Question

Arrange the following sulphates of alkaline earth metals in order of decreasing thermal stability:
$BeS{O_4}{\text{ }},{\text{ MgS}}{{\text{O}}_4}{\text{ , CaS}}{{\text{O}}_4}{\text{ , SrS}}{{\text{O}}_4}$
A. $BeS{O_4}{\text{ }} \succ {\text{ MgS}}{{\text{O}}_4}{\text{ }} \succ {\text{ CaS}}{{\text{O}}_4}{\text{ }} \succ {\text{ SrS}}{{\text{O}}_4}$
B. $BeS{O_4}{\text{ }} \prec {\text{ MgS}}{{\text{O}}_4}{\text{ }} \prec {\text{ CaS}}{{\text{O}}_4}{\text{ }} \prec {\text{ SrS}}{{\text{O}}_4}$
C. $BeS{O_4}{\text{ }} \succ {\text{ MgS}}{{\text{O}}_4}{\text{ }} \succ {\text{ CaS}}{{\text{O}}_4}{\text{ }} \succ {\text{ SrS}}{{\text{O}}_4}$
D. $SrS{O_4}{\text{ }} \succ {\text{ CsS}}{{\text{O}}_4}{\text{ }} \succ {\text{ BeS}}{{\text{O}}_4}{\text{ }} \succ {\text{ MgS}}{{\text{O}}_4}$

Answer 1

Thermal stability of a compound can be defined as the stability of a compound when it is thermally heated. The compound which gets easily decomposed is thermally unstable. Whereas the compound which does not get easily decomposed on thermal decomposition is a thermally stable compound.

Complete answer:
When a substance is heated then it decomposes into its constituent atoms or molecules. The substances which can decompose easily are known to be thermally unstable. Whereas the compound which does not get easily decomposed is known as thermally stable compound. We know that the sulphates of alkaline earth metals decompose on heating to produce its corresponding oxide. This can be shown as,
${\text{CaS}}{{\text{O}}_{\text{4}}}{\text{ }}\xrightarrow{\Delta }{\text{ CaO + S}}{{\text{O}}_2}{\text{ + }}\frac{1}{2}{O_2}$
Hence we can observe that sulphates of alkaline earth metals decompose to give its metal oxide, sulphide gas and half mole of oxygen gas. As we move down the group the size of metal atoms increases continuously. Also the stability increases as the basic nature of metal increases. The metal having larger size is more basic in nature rather than metal having smaller size. Therefore as we move down the group the thermal stability of sulphate of metal increases. Hence the correct order of increasing thermal stability of sulphate of alkaline earth metal is:
$BeS{O_4}{\text{ }} \prec {\text{ MgS}}{{\text{O}}_4}{\text{ }} \prec {\text{ CaS}}{{\text{O}}_4}{\text{ }} \prec {\text{ SrS}}{{\text{O}}_4}$.
And hence the correct answer is option B.

Note:
The increasing order of thermal stability can be found on the basis of size of cation and anion. Since the size of anion is the same for all compounds, the size of the cation varies. The ion having larger size of cation is more stable as the size of anion is also large. The size of sulphate ion is larger, therefore it will be thermally stable with larger size of cation.