy of an ion is the energy released when 1 mole of gaseous ions dissolve in water to form hydrated ions. It is a measure of the attraction between the ion and water molecules. Several factors influence the magnitude of hydration enthalpy, primarily the size and charge of the ion. 

In the case of alkali metal ions, which all have a +1 charge, the determining factor becomes the ionic size. The smaller the ion, the greater the charge density, which results in higher hydration enthalpy because the ion can interact more effectively with the polar water molecules.

The order of the alkali metal ions by increasing size is:

• \(\text{Li}^+\)
• \(\text{Na}^+\)
• \(\text{K}^+\)
• \(\text{Rb}^+\)
• \(\text{Cs}^+\)

Consequently, the correct order of hydration enthalpies, which is inversely proportional to size, is:

• \(\text{Li}^+ > \text{Na}^+ > \text{K}^+ > \text{Rb}^+ > \text{Cs}^+\)

This order confirms that lithum ions have the highest hydration enthalpy due to their small size and consequently high charge density, making them more effective at interacting with water molecules. As we move down the group, the ions increase in size, reducing charge density and thereby decreasing hydration enthalpy.

Therefore, the correct answer is: \(\text{Li}^+ > \text{Na}^+ > \text{K}^+ > \text{Rb}^+ > \text{Cs}^+\).