Question

How do the acidities of the aqueous solutions of the alkaline earth metal ions \(\left(\mathrm{M}^{2+}\right)\) change in going down the group?

Short answer

In summary, the acidity of aqueous solutions of alkaline earth metal ions (M²⁺) decreases when going down the group in the periodic table, primarily due to the decreasing charge density. The order of acidity is: \( \mathrm{Be^{2+} > Mg^{2+} > Ca^{2+} > Sr^{2+} > Ba^{2+}} \).

Step-by-step solution

Write the equations for alkaline earth metal ions interacting with water in aqueous solutions

In an aqueous solution, the alkaline earth metal ions (M²⁺) interact with water molecules and can either act as an acid (donating a proton) or a base (accepting a proton). The general equations for these interactions are: \[ \mathrm{M^{2+} + H_{2}O \rightleftharpoons MOH^{+} + H^{+}} \] (acting as an acid) and \[ \mathrm{M^{2+} + H_{2}O \rightleftharpoons M(OH)_{2} + 2H^{+}} \] (acting as a base) We are interested in the acidity trend, so we will focus on the first equation.

Discuss the factors affecting acidity

The acidity of the aqueous solution increases when the metal ion has a higher ability to donate a proton (H⁺). This depends on the charge density of the ion and its interaction with the water molecules. Smaller ions with higher charge densities polarize the O-H bond in the water molecule, which weakens the bond and allows for easier donation of the H⁺ ion.

Analyze the trend in ion size and charge density

As we go down the group in the periodic table, the ion size of the alkaline earth metal ions increases. The larger the ion, the lower its charge density (charge/volume). The order of the ion size and charge density is: \[ \mathrm{Be^{2+} > Mg^{2+} > Ca^{2+} > Sr^{2+} > Ba^{2+}} \] (for charge density) and \[ \mathrm{Be^{2+} < Mg^{2+} < Ca^{2+} < Sr^{2+} < Ba^{2+}} \] (for ion size)

Determine the trend in acidity

Considering the discussed factors affecting acidity, we can conclude that smaller alkaline earth metal ions with higher charge densities will result in a more acidic aqueous solution by donating protons more readily. Therefore, the acidity of aqueous solutions of alkaline earth metal ions will decrease when going down the group. The order of acidity in aqueous solutions is: \[ \mathrm{Be^{2+} > Mg^{2+} > Ca^{2+} > Sr^{2+} > Ba^{2+}} \] In summary, the acidity of aqueous solutions of alkaline earth metal ions decreases as we go down the group in the periodic table, mainly due to the decreasing charge density.