Problem 6 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The correct sequence of thermal stability of the following carbonates is : (a) \(\mathrm{BaCO}_{3}<\mathrm{CaCO}_{3}<\mathrm{SrCO}_{3}<\mathrm{MgCO}_{3}\) (b) \(\mathrm{MgCO}_{3}<\mathrm{CaCO}_{3}<\mathrm{SrCO}_{3}<\mathrm{BaCO}_{3}\) (c) \(\mathrm{MgCO}_{3}<\mathrm{SrCO}_{3}<\mathrm{CaCO}_{3}<\mathrm{BaCO}_{3}\) (d) \(\mathrm{BaCO}_{3}<\mathrm{SrCO}_{3}<\mathrm{CaCO}_{3}<\mathrm{MgCO}_{3}\)

Step-by-Step Solution

Answer

Option (b) is correct: \( \text{MgCO}_3 < \text{CaCO}_3 < \text{SrCO}_3 < \text{BaCO}_3 \).

Step 1: Understand the Concept of Thermal Stability

Thermal stability refers to the stability of a compound at high temperatures. In carbonates, thermal stability is related to how easily the compound decomposes upon heating. More stable compounds decompose less readily at higher temperatures.

Step 2: Identify Factors Affecting Thermal Stability

The thermal stability of carbonates primarily depends on the metal ion. In the alkaline earth metal carbonates, thermal stability typically increases with the increasing ionic size of the metal, which decreases up a group in the periodic table.

Step 3: Analyze the Involvement of Alkaline Earth Metals

The given carbonates (MgCO₃, CaCO₃, SrCO₃, BaCO₃) are those of metals from the alkaline earth group. As we move down the group from Mg to Ba, the ionic size increases.

Step 4: Sequence Based on Ionic Size and Stability

Thermal stability increases down the group due to the increase in ionic size, which stabilizes the carbonate ion more effectively. Therefore, the sequence of increasing thermal stability is: MgCO₃ < CaCO₃ < SrCO₃ < BaCO₃.

Step 5: Match the Correct Option to the Identified Sequence

Compare this sequence with the given options. The correct answer matches with option (b): \( \text{MgCO}_3 < \text{CaCO}_3 < \text{SrCO}_3 < \text{BaCO}_3 \).

Key Concepts

Alkaline Earth Metals

Alkaline earth metals are a group of elements found in Group 2 of the periodic table. These elements include beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). They are known for having two electrons in their outermost shell. This gives them certain characteristics: they are shiny, silvery-white and somewhat reactive metals.

• These metals tend to lose their two outer electrons easily. This forms cations with a charge of +2, like Mg²⁺ and Ca²⁺.
• They are less reactive than alkali metals found in Group 1, but still react with water, albeit not as vigorously.
• In general, alkaline earth metals have high melting points and are harder than the alkali metals.

In the context of carbonates, alkaline earth metals form compounds like MgCO3, CaCO3, and BaCO3. Each compound's properties, including thermal stability, are influenced by the particular metal ion involved.

Carbonates Decomposition

Carbonates are a group of compounds containing the carbonate ion, \[\mathrm{CO}_3^{2-}\]. These compounds often undergo decomposition when heated to produce metal oxides and carbon dioxide gas. For example, magnesium carbonate (\[\mathrm{MgCO}_3\]) decomposes into magnesium oxide (\[\mathrm{MgO}\]) and carbon dioxide (\[\mathrm{CO}_2\]): \[\text{MgCO}_3 (s) \rightarrow \text{MgO} (s) + \text{CO}_2 (g)\]The thermal stability of these carbonates varies based on several factors. At the heart of this is often the particular metal ion involved.

• Larger metal ions can better stabilize the carbonate ion due to their ability to weaken the C-O bond in the carbonate.
• This relationship explains why different alkaline earth metal carbonates decompose at different temperatures.

This understanding helps to explain why some carbonates require much higher temperatures to break down compared to others.

Ionic Size Effect

The concept of ionic size plays a crucial role in determining the thermal stability of alkaline earth metal carbonates. As we move down Group 2 in the periodic table, the ionic size of the metals increases. This has important implications:

• Larger ionic size can stabilize the carbonates more effectively by dispersing charge over a larger volume, which means a greater distance between the metal cation and carbonate anion.
• As a result, thermal stability increases because the compound can better maintain its structure at higher temperatures without decomposing.

In terms of decomposition: - MgCO3, with a relatively small magnesium ion, decomposes at a relatively lower temperature.- However, BaCO3, with a larger barium ion, can withstand a greater amount of heat before decomposing. This is why understanding ionic size is important—it directly influences how heat stable a carbonate compound like \[\text{BaCO}_3\]can be compared to others such as \[\text{MgCO}_3\].