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Group 2 elements
Concepts tested here
- Solubility of sulphates
- Solubility trends
All Questions
2010 AIPMT-PRE 2 questions
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Property of the alkaline earth metals that increases with their atomic number isDown group 2 the atomic size increases, so ionisation energy and electronegativity both decrease.
Hydroxides: the lattice enthalpy decreases down the group more rapidly than the hydration enthalpy (the small $OH^-$ ion makes lattice enthalpy very sensitive to cation size), so the solubility increases: $Be(OH)_2 < Mg(OH)_2 < Ca(OH)_2 < Sr(OH)_2 < Ba(OH)_2$.
Sulphates: the lattice enthalpy stays nearly constant (large $SO_4^{2-}$ ion) while the hydration enthalpy falls, so the solubility decreases: $BeSO_4 > MgSO_4 > CaSO_4 > SrSO_4 > BaSO_4$.
Hence the property that increases with atomic number is the solubility of their hydroxides in water. -
Which of the following alkaline earth metal sulphates has hydration enthalpy higher than the lattice enthalpy?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 $BeSO_4$.