A2 Chemistry
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Can you explain me the trend in the solubilities of group 2 sulphates and hydroxides with the reason?
Corrie writes ....
The Group 2 hydroxides become more soluble and the sulphates become less soluble as you down the Group.
Explanation of solubility trends is complicated because two factors are involved - the lattice energy of the ionic solid (the energy needed to separate the + and - ions) and the hydration energy of the ions (the energy released when the ions are surrounded (solvated) by water molecules.
The first of these changes is endothermic, the second exothermic, and the difference between them appears as the enthalpy change (heat) of solution.
Both these energies depend on the size of the ions -the bigger the ions, the lower the lattice energy, and the smaller (less negative) the hydration energy.
The hydroxides become more soluble because the lattice energy decreases more rapidly down the Group than the hydration energy. As a result the enthalpy of solution becomes more exothermic down the Group.
For the sulphates the negative ion is much bigger, and the lattice energy decreases less rapidly than the hydration energy. As a result the enthalpy of solution changes from exothermic to endothermic as you go from BeSO4 to BaSO4.
This is a rather simplified answer to what is quite a complicated problem.
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updated: 17 April 2007
