Problem 78 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is arranged in order of decreasing solubility in water? (a) \(\mathrm{CaSO}_{4}>\mathrm{BaSO}_{4}>\mathrm{BeSO}_{4}>\mathrm{MgSO}_{4}>\mathrm{SrSO}_{4}\) (b) \(\mathrm{BeSO}_{4}>\mathrm{MgSO}_{4}>\mathrm{CaSO}_{4}>\mathrm{SrSO}_{4}^{4}>\mathrm{BaSO}_{4}^{4}\) (c) \(\mathrm{BaSO}_{4}>\mathrm{SrSO}_{4}>\mathrm{CaSO}_{4}>\mathrm{MgSO}_{4}>\mathrm{BeSO}_{4}\) (d) \(\mathrm{BeSO}_{4}>\mathrm{CaSO}_{4}>\mathrm{MgSO}_{4}>\mathrm{SrSO}_{4}>\mathrm{BaSO}_{4}\)

Step-by-Step Solution

Answer

The correct answer is option (b): BeSO4 > MgSO4 > CaSO4 > SrSO4 > BaSO4.

Step 1: Understanding Solubility Trends

In the sulfate series, solubility decreases down the group. This means BeSO4 is most soluble, and BaSO4 is the least soluble among the sulfates listed.

Step 2: Evaluate Option (a)

In option (a), the order is CaSO4 > BaSO4 > BeSO4 > MgSO4 > SrSO4. This order incorrectly places CaSO4 and BaSO4 before BeSO4 and MgSO4, which doesn't match the general solubility trend.

Step 3: Evaluate Option (b)

Option (b) states BeSO4 > MgSO4 > CaSO4 > SrSO4 > BaSO4. This follows the correct trend where BeSO4 is the most soluble and BaSO4 is the least soluble.

Step 4: Evaluate Option (c)

Option (c) has BaSO4 > SrSO4 > CaSO4 > MgSO4 > BeSO4, which is the reverse order of the expected solubility trend, thus incorrect.

Step 5: Evaluate Option (d)

In option (d), the order is BeSO4 > CaSO4 > MgSO4 > SrSO4 > BaSO4. While it begins correctly with BeSO4, the order between MgSO4 and CaSO4 is not consistent with the decreasing solubility trend.

Step 6: Determine the Correct Answer

After evaluating all options, option (b) correctly arranges the compounds in decreasing order of solubility following the general trend observed for sulfates.

Key Concepts

Sulfate Solubility

Sulfate solubility plays a key role in inorganic chemistry, especially when examining how different elements interact with sulfate ions. Sulfates are the salts or esters of sulfuric acid, containing the anion \(\text{SO}_4^{2-}\). The solubility of sulfates varies greatly depending on the metal ion that forms the salt.

In general, the solubility of sulfate compounds decreases as you move down a group in the periodic table. This trend is particularly observed in the Group 2 elements, where the hydration enthalpy of cations plays a significant role. Because smaller ions like \(\text{Be}^{2+}\) have higher charge density, they form more soluble compounds with sulfate due to stronger interactions with water molecules. Therefore, among the common sulfates, beryllium sulfate \(\text{BeSO}_4\) tends to be more soluble, while barium sulfate \(\text{BaSO}_4\) is known for its low solubility, often used as a qualitative test for sulfate ions.

Understanding the solubility trends in sulfates helps in predicting the behavior of minerals and salts in natural processes and industrial applications.

Group 2 Elements

Group 2 elements, known as the alkaline earth metals, include beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). These elements are characterized by having two electrons in their outermost shell, which makes them quite reactive. They readily lose these two electrons to achieve a stable electronic configuration, making them form divalent cations \(\text{M}^{2+}\).

In the context of solubility, Group 2 sulfates display notable trends. As we progress from Be through to Ba, the solubility of their sulfates decreases. This is because the ionic size increases down the group. The larger cations further down the group have lower charge density, reducing their ability to interact with \(\text{SO}_4^{2-}\) ions and solvent water molecules effectively. Hence, \(\text{BeSO}_4\) is significantly more soluble than \(\text{BaSO}_4\).

This solubility pattern is critical in analyzing the behavior of Group 2 elements and when planning their application in processes such as metallurgy or pharmaceuticals.

Chemical Solubility Order

Understanding chemical solubility order is essential in inorganic chemistry for both synthesis and analytical purposes. Solubility patterns are influenced by several factors including ionic size, hydration energy, lattice energy, and crystal structure. These properties affect how readily a substance dissolves in a solvent, defining its solubility order.

For the Group 2 sulfate series, the solubility order is determined primarily by ionic size and the resulting lattice energy differences. As the ionic radius increases down the group, the lattice energy decreases, but hydration energy decreases more sharply because of larger ionic size, causing a net decrease in solubility.

When ranked by solubility, the order for the typical Group 2 sulfate salts is \(\text{BeSO}_4 > \text{MgSO}_4 > \text{CaSO}_4 > \text{SrSO}_4 > \text{BaSO}_4\). Recognizing the correct solubility order is crucial when predicting reaction outcomes and performing qualitative analysis in various chemical scenarios. This knowledge allows chemists to select appropriate procedures for separation, purification, and identification of compounds.