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Which of the following alkaline earth metal sulphates has hydration enthalpy higher than the lattice enthalpy?
A
$SrSO_4$
B
$CaSO_4$
C
$BeSO_4$
D
$BaSO_4$
Detailed Solution
A salt dissolves readily in water when its hydration enthalpy exceeds its lattice enthalpy.
For the sulphates of group 2, the lattice enthalpy is almost constant, because the $SO_4^{2-}$ ion is so large that a change in cation size makes little difference.
The hydration enthalpy, however, decreases down the group as the cation becomes larger: $Be^{2+} > Mg^{2+} > Ca^{2+} > Sr^{2+} > Ba^{2+}$.
The very small $Be^{2+}$ ion has an exceptionally high hydration enthalpy, which overcomes the lattice enthalpy; hence $BeSO_4$ (and $MgSO_4$) are readily soluble, whereas $CaSO_4$, $SrSO_4$ and $BaSO_4$ are sparingly soluble or insoluble.
Hence the sulphate with hydration enthalpy higher than lattice enthalpy is $BeSO_4$.
For the sulphates of group 2, the lattice enthalpy is almost constant, because the $SO_4^{2-}$ ion is so large that a change in cation size makes little difference.
The hydration enthalpy, however, decreases down the group as the cation becomes larger: $Be^{2+} > Mg^{2+} > Ca^{2+} > Sr^{2+} > Ba^{2+}$.
The very small $Be^{2+}$ ion has an exceptionally high hydration enthalpy, which overcomes the lattice enthalpy; hence $BeSO_4$ (and $MgSO_4$) are readily soluble, whereas $CaSO_4$, $SrSO_4$ and $BaSO_4$ are sparingly soluble or insoluble.
Hence the sulphate with hydration enthalpy higher than lattice enthalpy is $BeSO_4$.