Energy changes and bonding
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Why is enthalpy of hydration always exothermic?
Corrie writes .......
The process of hydration is a result of the attraction between the negative ends of polar water molecules and positive metal ions.
Sometimes this attraction is strong enough to lead to the formation of dative bonds between the water molecules and the metal ion, resulting in the formation of a complex ion, such as [Fe(H2O)6]2+ which is stable enough to exist not only in solution but also in crystals of iron(II) salts, such as iron(II) sulphate.
In other cases, e.g. for Na+, the attraction is only strong enough to surround the metal ion in solution with a 'coat' of water molecules, but these are lost when the metal compound comes out of solution as a crystalline solid, e.g. NaCl.
So, enthalpy of hydration is always exothermic because it involves attraction, and therefore bonding (however weak) between metal ions and water molecules. When bonding occurs, energy is released. Conversely, to break the 'bonding' by separating the water molecules and metal ions from one another would require energy, and therefore be an endothermic process.
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updated: 07 January 2008
