Problem 95 - A Complete Resource Book in Chemistry for JEE Main

Question

Thermal stability of \(\mathrm{BaCO}_{3}, \mathrm{CaCO}_{3}\) and \(\mathrm{MgCO}_{3}\) is (a) \(\mathrm{BaCO}_{3}>\mathrm{CaCO}_{3}>\mathrm{MgCO}_{3}\) (b) \(\mathrm{BaCO}_{3}>\mathrm{MgCO}_{3}>\mathrm{CaCO}_{3}\) (c) \(\mathrm{MgCO}_{3}>\mathrm{CaCO}_{3}>\mathrm{BaCO}_{3}\) (d) \(\mathrm{CaCO}_{3}>\mathrm{MgCO}_{3}>\mathrm{BaCO}_{3}\)

Step-by-Step Solution

Answer

The correct order of thermal stability is (a) \( \text{BaCO}_3 > \text{CaCO}_3 > \text{MgCO}_3 \).

Step 1: Understanding Thermal Stability

Thermal stability refers to how resistant a compound is to decomposition when subjected to heat. For carbonates like \( \text{BaCO}_3 \), \( \text{CaCO}_3 \), and \( \text{MgCO}_3 \), it means how likely they are to decompose into metal oxide and \( \text{CO}_2 \) on heating.

Step 2: Identifying Periodic Trends

Barium (Ba), Calcium (Ca), and Magnesium (Mg) are Group 2 elements in the periodic table. As we move down the group (from Mg to Ba), the thermal stability of the carbonates increases because larger cation size from Ba to Mg results in lower lattice energy and weaker bonding, which requires higher temperature for decomposition.

Step 3: Ranking Carbonates by Thermal Stability

Considering the periodic trends, \( \text{BaCO}_3 \) will be more thermally stable than \( \text{CaCO}_3 \), and \( \text{CaCO}_3 \) will be more stable than \( \text{MgCO}_3 \). This is because larger cations stabilize the carbonate structure more effectively.

Key Concepts

Group 2 Elements

Group 2 elements are also known as the alkaline earth metals. These elements include Beryllium, Magnesium, Calcium, Strontium, Barium, and Radium. They are located in the second column of the periodic table. These elements share similar properties, such as having two valence electrons. This characteristic makes them quite reactive, although not as reactive as their Group 1 counterparts, the alkali metals. Important properties of Group 2 elements include:

• Shiny and silvery-white appearance: Most of the Group 2 elements have a shiny, silver-like appearance when they are in their pure, metallic form.
• Relatively low density: With the exception of Beryllium, the elements in this group tend to be less dense than other metals.
• High melting and boiling points: These elements have higher melting and boiling points than the alkali metals.

Group 2 elements form stable compounds, and most of their oxides and hydroxides are basic. They also form important salts like carbonates and sulfates. As we go down the group from Magnesium to Barium, the elements become more reactive due to the decreasing ionization energy. This trend significantly affects their chemical behavior, including the thermal stability of their compounds like carbonates.

Periodic Trends

Periodic trends refer to patterns observed throughout the periodic table, related to the properties of the elements. In Group 2, these trends can be linked to atomic size, ionization energy, and electronegativity. Here are key periodic trends for Group 2 elements:

• Atomic size: As we move down the group from Magnesium to Barium, the atomic size increases. This happens because additional electron shells are added, increasing the distance between the nucleus and the outer electrons.
• Ionization energy: The energy required to remove the outermost electron decreases as you move down the group. This means elements become more reactive as you descend because it's easier to lose electrons.
• Electronegativity: Generally, the tendency of an atom to attract electrons decreases as you go down the group. Elements become less electronegative because the added electron shells provide a shielding effect that weakens the pull from the nucleus.

These trends influence how elements react, including their compounds' stability. Understanding these trends helps explain why compounds like carbonates of Barium, Calcium, and Magnesium differ in their thermal stability.

Carbonates Decomposition

Carbonates are compounds composed of carbonate ions (\[\text{CO}_3^{2-}\]) and metal cations. When heated, these compounds decompose into metal oxides and carbon dioxide (\[\text{CO}_2\]). The decomposition reaction can be represented as:\[\text{MCO}_3\; \rightarrow\; \text{MO}\; +\; \text{CO}_2\]where \( \text{M} \) represents the metal.The thermal stability of metal carbonates is a significant topic in chemistry because it explains how temperature impacts the breakdown of chemical compounds. For Group 2 carbonates:

• Larger cations (like Barium) make the carbonate structure more stable. This is because they result in lower lattice energy, making the compound less prone to breaking apart when heated.
• Smaller cations (like Magnesium) have a higher lattice energy, leading to more significant stability challenges at lower temperatures.

This trend aligns with the periodic properties of Group 2 elements, where thermal stability increases as we move from lighter to heavier elements. Therefore, Barium carbonate is the most thermally stable, followed by Calcium carbonate and then Magnesium carbonate. Understanding these decomposition processes is crucial for industrial applications where reliability and product stability are needed.