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 $\mathrm{Mg}$ to $\mathrm{Ba}$, 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 $\mathrm{MgCO}_{3}$.
