Periodic Classification and Periodic Properties 2 Question 7
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7. Which one of the following alkaline earth metal sulphates has its hydration enthalpy greater than its lattice enthalpy?
======= ####7. Which one of the following alkaline earth metal sulphates has its hydration enthalpy greater than its lattice enthalpy?
3e0f7ab6f6a50373c3f2dbda6ca2533482a77bed (a) $\mathrm{CaSO}_{4}$
(c) $\mathrm{BaSO}_{4}$
(b) $\mathrm{BeSO}_{4}$
(d) $\mathrm{SrSO}_{4}$
(2015 Main)
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Answer:
Correct Answer: 7. (b)
Solution:
- As we move down the group, size of metal increases. Be has lower size while $\mathrm{SO}{4}^{2-}$ has bigger size, that’s why $\mathrm{BeSO}{4}$ breaks easily and lattice energy becomes smaller but due to lower size of $\mathrm{Be}$, water molecules are gathered around and hence hydration energy increases.
On the other hand, rest of the metals, i.e $\mathrm{Ca}, \mathrm{Ba}, \mathrm{Sr}$ have bigger size and that’s why lattice energy is greater than hydration energy.
Time Saving Technique In the question of finding hydration energy only check the size of atom. Smaller sized atom has more hydration energy. Thus, in this question Be is placed upper most in the group has lesser size and not comparable with the size of sulphates. Hence, $\mathrm{BeSO}_{4}$ is the right response.
