What is hydration enthalphy?

Hydration enthalpy (ꕔHHyd) is the change in enthalpy when one mole of gaseous ion under a standard condition of 1 bar pressure dissolves in a sufficient amount of water to form an infinitely dilute solution (infinite dilution means a further addition of solute will not cause any heat change).

In simple terms, enthalpy of hydration is described as the amount of energy released on dilution of one mole of gaseous ions. It can be considered as enthalpy of solvation with the solvent being water. Hydration enthalpy is also called hydration energy and its values are always negative.

For a chemical reaction,

M+(g) + aq → M+(aq) Enthalpy change = ∆HHyd

Water is considered to be a polar solvent because it has a positive (H atom) and negative (O atom) poles. When an ionic compound (any salt, say NaCl) is dissolved in water the solid-state structure of the compound is destroyed and the Na+ and Cl– are separated.

In the solution, they exist as Na+ atoms surrounded by the negative ends of water molecules and similarly the Cl– is surrounded by the positive end of water molecules as shown in figure (i). Since additional bonds are formed between the atoms the process releases some energy.

This energy is expressed as the hydration enthalpy or enthalpy of hydration between M+(g) and M+(aq) is that in M+(aq) the ion is surrounded by water molecules forming a weak bond.

Explanation: