Question

Which of the following cations has the highest hydration energy?
A) $N{a^ + }$
B) $M{g^{2 + }}$
C) $A{l^{3 + }}$
D) $C{a^{2 + }}$

Answer 1

Hint: Hydration means the interaction of ions with water. The energy required for hydrating one mole of an ion is known as the hydration energy also known as hydration enthalpy. Hydration energy or enthalpy depends on the charge density of the ions.

Complete answer:
Hydration energy is the energy released or taken up when one mole of ion reacts with water i.e. undergo hydration. The general equation for this reaction can be given as follows.
${M^ + } + {H_2}O \to {M^ + }\left( {aq} \right)$
Here, we have M as an ion and it can be in either solid or gaseous state. Hydration energy also explains the solubility of a compound or an ion in water. When a compound is mixed with water, the lattice bonds between the compound break and they get separated as ions. These ions are surrounded by water molecules.

Hence, we can understand that if the hydration energy is equal or greater than the lattice energy, then the compound will be soluble in water. If the hydration energy is less than lattice energy then the compound is insoluble in water.

Given to us are four ions with different charges on them. We can establish a relationship between hydration energy and the charge present on the ions since we know that the hydration energy is dependent on the charge density.

Therefore, hydration energy is directly proportional to the charge on the ions. Among the given ions, $A{l^{3 + }}$ has the highest charge. This means that it would have the highest hydration energy.

Therefore, the correct answer is option C.

Note: It is to be noted that the hydration energy or hydration enthalpy also depends on the size of the ion. The hydration energy is inversely proportional to the size of the ion which means that ions that are smaller in size have higher hydration energies.