Question

Identify the incorrect order against the property given in brackets

• A. BeCO$_3$<MgCO$_3$<CaCO$_3$<SrCO$_3$ (Thermal stability)
• B. BeSO$_4$>MgSO$_4$>CaSO$_4$>SrSO$_4$ (Solubility in water)
• C. Li$_2$CO$_3$>Na$_2$CO$_3$>K$_2$CO$_3$>Rb$_2$CO$_3$ (Thermal stability)
• D. BeCO$_3$>MgCO$_3$>CaCO$_3$>SrCO$_3$ (Solubility in water)

Hint:

For Group 1 and 2 compounds: Thermal Stability of carbonates, nitrates, and hydroxides increases down the group. Solubility: For salts with small anions (e.g., F$^-$, OH$^-$, CO$_3^{2-}$), solubility generally decreases down the group. For salts with large anions (e.g., SO$_4^{2-}$, I$^-$, NO$_3^-$), solubility generally increases down the group (with the notable exception of Group 2 sulphates, which decrease).

Answer 1

The Correct Option is C

(A) Thermal stability of alkaline earth metal carbonates:
The thermal stability of carbonates increases down the group. This is due to the decreasing polarizing power of the cation with increasing size. Be$^{2+}$<Mg$^{2+}$<Ca$^{2+}$<Sr$^{2+}$.
Hence, thermal stability order: BeCO$_3$<MgCO$_3$<CaCO$_3$<SrCO$_3$. This statement is correct.
(B) Solubility of alkaline earth metal sulphates:
Solubility of sulphates decreases down the group as lattice enthalpy decreases less rapidly than hydration enthalpy.
Order: BeSO$_4$>MgSO$_4$>CaSO$_4$>SrSO$_4$. This statement is correct.
(C) Thermal stability of alkali metal carbonates:
Thermal stability increases down the group. Li$_2$CO$_3$ is less stable due to high polarizing power of Li$^+$.
Correct order: Li$_2$CO$_3$<Na$_2$CO$_3$<K$_2$CO$_3$<Rb$_2$CO$_3$.
The given order is reversed. Hence, this statement is incorrect.
(D) Solubility of alkaline earth metal carbonates:
For small anions like CO$_3^{2-}$, solubility decreases down the group due to lattice enthalpy.
Order: BeCO$_3$>MgCO$_3$>CaCO$_3$>SrCO$_3$. This statement is correct.