Question

Compare the solubility and thermal stability of the following compounds of the alkali metals with those of the alkaline earth metals. (a) Nitrates (b) Carbonates (c) Sulphates.

Short answer

Alkali metals generally have more soluble salts, while alkaline earth metals tend to be more thermally stable.

Step-by-step solution

Understanding Solubility Patterns in Alkali Metals

The solubility of nitrates, carbonates, and sulfates for alkali metals (e.g., lithium, sodium, potassium) generally increases down the group. Since the cation sizes increase, this allows for better solubilization and interaction with water molecules. All alkali metal nitrates and sulfates are soluble in water, whereas the solubility of their carbonates is relatively low due to their strong ionic bonds that make dissolution energetically unfavorable.

Understanding Solubility Patterns in Alkaline Earth Metals

For alkaline earth metals (e.g., magnesium, calcium, barium), solubility trends vary significantly compared to alkali metals. Nitrates are also typically soluble, but carbonates and sulfates often have lower solubility, particularly as the ionic size increases down the group. This is due to the stronger lattice energy that is not easily overcome by hydration energy in water.

Thermal Stability in Alkali Metals

Alkali metal compounds generally have high thermal stability. Their nitrates decompose upon heating to form nitrites and oxygen. Carbonates require significant heat to decompose into oxides and carbon dioxide due to their ionic nature. Sulfates are stable at high temperatures except at very extreme conditions where they decompose.

Thermal Stability in Alkaline Earth Metals

Alkaline earth metal compounds typically exhibit greater thermal stability than their alkali counterparts. Nitrates decompose into oxides, nitrogen dioxide, and oxygen when heated. Carbonates decompose readily into oxides and CO2, especially those of lighter elements like magnesium. Sulfates decompose into metal oxides and SO3 at higher temperatures compared to alkali sulfate decomposition.

Comparative Analysis

Comparing both groups, alkali metal nitrates and sulfates are more soluble than those of alkaline earth metals. Alkaline earth metal carbonates, however, have much lower solubility than alkali metal carbonates. For thermal stability, alkaline earth carbonates decompose more readily than alkali metal carbonates, whereas in general, both groups of compounds require high temperatures to decompose.

Key concepts

Alkali Metals

Alkali metals are elements found in Group 1 of the periodic table, which include lithium (Li), sodium (Na), and potassium (K). These metals are known for their high reactivity, especially with water. Their compounds show specific solubility and thermal stability trends.

• **Solubility:** Alkali metal nitrates and sulfates are predominantly soluble in water. For carbonates, however, the solubility is lower due to the strong ionic bonds present within these compounds. As we move down the group from lithium to potassium, the size of the cations increases, which leads to a trend of increasing solubility in nitrates and sulfates.
• **Thermal Stability:** Alkali metal compounds are thermally stable. Nitrates decompose to nitrites and oxygen upon heating, requiring significant energy due to their ionic nature. Carbonates are more stable, decomposing to oxides and carbon dioxide only at higher temperatures. Sulfates tend to resist decomposition except under very extreme conditions.

Alkaline Earth Metals

Alkaline earth metals belong to Group 2 of the periodic table and include elements such as magnesium (Mg), calcium (Ca), and barium (Ba). These elements have two electrons in their outer shell and are slightly less reactive than alkali metals. They have some distinctive solubility and thermal stability characteristics.

• **Solubility:** Alkaline earth metal nitrates are generally soluble, yet carbonates and sulfates exhibit lower solubility compared to their alkali metal counterparts. This is due to the stronger lattice energy in alkaline earth metal compounds, which is difficult to overcome by hydration energy. The solubility typically decreases as you move down the group.
• **Thermal Stability:** Generally more stable than alkali metal compounds, alkaline earth metal compounds like nitrates decompose to form oxides, nitrogen dioxide, and oxygen under heat. Carbonates also decompose readily to oxides and carbon dioxide, with those of the lighter alkaline earth metals (e.g., Mg) decomposing at even lower temperatures. Sulfates require relatively higher temperatures to decompose compared to alkali metal sulfates.

Nitrates

Nitrates are salts or esters of nitric acid and contain the nitrate ion ( O_3^- ). They are generally soluble in water, which represents a point of similarity between alkali and alkaline earth metals. However, their thermal stability varies between these groups.

• **Alkali Metal Nitrates:** These compounds decompose into nitrites and oxygen when subjected to heat. They maintain good solubility in water across all members of the alkali metal group.
• **Alkaline Earth Metal Nitrates:** Similar to their alkali counterparts, these nitrates also decompose when heated. Their decomposition products are typically oxides, nitrogen dioxide, and oxygen. These compounds are also water-soluble, although with minor variations across the group.

Carbonates

Carbonates are salts of carbonic acid containing the carbonate ion ( CO_3^{2-} ). These compounds differ notably in solubility and thermal stability across alkali and alkaline earth metal groups.

• **Alkali Metal Carbonates:** Generally have limited solubility in water, yet they require substantial heat to decompose into metal oxides and carbon dioxide. Their ionic bonds make them particularly stable under normal conditions.
• **Alkaline Earth Metal Carbonates:** These typically have lower solubility compared to alkali metal carbonates, predominantly due to strong lattice structures. Many alkaline earth metal carbonates decompose more readily when heated, especially for lighter metals like magnesium.

Sulfates

Sulfates are salts or esters of sulfuric acid containing the sulfate ion ( SO_4^{2-} ). Both alkali and alkaline earth metal sulfates exhibit distinct solubility and thermal behavior.

• **Alkali Metal Sulfates:** All sulfates of alkali metals are soluble in water, showing no significant resistance against dissolution. These compounds are stable under high temperatures, with decomposition only occurring under extreme conditions.
• **Alkaline Earth Metal Sulfates:** As you move down the group, the solubility tends to decrease. For instance, barium sulfate is notoriously insoluble in water. These sulfates generally require higher temperatures to decompose compared to alkali metal sulfates, often forming metal oxides and sulfur trioxide upon decomposition.