The standard enthalpy change of hydration is the enthalpy change that takes place when 1 mole of isolated gaseous ions dissolve in water forming 1 mole of aqueous ions under standard conditions.Hydration enthalpy is affected by size as the ionic radius becomes smaller the hydration enthalpy becomes more negative. A smaller ion is better able to attract and bond with water molecules. Smaller ions exert more attraction on the water molecules and more energy is released. As the charge increases, there is a greater attraction on the water molecule. The enthalpy of hydration is therefore more negative.