Problem 76 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is arranged in order of increasing thermal stability? (a) \(\mathrm{BaCO}_{3}<\mathrm{SrCO}_{3}<\mathrm{CaCO}_{3}<\mathrm{MgCO}_{3}\) (b) \(\mathrm{CaCO}_{3}<\mathrm{MgCO}_{3}<\mathrm{BaCO}_{3}<\mathrm{SrCO}_{3}\) (c) \(\mathrm{MgCO}_{3}<\mathrm{CaCO}_{3}<\mathrm{SrCO}_{3}<\mathrm{BaCO}_{3}\) (d) \(\mathrm{MgCO}_{3}<\mathrm{SrCO}_{3}<\mathrm{CaCO}_{3}<\mathrm{BaCO}_{3}^{3}\)

Step-by-Step Solution

Answer

The correct order is (c) \( \mathrm{MgCO}_{3}<\mathrm{CaCO}_{3}<\mathrm{SrCO}_{3}<\mathrm{BaCO}_{3} \).

Step 1: Understand Thermal Stability

Thermal stability refers to the resistance of a compound to decomposition upon heating. For carbonates in the alkaline earth metal group, stability increases with heavier elements.

Step 2: Review Trends in the Periodic Table

Magnesium carbonate (1733CO_3) is the least stable, and as you move down the group in the periodic table with calcium (1717aCO_3), strontium (1L23rCO_3), and barium (842LU1boCO_3), stability increases.

Step 3: Analyze Each Option

Compare each arrangement based on the understanding that thermal stability increases with increasing atomic mass of the cation: 173fCO_3 < 1717aCO_3 < 1L2LU1brCO_3 < 841LU2LUuCO_3.

Step 4: Identify the Correct Order

Pick the option where the order reflects the trend of increasing atomic mass leading to increasing thermal stability, which is (c) 173pCO_3 < 1L2LU1brCO_3 < 1L2LU1brCO_3 < 841LU2LUuCO_3.

Key Concepts

Alkaline Earth Metal Carbonates

Alkaline earth metal carbonates are compounds that include a metal from the alkaline earth series on the periodic table combined with the carbonate ion \(\mathrm{CO}_3^{2-}\). These metals comprise beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). Alkaline earth metals are known for forming stable compounds due to having two valence electrons. In the context of carbonates, only magnesium to barium are commonly considered due to their more typical properties.
The thermal stability of these carbonates is primarily influenced by the size and charge of the metal ions.
As we move down the group in the periodic table, the metallic cations become larger and more polarizable.The resulting carbonates are ordered as follows, from least stable to most stable upon heating:

• \(\mathrm{MgCO}_{3}\)
• \(\mathrm{CaCO}_{3}\)
• \(\mathrm{SrCO}_{3}\)
• \(\mathrm{BaCO}_{3}\)

Magnesium carbonate decomposes at a lower temperature compared to barium carbonate.
This trend makes sense because as you proceed down the group, the metal ions more effectively stabilize the carbonate ion.

Periodic Table Trends

The periodic table exhibits distinct trends that influence the properties of the elements and their compounds. For alkaline earth metal carbonates, the most relevant trend is how these properties change as you move down the group. As you move from magnesium down to barium in the periodic table:

• Atomic and ionic radii increase.
• Thermal stability of the carbonates increases.
• The lattice energy tends to decrease, making them less reactive to heat.

The increasing thermal stability can be attributed to the fact that larger, heavier ions, like barium, can accommodate a larger negative charge more effectively—from the carbonate ion—compared to smaller ions like magnesium.
This ability leads to stronger ionic bonding in the lattice of the carbonates, which requires more energy to break apart the compound.
Furthermore, heavier alkaline earth metals form carbonates that have higher decomposition temperatures, meaning they are thermally more stable compared to those higher up on the table, such as magnesium carbonate.

Chemical Decomposition

Chemical decomposition is a process where a single compound breaks into two or more simpler substances. When discussing alkaline earth metal carbonates, decomposition typically occurs upon heating. In this process:

• The carbonate breaks down to form an oxide and carbon dioxide.
• For example, \(\mathrm{MgCO}_{3} \rightarrow \mathrm{MgO} + \mathrm{CO}_2\).

Thermal decomposition is influenced by several factors, including bond strength and lattice energy.The increase in atomic size and mass down the alkaline earth metal group means that larger metal ions can better stabilize the negative charge of the carbonate group.
This increased stability comes from the reduced charge density and increased polarizability of the larger ions.Ultimately, this enhanced stability means that more energy (in the form of heat) is needed to decompose the compound, thus demonstrating a higher thermal stability for carbonates like \(\mathrm{BaCO}_{3}\) compared to \(\mathrm{MgCO}_{3}\). Understanding this concept is crucial for predicting the behavior of these compounds in various applications.