Problem 141 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following carbonates cannot evolve \(\mathrm{CO}_{2}\) on heating? (a) \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) (b) \(\mathrm{Rb}_{2} \mathrm{CO}_{3}\) (c) \(\mathrm{Li}_{2} \mathrm{CO}_{3}\) (d) \(\mathrm{MgCO}_{3}\)

Step-by-Step Solution

Answer

(a) \( \mathrm{Na}_2 \mathrm{CO}_3 \) does not evolve \( \mathrm{CO}_2 \) on heating.

Step 1: Understanding Thermal Decomposition

To solve this problem, we need to understand that not all carbonates decompose to release \( \mathrm{CO}_2 \) when heated. Generally, carbonates of alkali metals (except lithium carbonate) do not decompose upon heating. On the other hand, carbonates of other metals such as magnesium carbonate do decompose and release \( \mathrm{CO}_2 \).

Step 2: Analyzing Sodium Carbonate

\( \mathrm{Na}_2 \mathrm{CO}_3 \) is a carbonate of an alkali metal (sodium). Alkali metal carbonates (except lithium) are stable and do not decompose to release \( \mathrm{CO}_2 \) when heated. Therefore, \( \mathrm{Na}_2 \mathrm{CO}_3 \) does not evolve \( \mathrm{CO}_2 \) upon heating.

Step 3: Analyzing Rubidium Carbonate

Like sodium carbonate, \( \mathrm{Rb}_2 \mathrm{CO}_3 \) is also a carbonate of an alkali metal (rubidium). It, too, does not decompose to release \( \mathrm{CO}_2 \) when heated due to the stability of alkali metal carbonates.

Step 4: Analyzing Lithium Carbonate

\( \mathrm{Li}_2 \mathrm{CO}_3 \) is an exception among alkali metal carbonates. Lithium carbonate does decompose upon heating and releases \( \mathrm{CO}_2 \). So, it contradicts the general trend of alkali metal carbonates.

Step 5: Analyzing Magnesium Carbonate

\( \mathrm{MgCO}_3 \) is not a carbonate of an alkali metal. It is known to decompose upon heating, evolving \( \mathrm{CO}_2 \) into magnesium oxide and carbon dioxide.

Step 6: Identifying the Exception

Considering the behavior of each carbonate upon heating, \( \mathrm{Na}_2 \mathrm{CO}_3 \) and \( \mathrm{Rb}_2 \mathrm{CO}_3 \) do not evolve \( \mathrm{CO}_2 \). However, because we need to find which WILL NOT evolve \( \mathrm{CO}_2 \), the correct answer is \( \mathrm{Na}_2 \mathrm{CO}_3 \).

Key Concepts

Alkali Metal Carbonates

Alkali metal carbonates, except for lithium carbonate, are quite unique in their thermal stability. Generally, the carbonates of alkali metals are stable and do not decompose to release carbon dioxide \( \mathrm{CO}_2 \) when heated. This stems from their strong ionic bonds, which provide high thermal stability. Here's why:

• The ions in these compounds are larger and less tightly packed, allowing such compounds as \( \mathrm{Na}_2 \mathrm{CO}_3 \) to resist heat-induced breakdown.
• This stability is in contrast to the behaviour of carbonates of other metals which usually decompose upon heating.

Understanding this property is crucial, especially when comparing the thermal stability of different metal carbonates. Recognizing that alkali metal carbonates don't generally give off \( \mathrm{CO}_2 \) on heating can be a vital piece of knowledge for students studying thermal decomposition processes.

Lithium Carbonate Decomposition

Interestingly, lithium carbonate \( \mathrm{Li}_2 \mathrm{CO}_3 \) behaves differently from other alkali metal carbonates. Unlike sodium or rubidium carbonates, lithium carbonate decomposes when heated, contrary to the general rule for alkali metals.

• When heated, lithium carbonate decomposes into lithium oxide \( \mathrm{Li}_2 \mathrm{O} \) and carbon dioxide \( \mathrm{CO}_2 \). This process can be represented by the following chemical equation: \( \mathrm{Li}_2 \mathrm{CO}_3 \rightarrow \mathrm{Li}_2 \mathrm{O} + \mathrm{CO}_2 \).
• This exception is due to the smaller size of the lithium ion, which results in more covalent character and less thermal stability.

Such characteristics make \( \mathrm{Li}_2 \mathrm{CO}_3 \) an important exception among the alkali metal carbonates, displaying distinctive behavior that students need to pay attention to when learning about carbonate decomposition.

Magnesium Carbonate Decomposition

Magnesium carbonate \( \mathrm{MgCO}_3 \) stands out as a classic example of a carbonate that regularly undergoes decomposition upon heating, unlike its alkali metal counterparts such as sodium or rubidium carbonates. Here's how it behaves:

• Magnesium carbonate decomposes into magnesium oxide \( \mathrm{MgO} \) and carbon dioxide \( \mathrm{CO}_2 \), illustrated by the equation: \( \mathrm{MgCO}_3 \rightarrow \mathrm{MgO} + \mathrm{CO}_2 \).
• This decomposition is typical for carbonates of metals that are not alkali metals, primarily due to their lower thermal stability.

Understanding the decomposition of magnesium carbonate provides insight into common decomposition reactions and helps in contrasting the properties of various metal carbonates, highlighting the unique behavior of the alkali metal carbonates.