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 question asks for the correct order of hydration enthalpies of alkali metal ions. In order to determine this, we need to understand the concept of hydration enthalpy and how it varies with alkali metals.

Hydration enthalpy is the energy released when ions are surrounded by water molecules. For alkali metal ions, hydration enthalpy is influenced by the size of the ion; smaller ions have higher charge density and attract water molecules more strongly, resulting in higher hydration enthalpy.

Alkali metals in the periodic table are arranged as follows based on increasing atomic number: Li, Na, K, Rb, Cs.

The size of the ion increases as we move down the group from Li⁺ to Cs⁺. Thus, the charge density decreases, meaning the hydration enthalpy also decreases.

Based on this understanding, the correct order of hydration enthalpies from highest to lowest is:

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

This order corresponds to the first option provided in the question, which is:

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

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

Other options are incorrect as they do not follow the proper trend based on ionic size and charge density. Remember, in general, smaller ions release more energy upon hydration due to their greater attraction to the polar water molecules.