Q.In which of the following the hydration energy is higher than the lattice energy?

Stability of alkali metal chlorides increases as the cation size increases: CsCl > KCl > NaCl > LiCl.

As is typical for alkali metal salts, when the anion is comparatively large the compound's stability (lattice/thermal stability) tends to increase as the cation size increases, since a larger cation matches the anion's size more closely and minimises lattice strain (the same reasoning NCERT uses for the increasing thermal stability of alkali metal carbonates/nitrates down the gr …

Na2CO3 is thermally very stable and does not decompose on ordinary heating.

• CaCO3 and BaCO3 are alkaline earth metal carbonates; alkaline earth carbonates readily decompose on heating (e.g. CaCO3 → CaO + CO2), because the smaller, more highly-charged alkaline earth cations polarise the large carbonate ion strongly.
• Li2CO3 (an alkali metal carbonate) is an exception among alkali carbonates: Li+ is unusually small and polarising (similar in behaviour to Group 2 due to its diagonal relationship with Mg), so Li2CO3 also decomposes on heating. …

MgSO4 is the sulphate whose hydration energy exceeds its lattice energy.

For alkaline earth metal sulphates, solubility decreases down the group (MgSO4 is soluble, but SrSO4, BaSO4 and RaSO4 are essentially insoluble). This is because, as the cation gets larger going down the group, the lattice energy (dominated by the large, fixed-size sulfate anion) changes only slowly, while the hydration energy of the cation falls off sharply with increasing cation size. Mg2+, being the smallest cation among the four, has a hydration energy …

Thermal stability of Group 2 metal carbonates increases down the group; BaCO3 is the most stable because Ba^2+ is the largest and least polarizing cation.

Carbonates decompose on heating as: MCO3(s) → MO(s) + CO2(g). The ease of this decomposition depends on how strongly the metal cation polarizes (distorts) the electron cloud of the large CO3^2- ion — greater polarization weakens the C-O bonds within the carbonate ion and makes it decompose at a lower temperature.

Down Group 2 (Mg → Ca → Sr → Ba), ionic radius increases and charge density (polarizing power) decreases (Fajans' rules). So: …

Down Group 2, cation size increases, polarising power decreases, and carbonate thermal stability increases — so BaCO3 (largest cation, bottom of this set) is the most stable.

Note on syllabus scope: thermal stability trends of Group 2 metal carbonates were taught under the s-Block Elements chapter of the older NCERT syllabus, which has been dropped from the current rationalised CBSE Class 11 syllabus — flagged out-of-syllabus, answered honestly below.

The thermal decomposition of a metal carbonate (MCO3 → MO + CO2) becomes easier when the metal cation strongly polarises (distorts) the large, polarisable carbonate anion, weakening the C-O bonds within it and helping CO2 leave. This is governed by Fajans' rules: a smaller, more highly charged cation polarises an anion more strongly.

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