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) \(\mathrm{BaCO}_{3}>\mathrm{MgCO}_{3}>\mathrm{CaCO}_{3}^{3}\) (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

(a) Thermally, \\(\mathrm{BaCO}_{3} > \mathrm{CaCO}_{3} > \mathrm{MgCO}_{3}\\).

Step 1: Understanding Thermal Stability

Thermal stability of carbonates is the tendency of a compound to resist decomposition when heated. For metal carbonates, this depends on the metal's position in the periodic table and the lattice energy.

Step 2: Identifying the Trend in the Group

In the alkaline earth metal group (Group 2), thermal stability increases down the group. This means that beryllium carbonate decomposes more easily than magnesium carbonate, and so on down to barium carbonate.

Step 3: Relating Thermal Stability to Carbonates

Since \(\mathrm{BaCO}_{3}\), \(\mathrm{CaCO}_{3}\), and \(\mathrm{MgCO}_{3}\) are all group 2 metal carbonates, we expect their stability to follow the trend \(\mathrm{BaCO}_{3} > \mathrm{CaCO}_{3} > \mathrm{MgCO}_{3}\) based on their position in the periodic table.

Step 4: Final Comparison and Conclusion

Based on the trend described, \(\mathrm{BaCO}_{3}\) is the most thermally stable, followed by \(\mathrm{CaCO}_{3}\), and finally \(\mathrm{MgCO}_{3}\). The correct answer is option (a): \(\mathrm{BaCO}_{3} > \mathrm{CaCO}_{3} > \mathrm{MgCO}_{3}\).

Key Concepts

Alkaline Earth Metals

The alkaline earth metals are a fascinating group on the periodic table. They occupy Group 2, and include elements like beryllium, magnesium, calcium, strontium, barium, and radium. These elements share common characteristics owing to their electronic configuration. Each has two electrons in their outermost shell, which makes them reactive, though not as reactive as Group 1 metals, which are the alkali metals.

Alkaline earth metals form strong ionic bonds and their compounds are known for their distinctive properties. For instance, due to these ionic bonds, compounds of alkaline earth metals, like carbonates, are often insoluble in water and decompose at high temperatures. The thermal stability of these carbonates is a key consideration when assessing their reactivity and potential applications in industrial processes. As we move down the group, the size of the metal ions increases, leading to variations in their physical and chemical properties, including how they interact with heat.

This simple positional arrangement makes them perfect candidates for studying trends across the periodic table, including changes in thermal stability, which can help us predict the behavior of these compounds under varying conditions.

Periodic Table Trends

The periodic table is not just a collection of elements but a map of chemical trends. One such trend is the variation in thermal stability as you move down a group. Let's focus on Group 2, the alkaline earth metals. As you descend from magnesium to calcium to barium, a clear trend appears: thermal stability increases.

This trend can be attributed to several factors:

• **Increasing Ionic Size**: As the atomic number increases, the ionic size of these elements increases. Larger ions form more stable ionic compounds because the lattice energy needed to overcome the attraction between the ions also increases.
• **Lattice Energy**: This refers to the energy required to separate one mole of an ionic solid into its gaseous ions. As we go down the group, lattice energy decreases slightly due to increased inter-ionic distances, but it is compensated by a greater reduction in the acidic nature of the resulting metal oxides.

Understanding these trends helps predict and explain why barium carbonate is more thermally stable compared to magnesium carbonate, an observation crucial for many chemical applications.

Lattice Energy

Lattice energy is a critical factor in determining the thermal stability of ionic compounds like metal carbonates. It is the energy released when ions are combined to form a crystalline lattice. Generally, higher lattice energy means a more stable compound.

For the alkaline earth metals carbonates, as you move down the group from magnesium carbonate ( MgCO_3 ) to barium carbonate ( BaCO_3 ), lattice energy tends to decrease. However, larger cations like barium overcome this by forming more stable carbonates due to less distortion of the carbonate ion.

This is because:

• **Ion Size**: A larger cation like barium has a reduced charge density compared to smaller cations like magnesium. This results in a stronger attraction with the carbonate anion, despite moderately lower lattice energy.
• **Stability against Decomposition**: The arrangement and size of the ions make the lattice more resistant to the disruption caused by heating.
• **Compensating Factors**: Despite lower lattice energy in larger cations, factors like reduced polarization effects and increased overall size of ions help maintain or enhance stability.

Lattice energy, therefore, plays a vital role in defining why BaCO_3 is the most stable against thermal decomposition among its group counterparts.