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Solubility of Alkaline Earth Metal Sulphates
Concepts tested here
- Hydration energy versus lattice energy
All Questions
2007 AIPMT-PRE 1 question
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In which of the following the hydration energy is higher than the lattice energy?A salt dissolves in water when its hydration energy is higher than its lattice energy.
For the sulphates of the alkaline earth metals, the lattice energy remains almost constant down the group, because the sulphate ion is so large that small changes in the size of the cation do not make any difference.
The hydration energy, however, decreases down the group from $Be^{2+}$ to $Ba^{2+}$ as the ionic size increases.
So the solubility of the sulphates decreases down the group: $BeSO_4 \gt MgSO_4 \gt CaSO_4 \gt SrSO_4 \gt BaSO_4$
$BeSO_4$ and $MgSO_4$ are readily soluble in water because the high hydration energy of the small $Be^{2+}$ and $Mg^{2+}$ ions exceeds the lattice energy.
$SrSO_4$, $BaSO_4$ and $RaSO_4$ are practically insoluble; their lattice energy is higher than the hydration energy.
So the hydration energy is higher than the lattice energy in $MgSO_4$.