Which of the following ions has the highest heat of hydration?

Heat of hydration is the amount of energy released when ions are surrounded by water molecules in an aqueous solution. It depends on two main factors: the charge of the ion and its size. Higher charged ions and smaller ions have stronger interactions with water molecules, leading to a higher heat of hydration.

Among the given ions, Al³⁺ has a +3 charge and a very small ionic radius. The combination of high charge and small size results in very strong attraction between the Al³⁺ ion and water molecules. This strong electrostatic interaction releases a large amount of energy, giving Al³⁺ the highest heat of hydration.

Mg²⁺ and Ba²⁺ have a +2 charge. Magnesium ion is smaller than barium, so Mg²⁺ has a higher heat of hydration than Ba²⁺, but still much lower than Al³⁺ because of the smaller charge. Cs⁺ has a +1 charge and a very large ionic radius, so it has the lowest heat of hydration among the listed ions.

The concept of heat of hydration is crucial in inorganic chemistry for understanding solubility, ionic interactions, and lattice energy. Hydration energy also helps explain why some salts are highly soluble in water while others are not. For example, Al³⁺ salts like AlCl₃ dissolve easily due to the large hydration energy of the ion.

In summary, the high charge-to-size ratio of Al³⁺ makes it strongly attract water molecules. This property is a fundamental concept in physical and inorganic chemistry, illustrating how ionic charge and size determine hydration behavior in aqueous solutions.