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) \(\mathrm{CaCO}_{3}^{3}<\mathrm{MgCO}_{3}<\mathrm{BaCO}_{3}<\mathrm{SrCO}_{3}^{3}\) (c) \(\mathrm{MgCO}_{3}<\mathrm{CaCO}_{3}<\mathrm{SrCO}_{3}<\mathrm{BaCO}_{3}\) (d) \(\mathrm{MgCO}_{3}<\mathrm{SrCO}_{3}^{3}<\mathrm{CaCO}_{3}^{3}<\mathrm{BaCO}_{3}^{3}\)

Step-by-Step Solution

Answer

Option (c) is arranged in order of increasing thermal stability.

Step 1: Understand Thermal Stability

Thermal stability refers to the stability of a compound when exposed to heat. Compounds decompose when heated, and those that require higher temperatures to decompose are considered to have higher thermal stability.

Step 2: Analyze the Carbonate Compounds

The carbonates in question are MgCO₃ (Magnesium Carbonate), CaCO₃ (Calcium Carbonate), SrCO₃ (Strontium Carbonate), and BaCO₃ (Barium Carbonate). In general, the thermal stability of group 2 carbonates increases as you move down the group.

Step 3: Predict the Order of Stability

In the Group 2 elements (alkaline earth metals), heavier elements form more thermally stable carbonates. Thus, the order of increasing thermal stability among the carbonates is: MgCO₃ < CaCO₃ < SrCO₃ < BaCO₃, because BaCO₃ is more stable than SrCO₃, SrCO₃ is more stable than CaCO₃, and CaCO₃ is more stable than MgCO₃.

Step 4: Identify the Correct Option

Compare this order with the options given: (a) BaCO₃ < SrCO₃ < CaCO₃ < MgCO₃ is incorrect. (b) CaCO₃ < MgCO₃ < BaCO₃ < SrCO₃ is incorrect. (c) MgCO₃ < CaCO₃ < SrCO₃ < BaCO₃ matches our predicted order. (d) MgCO₃ < SrCO₃ < CaCO₃ < BaCO₃ is incorrect. Therefore, the correct answer is option (c).

Key Concepts

Group 2 Elements

Group 2 elements, also known as the alkaline earth metals, include beryllium, magnesium, calcium, strontium, barium, and radium. These metals are located in the second column of the periodic table. They are highly characterized by their shiny appearance and are less reactive than the alkali metals of Group 1.
These metals have two electrons in their outer shell, which they readily lose to form divalent cations ( +2 charge). This characteristic makes them quite similar to each other in chemical behavior.
Some notable properties of Group 2 elements are:

• They are usually harder than alkali metals.
• They have higher melting points and boiling points compared to alkali metals.
• These elements are good conductors of electricity.

Understanding the general trends and properties of these metals helps in studying their compounds, such as carbonates, and predicting their behavior under various conditions, including thermal stability.

Thermal Decomposition

Thermal decomposition refers to the process in which a chemical compound breaks down into simpler substances when heated. This process is often endothermic, requiring the input of heat to proceed.
In the case of carbonates, the general decomposition reaction for Group 2 carbonates when heated is:\[\text{MCO}_{3(s)} \rightarrow \text{MO}_{(s)} + \text{CO}_{2(g)}\]where \(M\) is a Group 2 metal.
This process involves breaking the bonds in the carbonate compound to form the metal oxide and carbon dioxide gas.

Some key points about thermal decomposition include:

• The temperature required for decomposition varies depending on the metal present.
• Generally, heavier alkaline earth metals form carbonates that are more stable and require higher temperatures to decompose.

This knowledge is crucial when predicting and understanding the order of thermal stability among different carbonates.

Order of Stability

The order of thermal stability refers to how resistant certain compounds are to decomposition when exposed to heat. In terms of Group 2 carbonates, a distinct trend can be observed as you move down the group in the periodic table.
This pattern shows that

• Heavier metals like barium form carbonates that are more thermally stable.
• The stability increases from magnesium carbonate to barium carbonate.

The reason for this trend lies in the size of the metal ions and their bonding characteristics with carbonate ions. Larger metal ions, such as those found in barium, result in a more stable lattice structure that withstands heat better.
This increasing stability can be represented as \( \text{MgCO}_{3} < \text{CaCO}_{3} < \text{SrCO}_{3} < \text{BaCO}_{3} \), which explains the correct step-by-step solution for the thermal stability question.

Alkaline Earth Metals

Alkaline earth metals are a group of elements with similar properties, forming Group 2 of the periodic table. These include well-known metals such as magnesium and calcium.
Alkaline earth metals are crucial in various industrial applications and biological processes. They feature prominently in forming compounds such as oxides and carbonates.

Key characteristics of alkaline earth metals:

• Their oxides are termed 'earths' and are alkaline when dissolved in water, hence the name 'alkaline earth.'
• They are less reactive than alkali metals but more reactive than transition metals.
• They have a shiny luster and can also form strong ionic bonds in compounds.

The understanding of alkaline earth metals extends to their compounds' behavior, including thermal stability affecting how such compounds decompose when heated.
Their predictability helps in various chemical applications, particularly regarding their stability at high temperatures, which is essential when selecting materials for heat-resilient applications.