Question

Ionic mobility of which of the following alkali metal ions is lowest when aqueous solution of their salts are put under an electric field?

• A. Na
• B. K
• C. Rb
• D. Li

Answer 1

The Correct Option is D

The question asks about which alkali metal ion has the lowest ionic mobility when its aqueous solution is placed under an electric field. To answer this, we need to understand the concept of ionic mobility in aqueous solutions.

Ionic Mobility Explanation:

Ionic mobility refers to the speed at which an ion moves through a solvent when subjected to an electric field. It is influenced by the size of the ion and its charge. In general, smaller ions with higher charge density have higher interactions with the surrounding water molecules, which results in lower mobility. On the contrary, larger ions have lower charge density and, thus, move more freely in the solvent.

In the case of alkali metals, the major factor affecting ionic mobility in water is the hydration or the number of water molecules attracted and attached to the ion. This hydrates the ion, effectively increasing its size and altering its mobility. Alkali metal ions are in the order of increasing ionic radii as follows: Li⁺ < Na⁺ < K⁺ < Rb⁺.

Analysis of Options:

1. Li⁺: Lithium ions are smallest in size but have strongly bound hydration shells, significantly increasing their effective ionic size in solution. Thus, Li⁺ has high interaction with water, resulting in the lowest mobility.
2. Na⁺: Sodium ions are larger than lithium ions and have less extensive hydration, leading to increased mobility compared to Li⁺.
3. K⁺: Potassium ions are larger than sodium ions, hence with even less hydration of the ions, they possess even higher ionic mobility.
4. Rb⁺: Rubidium ions are the largest among the options mentioned and have the least hydration, thus the highest mobility among the options.

Conclusion: Based on the above analysis, Li⁺ has the lowest ionic mobility due to its strong hydration leading to a larger effective size compared to other alkali metal ions. Thus, the correct answer is:

Li