Problem 9 - Chemistry in 30 days

Question

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

Step-by-Step Solution

Answer

Option (c) is arranged in order of decreasing solubility.

Step 1: Identify the Trend

For sulfates of alkaline earth metals, the solubility generally decreases as the size of the metal cation increases. This means that the order of solubility from highest to lowest is typically \[ \text{BeSO}_4 > \text{MgSO}_4 > \text{CaSO}_4 > \text{SrSO}_4 > \text{BaSO}_4 \] because beryllium sulfate is very soluble in water, while barium sulfate is the least soluble.

Step 2: Compare Given Options

Examine the options provided: (a) \( \text{BaSO}_4 > \text{SrSO}_4 > \text{CaSO}_4 > \text{MgSO}_4 > \text{BeSO}_4 \) (b) \( \text{BeSO}_4 > \text{CaSO}_4 > \text{MgSO}_4 > \text{SrSO}_4 > \text{BaSO}_4 \) (c) \( \text{BeSO}_4 > \text{MgSO}_4 > \text{CaSO}_4 > \text{SrSO}_4 > \text{BaSO}_4 \) (d) \( \text{CaSO}_4 > \text{BaSO}_4 > \text{BeSO}_4 > \text{MgSO}_4 > \text{SrSO}_4 \) Our task is to find the option that matches the trend \( \text{BeSO}_4 > \text{MgSO}_4 > \text{CaSO}_4 > \text{SrSO}_4 > \text{BaSO}_4 \).

Step 3: Find the Matching Sequence

Comparing the general trend with options available, option (c) matches the expected order of decreasing solubility: (c) \( \text{BeSO}_4 > \text{MgSO}_4 > \text{CaSO}_4 > \text{SrSO}_4 > \text{BaSO}_4 \).

Key Concepts

Alkaline Earth Metals

Alkaline earth metals are a group of metallic elements found in Group 2 of the periodic table. These metals include beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). A defining trait of these metals is their two valence electrons, which they readily lose to form

• divalent cations (\( ext{M}^{2+} \)).

This loss leads them to bond easily with nonmetals, such as sulfate (SO₄²⁻), forming ionic compounds.

The reactivity of alkaline earth metals is notable, although it is less than that of the more reactive alkali metals found in Group 1. As you move down the group, the reactivity increases. However, the first ionization energies decrease. This trend occurs because the atomic size increases, as it is

• more challenging to hold the outer electrons close to the nucleus.

As a result, it becomes easier for them to lose an electron, enhancing their ability to form different

• compounds with varying solubilities.

Solubility of Sulfates

The solubility of sulfates of alkaline earth metals decreases as you move down the group in the periodic table. This means compounds like

• beryllium sulfate (BeSO₄) are very soluble
• whereas barium sulfate (BaSO₄) is scarcely soluble in water.

The difference in solubility is due to the size of the cation.

In general, smaller cations, when paired with a sulfate anion, lead to higher solubility because of the stronger attraction and ability to stabilize in water. As the cation size increases

• (from Be
  to Ba),

this stabilization diminishes, resulting in less soluble compounds. Because of this trend, BeSO₄ and MgSO₄ are highly soluble. However, the solubility sharply declines with calcium sulfate (
CaSO₄)

and becomes significantly low with BaSO₄. For chemistry students, recognizing this pattern allows for predicting the solubility
• of new unknown sulfate compounds.

Chemical Trends in Periodic Table

Chemical trends in the periodic table allow us to predict the properties and behaviors of elements, like solubility, reactivity, and atomic radius. Alkaline earth metals exemplify these trends vividly with decreasing solubility in sulfates as we move from beryllium to barium. This trend can also be explained by looking at the chemical properties and atomic structure changes

• within the group.

For instance, the atomic radius gets larger as you progress down the group. This increase means valence electrons are further from the nucleus, weakening the attraction between electrons and enabling easier ionization. Simultaneously, the increased distance in atomic radius also reduces the attraction

• exerted by the sulfate ions, resulting in lower solubility.

Understanding such trends not only highlights existing element properties but assists students in anticipating how elements might interact in untested scenarios.