sblock Elements 2 Question 2
2. The correct sequence of thermal stability of the following carbonates is
(2019 Main, 12 April I)
(a) $\mathrm{BaCO}_3<\mathrm{CaCO}_3<\mathrm{SrCO}_3<\mathrm{MgCO}_3$
(b) $\mathrm{MgCO}_3<\mathrm{CaCO}_3<\mathrm{SrCO}_3<\mathrm{BaCO}_3$
(c) $\mathrm{MgCO}_3<\mathrm{SrCO}_3<\mathrm{CaCO}_3<\mathrm{BaCO}_3$
(d) $\mathrm{BaCO}_3<\mathrm{SrCO}_3<\mathrm{CaCO}_3<\mathrm{MgCO}_3$
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Answer:
Correct Answer: 2. (b)
Solution: The correct sequence of thermal stability of carbonates is
$ \mathrm{MgCO}_3<\mathrm{CaCO}_3<\mathrm{SrCO}_3<\mathrm{BaCO}_3 $
On moving down the group, i.e. from Mg to Ba , atomic radius generally increases. It is due to the addition of shell. As a result, the atomic size increases. $\mathrm{CO}_3^{2-}$ is a large anion. Hence, more stabilised by $\mathrm{Ba}^{2+}$ (large cation) and less stabilised by $\mathrm{Mg}^{2+}$. Therefore, $\mathrm{BaCO}_3$ has highest thermal stability followed by $\mathrm{SrCO}_3, \mathrm{CaCO}_3$ and $\mathrm{MgCO}_3$.
BETA
