Enthalpy of hydration of group 2 metal oxides

[vijayram]

Arrange the energy released upon hydration of Magnesium Oxide,calcium oxide,barium oxide,strontium oxide.

I placed Magnesium oxide at the last,since magnesium hydroxide is a stable so lot of energy must be released in order to achieve that state,but the answer says calcium oxide.

 

[mjc123]

This is not simple or obvious, because it involves the balance between large positive and negative enthalpy terms. See for example this discussion: http://www.chemicalforums.com/index.php?topic=81896.msg297960#msg297960