Question

Which of the following will have the highest hydration enthalpy?
Options-
a.\[L{i^ + }\]
b.\[N{a^ + }\]
c.\[B{e^{2 + }}\]
d.\[M{g^{2 + }}\]

Answer 1

Hint: The enthalpy of hydration is dependent on factors like the charge density and the size of the ion. Lithium and sodium are monovalent atoms that can carry a single charge and beryllium and magnesium are divalent atoms that can carry a positive charge of magnitude two.

Complete answer:
Hydration enthalpy is defined as the heat released on dissolution of ions in water at constant pressure.
The enthalpy of hydration is strongly associated with the kind of interactions that the ion shows with water molecules. Water being a polar solvent aggregates around the positively charged cations and forms a hydration cage that stabilizes the ions in solution. This is an exothermic process and the amount of energy released depends on the extent of stabilization.
Smaller cations with higher charge are known to have a higher charge density. A high positive charge density attracts more water molecules due to the higher polarizing power. Thus, those cations will have higher enthalpy of hydration that have smaller ionic radii and higher magnitude of charge on them.
Out of all the cations given in the options, beryllium cation \[B{e^{2 + }}\] has the highest charge density on it because it is the smallest in size and has the highest charge.

\[ \Rightarrow \] Thus, \[B{e^{2 + }}\] has the highest enthalpy of hydration and option (c) is correct.

Note:
The size of atoms increases on descending a group and decreases on moving left to right in a period. Sodium and magnesium are the second members of the first and the second group respectively and are therefore larger than the first members. Beryllium is in the second group and lithium is the first group which makes beryllium the smallest atom.