The
ΔHf of alkali metal halides is shown through this graph.
This graph shows that formation of metal halide is negative, i.e. energy is released during the formation of metal halides.
(A)
↓ On moving down the group, the size of halogens increase which results in the lengthening of bond, making the bond weaker. Therefore,
ΔHf is less negative on moving down the group as the stability of metal halide decreases down the group.
(B)
↓ On moving down the group, the electropositivity of metal increases making the attraction between the oppositely charged ions even stronger, thus increasing the strength of bond. Increase in strength is more than the weakness produced due to lengthening of bond because of increase in size of metal ions down the group. This makes
ΔHf more negative on moving down the group. Option (a) is incorrect, due to reason produced in (B).
Option (b) is incorrect. As CsI has
Cs⊕ and
I⊖ an constituent ions and both the ions are larger in size. The lattice enthalpy is not so high as shown in graph. But it has low solubility due to less hydration energy released when hydration of larger ions takes place. Option (c) is correct. As LiF has highest lattice enthalpy as shown in graph. This is due to the size of ions that is smallest in their respective groups. Smaller is the size, shorter is the bond length and stronger is the bond. The energy required is break the bond and release the constituents ions is more than the energy released in the hydration of ions. So, LiF is least soluble in water. Option (d) is incorrect. LiF has most negative enthalpy in all metal fluorides as shown in graph.